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OSO Optima OGC300 Combined Heat Pump Cylinder & Buffer Tank | 225L DHW + 65L Buffer
Heat Pump Cylinder
225L DHW + 65L Buffer
Single Footprint
Energy Rating A/A
36.5 kW Coil Output
Up to 15 kW Heat Pump
10-Year Warranty
The OSO Optima OGC300 is a unique combined domestic hot water cylinder and buffer/volumiser tank in a single footprint, purpose-built for air source and ground source heat pump installations. Rather than requiring a separate cylinder and buffer tank with linking pipework between them, the OGC300 integrates a 225-litre DHW cylinder (upper section) and a 65-litre buffer/volumiser vessel (lower section) within a single insulated stainless steel casing, 595mm in diameter and 1,776mm tall. This dramatically reduces the space, pipework, and labour required compared to a conventional two-tank heat pump system. The buffer pre-heats the DHW cylinder continuously, which keeps the heat pump's Coefficient of Performance (COP) high by maintaining a steady load. When domestic hot water is called for, the heat pump reaches the required temperature faster because the cylinder is never fully cold — reducing the time the property is deprived of space heating during DHW mode. The OGC300 can serve heat pumps up to 15 kW output, supports a 36.5 kW rated coil, and is backed by a 10-year cylinder warranty. A 2.8 kW INCOTEC backup immersion heater — connectable to solar PV — provides supplementary DHW heating when required.
The Four Key Advantages
Space Saving
One unit replaces a separate cylinder and buffer tank — halving the footprint and eliminating inter-tank pipework.
Cost Saving
Lower installation cost than buying two separate tanks, less labour, fewer fittings, and simpler pipework routing.
Higher Efficiency
Buffer pre-heating keeps the heat pump COP high and reduces heat loss from exposed inter-tank pipework.
Greater Comfort
Pre-warmed DHW cylinder means shorter DHW-mode cycles — less time without space heating, faster hot water delivery.
Key Features
- Single-footprint, two-tank design — 225L DHW cylinder and 65L buffer/volumiser in one 595mm diameter unit, eliminating a separate buffer tank and all the pipework between them
- Pre-heating keeps COP high — the buffer continuously pre-warms the DHW cylinder, maintaining a steady heat pump load, avoiding cold-start efficiency losses and preserving the optimal Coefficient of Performance
- Shorter DHW-mode cycles — because the cylinder is never fully cold, the heat pump reaches the required DHW temperature faster, reducing the time the property goes without space heating
- Compatible with heat pumps up to 15 kW — suitable for the majority of domestic ASHP and GSHP systems currently installed in the UK
- 36.5 kW rated heating coil — high-capacity primary coil for fast heat-up and recovery (19-minute heat-up from 10°C; 13-minute 70% recovery time)
- 355-litre effective hot water capacity at 40°C — significantly more usable hot water than the nominal 225L cylinder volume, thanks to OSO's high-temperature stratification
- Dual A energy ratings — both DHW cylinder (upper) and buffer (lower) achieve ErP A class, supporting Building Regulations compliance and SAP calculations
- 2.8 kW INCOTEC backup immersion heater — provides DHW boost or backup when the heat pump is in maintenance, service, or defrost mode; connectable to solar PV for maximum self-consumption
- Solar PV ready — immersion heater position allows 100% of the DHW cylinder to be heated from solar PV or mains, maximising renewable self-consumption
- NANOPUR + VIP insulation — combination of PUR foam and Vacuum Insulation Panels (VIP) for exceptional standby heat retention, achieving just 45W / 1.08 kWh per day (DHW) and 20W / 0.74 kWh per day (buffer)
- ULTRAWELD stainless steel construction — laser-welded Duplex stainless steel cylinder resists corrosion in all water conditions; 10-year cylinder warranty
- S-DESIGN hidden connections — all pipework connections are concealed, keeping the plant room installation clean and professional
- Integrated G3 Building Regulations kit — 18L expansion vessel, inlet control valve (strainer, PRV, expansion relief), and tundish all factory-fitted, simplifying compliance
- Separate pressure circuits — upper DHW cylinder rated to 10 bar; lower buffer circuit rated to 2.5 bar (0.20 MPa), correctly matched to heat pump system pressures
- Temperature sensor slots — two sensor pockets (6mm and 8mm) pre-fitted for heat pump controller compatibility, supporting most ASHP and GSHP control systems
- TARIFF-READY immersion — backup immersion heater can be connected to off-peak electricity tariffs for cost-effective DHW top-up
OSO Technology Advantages
S-DESIGN — All Connections Hidden
NANOPUR + VIP — Premium Insulation
ULTRAWELD — Superior Corrosion Resistance
INCOTEC — Hard Water Durability
TARIFF-READY — Solar & Off-Peak Compatible
What's Included
OGC300 combined cylinder/buffer unit
2.8 kW INCOTEC immersion heater (factory-fitted)
T&P safety valve — 7 bar / 90–95°C (factory-fitted)
18L expansion vessel (factory-fitted)
G3 inlet group kit (strainer, PRV, expansion relief)
Tundish
Safety cut-out — upper and lower circuits
Temperature sensor pockets × 2 (6mm & 8mm)
Adjustable feet (factory-fitted, 0–40mm range)
Strain relievers × 2
Installation & commissioning manual
Service & commissioning logbook
How the OGC300 Works with a Heat Pump
ℹ️ Two vessels, one system: The lower 65L buffer acts as the heat pump's primary thermal store — absorbing heat from the pump and smoothing out cycling. The upper 225L DHW vessel is pre-warmed by the buffer through its internal coil, so it's never starting from cold when the household calls for hot water. When the heat pump switches into DHW mode via its three-way valve, the cylinder reaches temperature quickly — and space heating resumes sooner.
- Heat pump runs in space heating mode: Heat is circulated through the lower buffer vessel, maintaining it at heat pump flow temperature. The buffer is slowly pre-heating the DHW cylinder via the internal coil.
- DHW demand is triggered (by timer or temperature drop): The heat pump's three-way valve diverts flow into dedicated DHW mode, now heating the pre-warmed DHW cylinder via the primary coil. Because the cylinder is already partially warm, this cycle is significantly shorter than with a cold cylinder.
- DHW cylinder reaches setpoint: The heat pump returns to space heating mode. The shorter DHW cycle means less time the property goes without space heating — improving comfort, particularly in colder weather.
- Backup immersion provides top-up or boost: If the heat pump cannot meet DHW demand (e.g. during defrost, maintenance, or extreme cold), the 2.8 kW INCOTEC immersion heater activates to maintain DHW temperature. This can also be triggered by solar PV export for maximum self-consumption.
- COP remains high throughout: Because the buffer always provides a heat pump load, the heat pump avoids short-cycling and operates at larger, more efficient run cycles — maintaining peak COP rather than degrading through frequent starts and stops.
Technical Specifications
| Specification | Unit | DHW Cylinder (Upper) | Buffer / Volumiser (Lower) |
|---|---|---|---|
| Product Number | OSO | 11003523 | |
| Product Name | OGC300 — Optima | ||
| Volume | L | 225 | 65 |
| Effective Capacity at 40°C | L | 355 (combined) | — |
| Outer Diameter | mm | 595 | |
| Overall Height | mm | 1,776 | |
| Weight (empty) | kg | 65 | |
| Weight (full / net) | kg | 355 | |
| Energy Rating (ErP) | A | A | |
| Standby Heat Loss | W | 45 | 20 |
| Standby Heat Loss / 24 hrs | kWh/day | 1.08 | 0.74 |
| Thermostat Setpoint (immersion) | °C | 40–70 (adjustable) | — |
| Safety Thermostat Cut-out | °C | 85 | — |
| Max Operating Temperature | °C | 70 | — |
| Max Design Pressure | bar | 10 | 2.5 |
| T&P Valve Rating | bar / °C | 7 / 90–95 | — |
| Primary Coil Power Output | kW | 36.5 | — |
| Primary Coil Surface Area | m² | 2.6 | — |
| Primary Flow Rate (design) | L/h | 900 | — |
| Heat-up Time from 10°C (coil) | min | 19 | — |
| Recovery Time to 70% (coil) | min | 13 | — |
| Backup Immersion Power | kW | 2.8 @ 230V / 3.0 @ 240V | — |
| Immersion Heater Material | Incoloy 825 (INCOTEC) | — | |
| Electrical Supply | V / Hz | 220–240 VAC / 50 Hz | — |
| IP Classification | IP21 | ||
| Insulation Material | PUR + Vacuum Insulation Panels (VIP) | ||
| Insulation Thickness (average) | mm | 50 | |
| Expansion Vessel (supplied) | L | 18 | — |
| Buffer Connections | inch BSP | — | G 1" female (upper & lower) |
| DHW Cold Water Inlet | inch BSP | G ¾" female | — |
| DHW Hot Water Outlet | inch BSP | G ¾" female | — |
| Coil Connections (upper & lower) | inch BSP | G ¾" female | — |
| Immersion Heater Connection | inch BSP | G 1¼" female | — |
| T&P Valve Connection | inch BSP | G ½" female (factory-fitted) | — |
| Temperature Sensor Pockets | 2 × (6mm and 8mm compatible) | ||
| Adjustable Feet Range | mm | 0–40 (factory-fitted) | |
| Min Servicing Clearance (front) | mm | 400 | |
| Min Servicing Clearance (above) | mm | 150 | |
| Certification | KIWA Watertec — Building & Water Regulations approved | ||
| UKCA Marking | Yes | ||
| Quality / Environmental | ISO 9001 / ISO 14001 / ISO 45001 | ||
| Cylinder Warranty | 10 years | ||
| Components Warranty | 2 years | ||
Installation Overview
⚠️ Qualified installer required: The OGC300 is an unvented hot water cylinder. Installation of the DHW section must be carried out by a G3-qualified unvented hot water installer. Heat pump connections must be carried out by a competent heat pump engineer. All electrical work must comply with BS 7671 and IET Wiring Regulations (minimum 16A dedicated supply, 20A double-pole isolation switch with 3mm contact separation). Mains cable must be rated to 90°C continuously.
- Siting and floor preparation: Position the OGC300 on a solid, level floor capable of supporting 355 kg (full weight). Allow 400mm clearance in front of the junction box covers and 150mm above the top of the unit. Unscrew the adjustable feet a minimum of 15mm from the base before levelling.
- Fill the upper DHW cylinder first: Open the furthest hot water outlet and the mains cold supply stop cock. Allow water to flow until air is purged from the upper cylinder, then close taps and check all connections for leaks. The upper cylinder must be filled with water before the power is switched on.
- Connect the heat pump primary circuit: Connect the heat pump flow and return to the coil connections (E and F — G¾" female). Connect the buffer tank connections (C and D — G1" female) to the heat pump heating circuit as applicable. Fit a drain cock at the lowest point of the buffer circuit (not supplied). Fill and bleed the buffer/heating circuit in accordance with the heat pump manufacturer's instructions.
- Install temperature sensors: Fit 6mm or 8mm heat pump temperature sensors into the sensor pockets as required by your heat pump controller. Pull out the sensor bracket, insert the sensor, and refit ensuring full contact with the inner stainless steel wall surface.
- Connect DHW cold water inlet and hot water outlet: Connect mains cold water supply to connection H (G¾" female). Connect hot water outlet to connection K (G¾" female). A mixing valve (not supplied) is strongly recommended to prevent scalding at draw-off points — maximum stored temperature is 70°C.
- Connect T&P valve discharge and tundish: Connect the T&P valve discharge to a tundish and discharge pipe in accordance with Building Regulations Part G. Ensure a continuous fall to drain and a clear, visible discharge point. Discharge pipe must be metal rated to the relevant temperature.
- Electrical connections: Wire the 2.8 kW immersion heater to a dedicated 16A+ supply with a 20A double-pole isolation switch (3mm separation). Mains cable must withstand 90°C. Fit strain relievers (supplied) to both junction box cable entries. Do not switch on power until both vessels are filled with water.
- Commission and test: Verify all pipe connections for tightness and leaks. Test T&P valve by turning the knob counter-clockwise and confirming free flow to drain. Adjust immersion thermostat (40–70°C, minimum 60°C for Legionella prevention). Complete the G3 commissioning logbook (supplied) and hand all documentation to the customer.
💡 Mixer valve recommendation: The OGC300 stores DHW at up to 70°C. A thermostatic mixing valve (TMV) preset to a maximum of 60°C at draw-off is strongly recommended to prevent scalding risk, particularly where children or elderly users are present. This is a requirement under Part G of the Building Regulations for most domestic installations.
ℹ️ Annual servicing: The OGC300 requires annual inspection and maintenance by a competent person to validate the 10-year cylinder warranty. This includes testing both safety valves, checking and recharging the expansion vessel (target 3 bar pre-charge), cleaning the line strainer, and recording service in the commissioning logbook. The immersion heater element must be removed for inspection after 5 years, then every 3 years thereafter — particularly important in hard water areas.
Frequently Asked Questions
What is the difference between the DHW cylinder, the buffer, and the coil in the OGC300?
The OGC300 contains three distinct but connected elements. The upper 225L DHW cylinder stores potable domestic hot water for showers, baths, and taps. The lower 65L buffer/volumiser is a closed heating circuit vessel — it stores heat pump flow water (non-potable) and acts as a thermal flywheel, smoothing heat pump cycling. The 36.5 kW internal coil transfers heat from the heat pump's primary circuit into the DHW cylinder — it is a heat exchanger, not a separate vessel. The three work together so that buffer heat continuously pre-warms the DHW cylinder through the coil.
Why does combining the cylinder and buffer in one unit improve efficiency?
Two reasons. First, heat loss is reduced — in a conventional two-tank system, the pipework between the cylinder and buffer loses heat to the surrounding air (even if well lagged), whereas the OGC300 encapsulates both tanks within the same foam-insulated casing. Second, COP is maintained — because the buffer always provides a steady load to the heat pump, the heat pump runs in longer, more efficient cycles rather than short-cycling, which degrades efficiency. The OGC300 achieves both A-class ErP ratings for DHW and buffer.
Does the OGC300 work with both air source and ground source heat pumps?
Yes. The OGC300 is compatible with any heat pump — ASHP (air source) or GSHP (ground source) — up to 15 kW output that uses a three-way valve to switch between space heating and DHW modes. This covers the vast majority of domestic heat pump installations in the UK. Check your heat pump's flow rate and connection sizes against the OGC300's coil connections (G¾" female, 900 L/h design flow rate).
How much usable hot water does the OGC300 actually provide?
The nominal DHW cylinder volume is 225L, but the effective usable hot water capacity at 40°C draw-off temperature is 355 litres. This is because the cylinder stores water at up to 70°C — when blended down to 40°C comfort temperature, the volume of usable water is significantly greater than the nominal 225L. OSO rates this as sufficient for approximately 5.5 persons, making the OGC300 appropriate for larger family homes.
What is the maximum heat pump capacity the OGC300 can work with?
The OGC300 is designed for heat pumps up to 15 kW output. The primary heating coil has a rated power output of 36.5 kW at the design flow rate of 900 L/h, which comfortably accommodates the thermal output of domestic heat pumps in this range. For heat pumps larger than 15 kW, consult OSO for an appropriate commercial cylinder specification.
Can I connect solar PV to the OGC300?
Yes. The 2.8 kW backup immersion heater can be connected to a solar PV diverter device, allowing excess solar generation to heat the DHW cylinder instead of being exported to the grid. The immersion heater position within the OGC300 is designed so that it can heat 100% of the DHW cylinder volume when operating from solar or mains — maximising self-consumption and reducing running costs.
What are the two separate pressure ratings, and why do they differ?
The upper DHW cylinder is rated to 10 bar — a standard mains-pressure unvented rating for potable hot water systems. The lower buffer circuit is rated to 2.5 bar — appropriate for a closed heating system connected to a heat pump, where operating pressures are typically 1–2 bar. These are physically separate circuits with no direct water connection, so the pressure difference is not a problem. Never attempt to connect mains water pressure to the buffer circuit.
How does the OGC300 compare to installing a separate cylinder and buffer tank?
A conventional setup requires a DHW cylinder, a separate buffer tank, interconnecting pipework (and lagging), fittings for two G3 kits, and installation time for both units plus the pipework between them. The OGC300 eliminates all of this — single delivery, single installation footprint, single set of pipework connections, and one G3 kit included in the price. OSO reports significant reductions in both material cost and installation time. The combined insulation of both tanks within one casing also means less heat loss than two separately insulated tanks.
What warranty comes with the OGC300?
The stainless steel cylinder carries a 10-year warranty against defects in materials and manufacture — provided the cylinder is serviced annually by a competent person and the service logbook (supplied) is updated at each service visit. Electrical components and valve gear carry a 2-year warranty. Warranty conditions include correct installation by a professional, use with compliant mains water (chloride <250 mg/L, conductivity <750 µS/cm at 25°C, pH 6.0–9.5), and hard water protection (max 10°dH hardness — a water softener is recommended above this).
Does the OGC300 come with Building Regulations compliance documentation?
Yes. The OGC300 is approved to Building Regulations and Water Regulations by KIWA Watertec Ltd, the relevant UK notified body for unvented hot water systems. It carries UKCA marking and is supplied with a G3 inlet group kit (strainer, PRV, expansion relief, tundish) and an 18L expansion vessel, all factory-fitted. The supplied commissioning logbook, when completed by the G3-qualified installer, serves as the compliance record for Part G3 of the Building Regulations.
$968.86
Original: $2,768.17
-65%OSO Optima OGC300 Combined Heat Pump Cylinder & Buffer Tank | 225L DHW + 65L Buffer—
$2,768.17
$968.86More Images
OSO Optima OGC300 Combined Heat Pump Cylinder & Buffer Tank | 225L DHW + 65L Buffer
Heat Pump Cylinder
225L DHW + 65L Buffer
Single Footprint
Energy Rating A/A
36.5 kW Coil Output
Up to 15 kW Heat Pump
10-Year Warranty
The OSO Optima OGC300 is a unique combined domestic hot water cylinder and buffer/volumiser tank in a single footprint, purpose-built for air source and ground source heat pump installations. Rather than requiring a separate cylinder and buffer tank with linking pipework between them, the OGC300 integrates a 225-litre DHW cylinder (upper section) and a 65-litre buffer/volumiser vessel (lower section) within a single insulated stainless steel casing, 595mm in diameter and 1,776mm tall. This dramatically reduces the space, pipework, and labour required compared to a conventional two-tank heat pump system. The buffer pre-heats the DHW cylinder continuously, which keeps the heat pump's Coefficient of Performance (COP) high by maintaining a steady load. When domestic hot water is called for, the heat pump reaches the required temperature faster because the cylinder is never fully cold — reducing the time the property is deprived of space heating during DHW mode. The OGC300 can serve heat pumps up to 15 kW output, supports a 36.5 kW rated coil, and is backed by a 10-year cylinder warranty. A 2.8 kW INCOTEC backup immersion heater — connectable to solar PV — provides supplementary DHW heating when required.
The Four Key Advantages
Space Saving
One unit replaces a separate cylinder and buffer tank — halving the footprint and eliminating inter-tank pipework.
Cost Saving
Lower installation cost than buying two separate tanks, less labour, fewer fittings, and simpler pipework routing.
Higher Efficiency
Buffer pre-heating keeps the heat pump COP high and reduces heat loss from exposed inter-tank pipework.
Greater Comfort
Pre-warmed DHW cylinder means shorter DHW-mode cycles — less time without space heating, faster hot water delivery.
Key Features
- Single-footprint, two-tank design — 225L DHW cylinder and 65L buffer/volumiser in one 595mm diameter unit, eliminating a separate buffer tank and all the pipework between them
- Pre-heating keeps COP high — the buffer continuously pre-warms the DHW cylinder, maintaining a steady heat pump load, avoiding cold-start efficiency losses and preserving the optimal Coefficient of Performance
- Shorter DHW-mode cycles — because the cylinder is never fully cold, the heat pump reaches the required DHW temperature faster, reducing the time the property goes without space heating
- Compatible with heat pumps up to 15 kW — suitable for the majority of domestic ASHP and GSHP systems currently installed in the UK
- 36.5 kW rated heating coil — high-capacity primary coil for fast heat-up and recovery (19-minute heat-up from 10°C; 13-minute 70% recovery time)
- 355-litre effective hot water capacity at 40°C — significantly more usable hot water than the nominal 225L cylinder volume, thanks to OSO's high-temperature stratification
- Dual A energy ratings — both DHW cylinder (upper) and buffer (lower) achieve ErP A class, supporting Building Regulations compliance and SAP calculations
- 2.8 kW INCOTEC backup immersion heater — provides DHW boost or backup when the heat pump is in maintenance, service, or defrost mode; connectable to solar PV for maximum self-consumption
- Solar PV ready — immersion heater position allows 100% of the DHW cylinder to be heated from solar PV or mains, maximising renewable self-consumption
- NANOPUR + VIP insulation — combination of PUR foam and Vacuum Insulation Panels (VIP) for exceptional standby heat retention, achieving just 45W / 1.08 kWh per day (DHW) and 20W / 0.74 kWh per day (buffer)
- ULTRAWELD stainless steel construction — laser-welded Duplex stainless steel cylinder resists corrosion in all water conditions; 10-year cylinder warranty
- S-DESIGN hidden connections — all pipework connections are concealed, keeping the plant room installation clean and professional
- Integrated G3 Building Regulations kit — 18L expansion vessel, inlet control valve (strainer, PRV, expansion relief), and tundish all factory-fitted, simplifying compliance
- Separate pressure circuits — upper DHW cylinder rated to 10 bar; lower buffer circuit rated to 2.5 bar (0.20 MPa), correctly matched to heat pump system pressures
- Temperature sensor slots — two sensor pockets (6mm and 8mm) pre-fitted for heat pump controller compatibility, supporting most ASHP and GSHP control systems
- TARIFF-READY immersion — backup immersion heater can be connected to off-peak electricity tariffs for cost-effective DHW top-up
OSO Technology Advantages
S-DESIGN — All Connections Hidden
NANOPUR + VIP — Premium Insulation
ULTRAWELD — Superior Corrosion Resistance
INCOTEC — Hard Water Durability
TARIFF-READY — Solar & Off-Peak Compatible
What's Included
OGC300 combined cylinder/buffer unit
2.8 kW INCOTEC immersion heater (factory-fitted)
T&P safety valve — 7 bar / 90–95°C (factory-fitted)
18L expansion vessel (factory-fitted)
G3 inlet group kit (strainer, PRV, expansion relief)
Tundish
Safety cut-out — upper and lower circuits
Temperature sensor pockets × 2 (6mm & 8mm)
Adjustable feet (factory-fitted, 0–40mm range)
Strain relievers × 2
Installation & commissioning manual
Service & commissioning logbook
How the OGC300 Works with a Heat Pump
ℹ️ Two vessels, one system: The lower 65L buffer acts as the heat pump's primary thermal store — absorbing heat from the pump and smoothing out cycling. The upper 225L DHW vessel is pre-warmed by the buffer through its internal coil, so it's never starting from cold when the household calls for hot water. When the heat pump switches into DHW mode via its three-way valve, the cylinder reaches temperature quickly — and space heating resumes sooner.
- Heat pump runs in space heating mode: Heat is circulated through the lower buffer vessel, maintaining it at heat pump flow temperature. The buffer is slowly pre-heating the DHW cylinder via the internal coil.
- DHW demand is triggered (by timer or temperature drop): The heat pump's three-way valve diverts flow into dedicated DHW mode, now heating the pre-warmed DHW cylinder via the primary coil. Because the cylinder is already partially warm, this cycle is significantly shorter than with a cold cylinder.
- DHW cylinder reaches setpoint: The heat pump returns to space heating mode. The shorter DHW cycle means less time the property goes without space heating — improving comfort, particularly in colder weather.
- Backup immersion provides top-up or boost: If the heat pump cannot meet DHW demand (e.g. during defrost, maintenance, or extreme cold), the 2.8 kW INCOTEC immersion heater activates to maintain DHW temperature. This can also be triggered by solar PV export for maximum self-consumption.
- COP remains high throughout: Because the buffer always provides a heat pump load, the heat pump avoids short-cycling and operates at larger, more efficient run cycles — maintaining peak COP rather than degrading through frequent starts and stops.
Technical Specifications
| Specification | Unit | DHW Cylinder (Upper) | Buffer / Volumiser (Lower) |
|---|---|---|---|
| Product Number | OSO | 11003523 | |
| Product Name | OGC300 — Optima | ||
| Volume | L | 225 | 65 |
| Effective Capacity at 40°C | L | 355 (combined) | — |
| Outer Diameter | mm | 595 | |
| Overall Height | mm | 1,776 | |
| Weight (empty) | kg | 65 | |
| Weight (full / net) | kg | 355 | |
| Energy Rating (ErP) | A | A | |
| Standby Heat Loss | W | 45 | 20 |
| Standby Heat Loss / 24 hrs | kWh/day | 1.08 | 0.74 |
| Thermostat Setpoint (immersion) | °C | 40–70 (adjustable) | — |
| Safety Thermostat Cut-out | °C | 85 | — |
| Max Operating Temperature | °C | 70 | — |
| Max Design Pressure | bar | 10 | 2.5 |
| T&P Valve Rating | bar / °C | 7 / 90–95 | — |
| Primary Coil Power Output | kW | 36.5 | — |
| Primary Coil Surface Area | m² | 2.6 | — |
| Primary Flow Rate (design) | L/h | 900 | — |
| Heat-up Time from 10°C (coil) | min | 19 | — |
| Recovery Time to 70% (coil) | min | 13 | — |
| Backup Immersion Power | kW | 2.8 @ 230V / 3.0 @ 240V | — |
| Immersion Heater Material | Incoloy 825 (INCOTEC) | — | |
| Electrical Supply | V / Hz | 220–240 VAC / 50 Hz | — |
| IP Classification | IP21 | ||
| Insulation Material | PUR + Vacuum Insulation Panels (VIP) | ||
| Insulation Thickness (average) | mm | 50 | |
| Expansion Vessel (supplied) | L | 18 | — |
| Buffer Connections | inch BSP | — | G 1" female (upper & lower) |
| DHW Cold Water Inlet | inch BSP | G ¾" female | — |
| DHW Hot Water Outlet | inch BSP | G ¾" female | — |
| Coil Connections (upper & lower) | inch BSP | G ¾" female | — |
| Immersion Heater Connection | inch BSP | G 1¼" female | — |
| T&P Valve Connection | inch BSP | G ½" female (factory-fitted) | — |
| Temperature Sensor Pockets | 2 × (6mm and 8mm compatible) | ||
| Adjustable Feet Range | mm | 0–40 (factory-fitted) | |
| Min Servicing Clearance (front) | mm | 400 | |
| Min Servicing Clearance (above) | mm | 150 | |
| Certification | KIWA Watertec — Building & Water Regulations approved | ||
| UKCA Marking | Yes | ||
| Quality / Environmental | ISO 9001 / ISO 14001 / ISO 45001 | ||
| Cylinder Warranty | 10 years | ||
| Components Warranty | 2 years | ||
Installation Overview
⚠️ Qualified installer required: The OGC300 is an unvented hot water cylinder. Installation of the DHW section must be carried out by a G3-qualified unvented hot water installer. Heat pump connections must be carried out by a competent heat pump engineer. All electrical work must comply with BS 7671 and IET Wiring Regulations (minimum 16A dedicated supply, 20A double-pole isolation switch with 3mm contact separation). Mains cable must be rated to 90°C continuously.
- Siting and floor preparation: Position the OGC300 on a solid, level floor capable of supporting 355 kg (full weight). Allow 400mm clearance in front of the junction box covers and 150mm above the top of the unit. Unscrew the adjustable feet a minimum of 15mm from the base before levelling.
- Fill the upper DHW cylinder first: Open the furthest hot water outlet and the mains cold supply stop cock. Allow water to flow until air is purged from the upper cylinder, then close taps and check all connections for leaks. The upper cylinder must be filled with water before the power is switched on.
- Connect the heat pump primary circuit: Connect the heat pump flow and return to the coil connections (E and F — G¾" female). Connect the buffer tank connections (C and D — G1" female) to the heat pump heating circuit as applicable. Fit a drain cock at the lowest point of the buffer circuit (not supplied). Fill and bleed the buffer/heating circuit in accordance with the heat pump manufacturer's instructions.
- Install temperature sensors: Fit 6mm or 8mm heat pump temperature sensors into the sensor pockets as required by your heat pump controller. Pull out the sensor bracket, insert the sensor, and refit ensuring full contact with the inner stainless steel wall surface.
- Connect DHW cold water inlet and hot water outlet: Connect mains cold water supply to connection H (G¾" female). Connect hot water outlet to connection K (G¾" female). A mixing valve (not supplied) is strongly recommended to prevent scalding at draw-off points — maximum stored temperature is 70°C.
- Connect T&P valve discharge and tundish: Connect the T&P valve discharge to a tundish and discharge pipe in accordance with Building Regulations Part G. Ensure a continuous fall to drain and a clear, visible discharge point. Discharge pipe must be metal rated to the relevant temperature.
- Electrical connections: Wire the 2.8 kW immersion heater to a dedicated 16A+ supply with a 20A double-pole isolation switch (3mm separation). Mains cable must withstand 90°C. Fit strain relievers (supplied) to both junction box cable entries. Do not switch on power until both vessels are filled with water.
- Commission and test: Verify all pipe connections for tightness and leaks. Test T&P valve by turning the knob counter-clockwise and confirming free flow to drain. Adjust immersion thermostat (40–70°C, minimum 60°C for Legionella prevention). Complete the G3 commissioning logbook (supplied) and hand all documentation to the customer.
💡 Mixer valve recommendation: The OGC300 stores DHW at up to 70°C. A thermostatic mixing valve (TMV) preset to a maximum of 60°C at draw-off is strongly recommended to prevent scalding risk, particularly where children or elderly users are present. This is a requirement under Part G of the Building Regulations for most domestic installations.
ℹ️ Annual servicing: The OGC300 requires annual inspection and maintenance by a competent person to validate the 10-year cylinder warranty. This includes testing both safety valves, checking and recharging the expansion vessel (target 3 bar pre-charge), cleaning the line strainer, and recording service in the commissioning logbook. The immersion heater element must be removed for inspection after 5 years, then every 3 years thereafter — particularly important in hard water areas.
Frequently Asked Questions
What is the difference between the DHW cylinder, the buffer, and the coil in the OGC300?
The OGC300 contains three distinct but connected elements. The upper 225L DHW cylinder stores potable domestic hot water for showers, baths, and taps. The lower 65L buffer/volumiser is a closed heating circuit vessel — it stores heat pump flow water (non-potable) and acts as a thermal flywheel, smoothing heat pump cycling. The 36.5 kW internal coil transfers heat from the heat pump's primary circuit into the DHW cylinder — it is a heat exchanger, not a separate vessel. The three work together so that buffer heat continuously pre-warms the DHW cylinder through the coil.
Why does combining the cylinder and buffer in one unit improve efficiency?
Two reasons. First, heat loss is reduced — in a conventional two-tank system, the pipework between the cylinder and buffer loses heat to the surrounding air (even if well lagged), whereas the OGC300 encapsulates both tanks within the same foam-insulated casing. Second, COP is maintained — because the buffer always provides a steady load to the heat pump, the heat pump runs in longer, more efficient cycles rather than short-cycling, which degrades efficiency. The OGC300 achieves both A-class ErP ratings for DHW and buffer.
Does the OGC300 work with both air source and ground source heat pumps?
Yes. The OGC300 is compatible with any heat pump — ASHP (air source) or GSHP (ground source) — up to 15 kW output that uses a three-way valve to switch between space heating and DHW modes. This covers the vast majority of domestic heat pump installations in the UK. Check your heat pump's flow rate and connection sizes against the OGC300's coil connections (G¾" female, 900 L/h design flow rate).
How much usable hot water does the OGC300 actually provide?
The nominal DHW cylinder volume is 225L, but the effective usable hot water capacity at 40°C draw-off temperature is 355 litres. This is because the cylinder stores water at up to 70°C — when blended down to 40°C comfort temperature, the volume of usable water is significantly greater than the nominal 225L. OSO rates this as sufficient for approximately 5.5 persons, making the OGC300 appropriate for larger family homes.
What is the maximum heat pump capacity the OGC300 can work with?
The OGC300 is designed for heat pumps up to 15 kW output. The primary heating coil has a rated power output of 36.5 kW at the design flow rate of 900 L/h, which comfortably accommodates the thermal output of domestic heat pumps in this range. For heat pumps larger than 15 kW, consult OSO for an appropriate commercial cylinder specification.
Can I connect solar PV to the OGC300?
Yes. The 2.8 kW backup immersion heater can be connected to a solar PV diverter device, allowing excess solar generation to heat the DHW cylinder instead of being exported to the grid. The immersion heater position within the OGC300 is designed so that it can heat 100% of the DHW cylinder volume when operating from solar or mains — maximising self-consumption and reducing running costs.
What are the two separate pressure ratings, and why do they differ?
The upper DHW cylinder is rated to 10 bar — a standard mains-pressure unvented rating for potable hot water systems. The lower buffer circuit is rated to 2.5 bar — appropriate for a closed heating system connected to a heat pump, where operating pressures are typically 1–2 bar. These are physically separate circuits with no direct water connection, so the pressure difference is not a problem. Never attempt to connect mains water pressure to the buffer circuit.
How does the OGC300 compare to installing a separate cylinder and buffer tank?
A conventional setup requires a DHW cylinder, a separate buffer tank, interconnecting pipework (and lagging), fittings for two G3 kits, and installation time for both units plus the pipework between them. The OGC300 eliminates all of this — single delivery, single installation footprint, single set of pipework connections, and one G3 kit included in the price. OSO reports significant reductions in both material cost and installation time. The combined insulation of both tanks within one casing also means less heat loss than two separately insulated tanks.
What warranty comes with the OGC300?
The stainless steel cylinder carries a 10-year warranty against defects in materials and manufacture — provided the cylinder is serviced annually by a competent person and the service logbook (supplied) is updated at each service visit. Electrical components and valve gear carry a 2-year warranty. Warranty conditions include correct installation by a professional, use with compliant mains water (chloride <250 mg/L, conductivity <750 µS/cm at 25°C, pH 6.0–9.5), and hard water protection (max 10°dH hardness — a water softener is recommended above this).
Does the OGC300 come with Building Regulations compliance documentation?
Yes. The OGC300 is approved to Building Regulations and Water Regulations by KIWA Watertec Ltd, the relevant UK notified body for unvented hot water systems. It carries UKCA marking and is supplied with a G3 inlet group kit (strainer, PRV, expansion relief, tundish) and an 18L expansion vessel, all factory-fitted. The supplied commissioning logbook, when completed by the G3-qualified installer, serves as the compliance record for Part G3 of the Building Regulations.
Product Information
Product Information
Shipping & Returns
Shipping & Returns
Description
Heat Pump Cylinder
225L DHW + 65L Buffer
Single Footprint
Energy Rating A/A
36.5 kW Coil Output
Up to 15 kW Heat Pump
10-Year Warranty
The OSO Optima OGC300 is a unique combined domestic hot water cylinder and buffer/volumiser tank in a single footprint, purpose-built for air source and ground source heat pump installations. Rather than requiring a separate cylinder and buffer tank with linking pipework between them, the OGC300 integrates a 225-litre DHW cylinder (upper section) and a 65-litre buffer/volumiser vessel (lower section) within a single insulated stainless steel casing, 595mm in diameter and 1,776mm tall. This dramatically reduces the space, pipework, and labour required compared to a conventional two-tank heat pump system. The buffer pre-heats the DHW cylinder continuously, which keeps the heat pump's Coefficient of Performance (COP) high by maintaining a steady load. When domestic hot water is called for, the heat pump reaches the required temperature faster because the cylinder is never fully cold — reducing the time the property is deprived of space heating during DHW mode. The OGC300 can serve heat pumps up to 15 kW output, supports a 36.5 kW rated coil, and is backed by a 10-year cylinder warranty. A 2.8 kW INCOTEC backup immersion heater — connectable to solar PV — provides supplementary DHW heating when required.
The Four Key Advantages
Space Saving
One unit replaces a separate cylinder and buffer tank — halving the footprint and eliminating inter-tank pipework.
Cost Saving
Lower installation cost than buying two separate tanks, less labour, fewer fittings, and simpler pipework routing.
Higher Efficiency
Buffer pre-heating keeps the heat pump COP high and reduces heat loss from exposed inter-tank pipework.
Greater Comfort
Pre-warmed DHW cylinder means shorter DHW-mode cycles — less time without space heating, faster hot water delivery.
Key Features
- Single-footprint, two-tank design — 225L DHW cylinder and 65L buffer/volumiser in one 595mm diameter unit, eliminating a separate buffer tank and all the pipework between them
- Pre-heating keeps COP high — the buffer continuously pre-warms the DHW cylinder, maintaining a steady heat pump load, avoiding cold-start efficiency losses and preserving the optimal Coefficient of Performance
- Shorter DHW-mode cycles — because the cylinder is never fully cold, the heat pump reaches the required DHW temperature faster, reducing the time the property goes without space heating
- Compatible with heat pumps up to 15 kW — suitable for the majority of domestic ASHP and GSHP systems currently installed in the UK
- 36.5 kW rated heating coil — high-capacity primary coil for fast heat-up and recovery (19-minute heat-up from 10°C; 13-minute 70% recovery time)
- 355-litre effective hot water capacity at 40°C — significantly more usable hot water than the nominal 225L cylinder volume, thanks to OSO's high-temperature stratification
- Dual A energy ratings — both DHW cylinder (upper) and buffer (lower) achieve ErP A class, supporting Building Regulations compliance and SAP calculations
- 2.8 kW INCOTEC backup immersion heater — provides DHW boost or backup when the heat pump is in maintenance, service, or defrost mode; connectable to solar PV for maximum self-consumption
- Solar PV ready — immersion heater position allows 100% of the DHW cylinder to be heated from solar PV or mains, maximising renewable self-consumption
- NANOPUR + VIP insulation — combination of PUR foam and Vacuum Insulation Panels (VIP) for exceptional standby heat retention, achieving just 45W / 1.08 kWh per day (DHW) and 20W / 0.74 kWh per day (buffer)
- ULTRAWELD stainless steel construction — laser-welded Duplex stainless steel cylinder resists corrosion in all water conditions; 10-year cylinder warranty
- S-DESIGN hidden connections — all pipework connections are concealed, keeping the plant room installation clean and professional
- Integrated G3 Building Regulations kit — 18L expansion vessel, inlet control valve (strainer, PRV, expansion relief), and tundish all factory-fitted, simplifying compliance
- Separate pressure circuits — upper DHW cylinder rated to 10 bar; lower buffer circuit rated to 2.5 bar (0.20 MPa), correctly matched to heat pump system pressures
- Temperature sensor slots — two sensor pockets (6mm and 8mm) pre-fitted for heat pump controller compatibility, supporting most ASHP and GSHP control systems
- TARIFF-READY immersion — backup immersion heater can be connected to off-peak electricity tariffs for cost-effective DHW top-up
OSO Technology Advantages
S-DESIGN — All Connections Hidden
NANOPUR + VIP — Premium Insulation
ULTRAWELD — Superior Corrosion Resistance
INCOTEC — Hard Water Durability
TARIFF-READY — Solar & Off-Peak Compatible
What's Included
OGC300 combined cylinder/buffer unit
2.8 kW INCOTEC immersion heater (factory-fitted)
T&P safety valve — 7 bar / 90–95°C (factory-fitted)
18L expansion vessel (factory-fitted)
G3 inlet group kit (strainer, PRV, expansion relief)
Tundish
Safety cut-out — upper and lower circuits
Temperature sensor pockets × 2 (6mm & 8mm)
Adjustable feet (factory-fitted, 0–40mm range)
Strain relievers × 2
Installation & commissioning manual
Service & commissioning logbook
How the OGC300 Works with a Heat Pump
ℹ️ Two vessels, one system: The lower 65L buffer acts as the heat pump's primary thermal store — absorbing heat from the pump and smoothing out cycling. The upper 225L DHW vessel is pre-warmed by the buffer through its internal coil, so it's never starting from cold when the household calls for hot water. When the heat pump switches into DHW mode via its three-way valve, the cylinder reaches temperature quickly — and space heating resumes sooner.
- Heat pump runs in space heating mode: Heat is circulated through the lower buffer vessel, maintaining it at heat pump flow temperature. The buffer is slowly pre-heating the DHW cylinder via the internal coil.
- DHW demand is triggered (by timer or temperature drop): The heat pump's three-way valve diverts flow into dedicated DHW mode, now heating the pre-warmed DHW cylinder via the primary coil. Because the cylinder is already partially warm, this cycle is significantly shorter than with a cold cylinder.
- DHW cylinder reaches setpoint: The heat pump returns to space heating mode. The shorter DHW cycle means less time the property goes without space heating — improving comfort, particularly in colder weather.
- Backup immersion provides top-up or boost: If the heat pump cannot meet DHW demand (e.g. during defrost, maintenance, or extreme cold), the 2.8 kW INCOTEC immersion heater activates to maintain DHW temperature. This can also be triggered by solar PV export for maximum self-consumption.
- COP remains high throughout: Because the buffer always provides a heat pump load, the heat pump avoids short-cycling and operates at larger, more efficient run cycles — maintaining peak COP rather than degrading through frequent starts and stops.
Technical Specifications
| Specification | Unit | DHW Cylinder (Upper) | Buffer / Volumiser (Lower) |
|---|---|---|---|
| Product Number | OSO | 11003523 | |
| Product Name | OGC300 — Optima | ||
| Volume | L | 225 | 65 |
| Effective Capacity at 40°C | L | 355 (combined) | — |
| Outer Diameter | mm | 595 | |
| Overall Height | mm | 1,776 | |
| Weight (empty) | kg | 65 | |
| Weight (full / net) | kg | 355 | |
| Energy Rating (ErP) | A | A | |
| Standby Heat Loss | W | 45 | 20 |
| Standby Heat Loss / 24 hrs | kWh/day | 1.08 | 0.74 |
| Thermostat Setpoint (immersion) | °C | 40–70 (adjustable) | — |
| Safety Thermostat Cut-out | °C | 85 | — |
| Max Operating Temperature | °C | 70 | — |
| Max Design Pressure | bar | 10 | 2.5 |
| T&P Valve Rating | bar / °C | 7 / 90–95 | — |
| Primary Coil Power Output | kW | 36.5 | — |
| Primary Coil Surface Area | m² | 2.6 | — |
| Primary Flow Rate (design) | L/h | 900 | — |
| Heat-up Time from 10°C (coil) | min | 19 | — |
| Recovery Time to 70% (coil) | min | 13 | — |
| Backup Immersion Power | kW | 2.8 @ 230V / 3.0 @ 240V | — |
| Immersion Heater Material | Incoloy 825 (INCOTEC) | — | |
| Electrical Supply | V / Hz | 220–240 VAC / 50 Hz | — |
| IP Classification | IP21 | ||
| Insulation Material | PUR + Vacuum Insulation Panels (VIP) | ||
| Insulation Thickness (average) | mm | 50 | |
| Expansion Vessel (supplied) | L | 18 | — |
| Buffer Connections | inch BSP | — | G 1" female (upper & lower) |
| DHW Cold Water Inlet | inch BSP | G ¾" female | — |
| DHW Hot Water Outlet | inch BSP | G ¾" female | — |
| Coil Connections (upper & lower) | inch BSP | G ¾" female | — |
| Immersion Heater Connection | inch BSP | G 1¼" female | — |
| T&P Valve Connection | inch BSP | G ½" female (factory-fitted) | — |
| Temperature Sensor Pockets | 2 × (6mm and 8mm compatible) | ||
| Adjustable Feet Range | mm | 0–40 (factory-fitted) | |
| Min Servicing Clearance (front) | mm | 400 | |
| Min Servicing Clearance (above) | mm | 150 | |
| Certification | KIWA Watertec — Building & Water Regulations approved | ||
| UKCA Marking | Yes | ||
| Quality / Environmental | ISO 9001 / ISO 14001 / ISO 45001 | ||
| Cylinder Warranty | 10 years | ||
| Components Warranty | 2 years | ||
Installation Overview
⚠️ Qualified installer required: The OGC300 is an unvented hot water cylinder. Installation of the DHW section must be carried out by a G3-qualified unvented hot water installer. Heat pump connections must be carried out by a competent heat pump engineer. All electrical work must comply with BS 7671 and IET Wiring Regulations (minimum 16A dedicated supply, 20A double-pole isolation switch with 3mm contact separation). Mains cable must be rated to 90°C continuously.
- Siting and floor preparation: Position the OGC300 on a solid, level floor capable of supporting 355 kg (full weight). Allow 400mm clearance in front of the junction box covers and 150mm above the top of the unit. Unscrew the adjustable feet a minimum of 15mm from the base before levelling.
- Fill the upper DHW cylinder first: Open the furthest hot water outlet and the mains cold supply stop cock. Allow water to flow until air is purged from the upper cylinder, then close taps and check all connections for leaks. The upper cylinder must be filled with water before the power is switched on.
- Connect the heat pump primary circuit: Connect the heat pump flow and return to the coil connections (E and F — G¾" female). Connect the buffer tank connections (C and D — G1" female) to the heat pump heating circuit as applicable. Fit a drain cock at the lowest point of the buffer circuit (not supplied). Fill and bleed the buffer/heating circuit in accordance with the heat pump manufacturer's instructions.
- Install temperature sensors: Fit 6mm or 8mm heat pump temperature sensors into the sensor pockets as required by your heat pump controller. Pull out the sensor bracket, insert the sensor, and refit ensuring full contact with the inner stainless steel wall surface.
- Connect DHW cold water inlet and hot water outlet: Connect mains cold water supply to connection H (G¾" female). Connect hot water outlet to connection K (G¾" female). A mixing valve (not supplied) is strongly recommended to prevent scalding at draw-off points — maximum stored temperature is 70°C.
- Connect T&P valve discharge and tundish: Connect the T&P valve discharge to a tundish and discharge pipe in accordance with Building Regulations Part G. Ensure a continuous fall to drain and a clear, visible discharge point. Discharge pipe must be metal rated to the relevant temperature.
- Electrical connections: Wire the 2.8 kW immersion heater to a dedicated 16A+ supply with a 20A double-pole isolation switch (3mm separation). Mains cable must withstand 90°C. Fit strain relievers (supplied) to both junction box cable entries. Do not switch on power until both vessels are filled with water.
- Commission and test: Verify all pipe connections for tightness and leaks. Test T&P valve by turning the knob counter-clockwise and confirming free flow to drain. Adjust immersion thermostat (40–70°C, minimum 60°C for Legionella prevention). Complete the G3 commissioning logbook (supplied) and hand all documentation to the customer.
💡 Mixer valve recommendation: The OGC300 stores DHW at up to 70°C. A thermostatic mixing valve (TMV) preset to a maximum of 60°C at draw-off is strongly recommended to prevent scalding risk, particularly where children or elderly users are present. This is a requirement under Part G of the Building Regulations for most domestic installations.
ℹ️ Annual servicing: The OGC300 requires annual inspection and maintenance by a competent person to validate the 10-year cylinder warranty. This includes testing both safety valves, checking and recharging the expansion vessel (target 3 bar pre-charge), cleaning the line strainer, and recording service in the commissioning logbook. The immersion heater element must be removed for inspection after 5 years, then every 3 years thereafter — particularly important in hard water areas.
Frequently Asked Questions
What is the difference between the DHW cylinder, the buffer, and the coil in the OGC300?
The OGC300 contains three distinct but connected elements. The upper 225L DHW cylinder stores potable domestic hot water for showers, baths, and taps. The lower 65L buffer/volumiser is a closed heating circuit vessel — it stores heat pump flow water (non-potable) and acts as a thermal flywheel, smoothing heat pump cycling. The 36.5 kW internal coil transfers heat from the heat pump's primary circuit into the DHW cylinder — it is a heat exchanger, not a separate vessel. The three work together so that buffer heat continuously pre-warms the DHW cylinder through the coil.
Why does combining the cylinder and buffer in one unit improve efficiency?
Two reasons. First, heat loss is reduced — in a conventional two-tank system, the pipework between the cylinder and buffer loses heat to the surrounding air (even if well lagged), whereas the OGC300 encapsulates both tanks within the same foam-insulated casing. Second, COP is maintained — because the buffer always provides a steady load to the heat pump, the heat pump runs in longer, more efficient cycles rather than short-cycling, which degrades efficiency. The OGC300 achieves both A-class ErP ratings for DHW and buffer.
Does the OGC300 work with both air source and ground source heat pumps?
Yes. The OGC300 is compatible with any heat pump — ASHP (air source) or GSHP (ground source) — up to 15 kW output that uses a three-way valve to switch between space heating and DHW modes. This covers the vast majority of domestic heat pump installations in the UK. Check your heat pump's flow rate and connection sizes against the OGC300's coil connections (G¾" female, 900 L/h design flow rate).
How much usable hot water does the OGC300 actually provide?
The nominal DHW cylinder volume is 225L, but the effective usable hot water capacity at 40°C draw-off temperature is 355 litres. This is because the cylinder stores water at up to 70°C — when blended down to 40°C comfort temperature, the volume of usable water is significantly greater than the nominal 225L. OSO rates this as sufficient for approximately 5.5 persons, making the OGC300 appropriate for larger family homes.
What is the maximum heat pump capacity the OGC300 can work with?
The OGC300 is designed for heat pumps up to 15 kW output. The primary heating coil has a rated power output of 36.5 kW at the design flow rate of 900 L/h, which comfortably accommodates the thermal output of domestic heat pumps in this range. For heat pumps larger than 15 kW, consult OSO for an appropriate commercial cylinder specification.
Can I connect solar PV to the OGC300?
Yes. The 2.8 kW backup immersion heater can be connected to a solar PV diverter device, allowing excess solar generation to heat the DHW cylinder instead of being exported to the grid. The immersion heater position within the OGC300 is designed so that it can heat 100% of the DHW cylinder volume when operating from solar or mains — maximising self-consumption and reducing running costs.
What are the two separate pressure ratings, and why do they differ?
The upper DHW cylinder is rated to 10 bar — a standard mains-pressure unvented rating for potable hot water systems. The lower buffer circuit is rated to 2.5 bar — appropriate for a closed heating system connected to a heat pump, where operating pressures are typically 1–2 bar. These are physically separate circuits with no direct water connection, so the pressure difference is not a problem. Never attempt to connect mains water pressure to the buffer circuit.
How does the OGC300 compare to installing a separate cylinder and buffer tank?
A conventional setup requires a DHW cylinder, a separate buffer tank, interconnecting pipework (and lagging), fittings for two G3 kits, and installation time for both units plus the pipework between them. The OGC300 eliminates all of this — single delivery, single installation footprint, single set of pipework connections, and one G3 kit included in the price. OSO reports significant reductions in both material cost and installation time. The combined insulation of both tanks within one casing also means less heat loss than two separately insulated tanks.
What warranty comes with the OGC300?
The stainless steel cylinder carries a 10-year warranty against defects in materials and manufacture — provided the cylinder is serviced annually by a competent person and the service logbook (supplied) is updated at each service visit. Electrical components and valve gear carry a 2-year warranty. Warranty conditions include correct installation by a professional, use with compliant mains water (chloride <250 mg/L, conductivity <750 µS/cm at 25°C, pH 6.0–9.5), and hard water protection (max 10°dH hardness — a water softener is recommended above this).
Does the OGC300 come with Building Regulations compliance documentation?
Yes. The OGC300 is approved to Building Regulations and Water Regulations by KIWA Watertec Ltd, the relevant UK notified body for unvented hot water systems. It carries UKCA marking and is supplied with a G3 inlet group kit (strainer, PRV, expansion relief, tundish) and an 18L expansion vessel, all factory-fitted. The supplied commissioning logbook, when completed by the G3-qualified installer, serves as the compliance record for Part G3 of the Building Regulations.
