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Baxi ASHP Cylinder 250L Stainless Steel Unvented Hot Water Cylinder for Air Source Heat Pumps
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Baxi ASHP Cylinder 250L Stainless Steel Unvented Hot Water Cylinder for Air Source Heat Pumps

Baxi ASHP Cylinder 250L Stainless Steel Unvented Hot Water Cylinder for Air Source Heat Pumps

The Baxi ASHP Cylinder is a duplex stainless steel unvented hot water cylinder engineered specifically for air source heat pump systems. Available in 210, 250 and 300 litre capacities, it features a high-surface-area 'coil-in-coil' heat exchanger for exceptionally fast recovery and includes two 3 kW immersion heaters — a lower main element and an upper boost — to support the heat pump for continuous hot water and Legionella protection. Manufactured by Megaflo and Heatrae Sadia, with 60 mm of high-efficiency PU insulation and supplied with a complete Immersion Heater Relay Kit for straightforward integration.

2 + 3 m²
Coil-in-coil surface area
2 × 3 kW
Dual immersion heaters
60 mm
PU foam insulation
Sec. return
Available on all sizes
Cylinder only — designed for installer-built control schemes
This is the cylinder vessel and immersion heater relay kit, intended for installations where the heating manifold, diverter valve and controls are designed and assembled separately by the installer. If you'd prefer an all-in-one solution with the diverter valve, expansion vessel and integrated controller already factory-fitted, look at the Baxi ASHP Pre-plumbed Cylinder instead.
Engineered for heat pumps
Coil-in-coil heat exchanger
A high-performance 'coil-in-coil' design delivers a large heat-transfer surface area in a compact footprint. The 250L and 300L models use a 3 m² coil; the 210L uses a 2 m² coil — both delivering exceptionally fast recovery times from heat pump flow temperatures.
Dual immersion heaters
A 3 kW lower immersion works with the heat pump for primary backup heating; a 3 kW upper immersion provides a high-temperature boost for fast top-up and the Legionella cycle. Total backup heating capacity is 6 kW when both elements run.
Immersion Heater Relay Kit included
Each cylinder ships with the Baxi Immersion Heater Relay Kit — relay, relay box and DHW temperature probe extension — so the immersion control logic and sensor wiring required to interface with the Baxi heat pump is included in the box.
Duplex stainless steel build
Inner vessel manufactured by Megaflo and Heatrae Sadia from corrosion-resistant duplex stainless steel for long, low-maintenance service life — the same proven build quality used across Baxi's residential cylinder range.
Secondary return and 60 mm insulation
All three capacities support a secondary return circuit for properties with long pipe runs to outlets. 60 mm of PU foam keeps standing losses low — from 1.5 kWh/24h on the 210L up to 2.05 kWh/24h on the 300L.
Air source heat pump compatibility

Designed to pair with the full Baxi air source heat pump portfolio — the HP40 standard-temperature monobloc, the Baxi Assure HP50, the HP55 high-temperature monobloc and the HP60 high-temperature monobloc. The cylinder is also suitable for use with other heat pump brands when sized correctly: refer to the heat pump's specification sheet for required coil rating and flow rates. Flow rates of up to 76 L/min at 3 bar can be achieved when the cylinder is supplied with a 3.5 bar pressure-reducing valve and adequate cold mains supply.

Specifications by capacity
Specification 210L 250L 300L
Product code 7837416 7837491 7837562
Nominal capacity 210 L 250 L 300 L
Storage volume 209 L 246 L 287 L
Hot water at >40 °C 204 L 242 L 289 L
Coil heat-up time 21.5 min 22.8 min 27.8 min
Coil surface area 2 m² 3 m² 3 m²
Coil rating 33.2 kW 36.8 kW 35.5 kW
Standing heat loss 1.5 kWh/24h 1.78 kWh/24h 2.05 kWh/24h
Energy efficiency class B C C
External expansion vessel 18 L 24 L 24 L
Height 1516 mm 1767 mm 2082 mm
Weight (empty) 46 kg 54 kg 62 kg
Weight (full) 255 kg 300 kg 349 kg
Common specifications (all capacities)
Diameter 579 mm
Insulation thickness 60 mm CFC/HCFC-free PU foam
Lower immersion heater 3 kW
Upper immersion (boost) 3 kW
Boost immersion heat-up time 67 minutes
Secondary return Yes — supported on all sizes
Connections 3/4" BSP male flow & return · 22 mm / 3/4" BSP cold inlet · 22 mm / 3/4" BSP DHW outlet
Inner vessel Duplex stainless steel — by Megaflo / Heatrae Sadia
Included accessories Immersion Heater Relay, Relay Box, DHW probe extension
Coil heat-up times measured at 15 L/min. Cylinders tested in conformance to Annex A, BS EN 12897:2016. Energy efficiency class measured per European Commission regulations 814/2013 and 812/2013. Manufacturer warranty applies — see baxi.co.uk/terms for details.
Installation requirements
Installation, commissioning and annual servicing must be carried out by a competent person — typically G3 unvented hot water qualified, and MCS registered for grant-funded heat pump installations. The unit must be installed in accordance with Building Regulations Part G, BS EN 12897:2016 and the Baxi Installation & Service Manual. An external expansion vessel (18 L on the 210L; 24 L on the 250L / 300L) and a 3.5 bar pressure-reducing valve are required and must be sized and fitted by the installer.
Frequently asked questions
Should I choose the standard ASHP cylinder or the pre-plumbed version?
Choose this standard cylinder if you want flexibility to design and assemble your own heating manifold, diverter valve scheme and controls — common on bespoke or larger installations. Choose the Baxi ASHP Pre-plumbed Cylinder if you want a faster, lower-labour install with the diverter valve, expansion vessel and integrated controller already factory-fitted. Both share the same duplex stainless steel inner vessel build.
What does the 'coil-in-coil' heat exchanger do?
A coil-in-coil arrangement places a finer secondary coil inside a primary coil, which significantly increases heat-transfer surface area within a compact footprint. This matters for heat pumps because they deliver heat at lower flow temperatures than a gas boiler, so a large, efficient coil is essential to recover hot water in a reasonable time — the design here delivers recovery times of 21–28 minutes depending on capacity.
Why does this cylinder have two immersion heaters?
The lower 3 kW immersion is positioned to heat the bulk of the cylinder volume, providing primary backup if the heat pump is unavailable. The upper 3 kW boost immersion sits higher in the vessel and only heats the top portion — useful for a fast hot-water top-up and for running the high-temperature Legionella cycle without consuming as much energy as heating the whole tank. Together they offer flexible backup heating tailored to the situation.
What size cylinder do I need?
As a guide: 210L matches a typical 3-bed, 1-bathroom home; 250L is a strong fit for a 3–4 bed property with two bathrooms; and 300L serves a 4-bed home with two or more bathrooms or an en-suite. For ASHP systems we recommend sizing slightly larger than you would for a gas boiler installation, because heat pumps reheat more slowly and benefit from drawing on a larger reservoir of pre-heated water.
What's included with the cylinder, and what do I need to source separately?
The cylinder ships with the Immersion Heater Relay Kit (relay, relay box and DHW temperature probe extension) and both factory-fitted immersion heaters. You'll typically need to supply separately: an external expansion vessel (18L for 210L cylinder; 24L for 250L/300L), a 3.5 bar pressure-reducing valve, a 6 bar pressure relief valve, a tundish, a 3-port motorised diverter valve, the heat pump's room/cylinder thermostat, and any zone valves or wiring centres needed for your control scheme.
$1,267.88

Original: $3,622.51

-65%
Baxi ASHP Cylinder 250L Stainless Steel Unvented Hot Water Cylinder for Air Source Heat Pumps—

$3,622.51

$1,267.88

Baxi ASHP Cylinder 250L Stainless Steel Unvented Hot Water Cylinder for Air Source Heat Pumps

The Baxi ASHP Cylinder is a duplex stainless steel unvented hot water cylinder engineered specifically for air source heat pump systems. Available in 210, 250 and 300 litre capacities, it features a high-surface-area 'coil-in-coil' heat exchanger for exceptionally fast recovery and includes two 3 kW immersion heaters — a lower main element and an upper boost — to support the heat pump for continuous hot water and Legionella protection. Manufactured by Megaflo and Heatrae Sadia, with 60 mm of high-efficiency PU insulation and supplied with a complete Immersion Heater Relay Kit for straightforward integration.

2 + 3 m²
Coil-in-coil surface area
2 × 3 kW
Dual immersion heaters
60 mm
PU foam insulation
Sec. return
Available on all sizes
Cylinder only — designed for installer-built control schemes
This is the cylinder vessel and immersion heater relay kit, intended for installations where the heating manifold, diverter valve and controls are designed and assembled separately by the installer. If you'd prefer an all-in-one solution with the diverter valve, expansion vessel and integrated controller already factory-fitted, look at the Baxi ASHP Pre-plumbed Cylinder instead.
Engineered for heat pumps
Coil-in-coil heat exchanger
A high-performance 'coil-in-coil' design delivers a large heat-transfer surface area in a compact footprint. The 250L and 300L models use a 3 m² coil; the 210L uses a 2 m² coil — both delivering exceptionally fast recovery times from heat pump flow temperatures.
Dual immersion heaters
A 3 kW lower immersion works with the heat pump for primary backup heating; a 3 kW upper immersion provides a high-temperature boost for fast top-up and the Legionella cycle. Total backup heating capacity is 6 kW when both elements run.
Immersion Heater Relay Kit included
Each cylinder ships with the Baxi Immersion Heater Relay Kit — relay, relay box and DHW temperature probe extension — so the immersion control logic and sensor wiring required to interface with the Baxi heat pump is included in the box.
Duplex stainless steel build
Inner vessel manufactured by Megaflo and Heatrae Sadia from corrosion-resistant duplex stainless steel for long, low-maintenance service life — the same proven build quality used across Baxi's residential cylinder range.
Secondary return and 60 mm insulation
All three capacities support a secondary return circuit for properties with long pipe runs to outlets. 60 mm of PU foam keeps standing losses low — from 1.5 kWh/24h on the 210L up to 2.05 kWh/24h on the 300L.
Air source heat pump compatibility

Designed to pair with the full Baxi air source heat pump portfolio — the HP40 standard-temperature monobloc, the Baxi Assure HP50, the HP55 high-temperature monobloc and the HP60 high-temperature monobloc. The cylinder is also suitable for use with other heat pump brands when sized correctly: refer to the heat pump's specification sheet for required coil rating and flow rates. Flow rates of up to 76 L/min at 3 bar can be achieved when the cylinder is supplied with a 3.5 bar pressure-reducing valve and adequate cold mains supply.

Specifications by capacity
Specification 210L 250L 300L
Product code 7837416 7837491 7837562
Nominal capacity 210 L 250 L 300 L
Storage volume 209 L 246 L 287 L
Hot water at >40 °C 204 L 242 L 289 L
Coil heat-up time 21.5 min 22.8 min 27.8 min
Coil surface area 2 m² 3 m² 3 m²
Coil rating 33.2 kW 36.8 kW 35.5 kW
Standing heat loss 1.5 kWh/24h 1.78 kWh/24h 2.05 kWh/24h
Energy efficiency class B C C
External expansion vessel 18 L 24 L 24 L
Height 1516 mm 1767 mm 2082 mm
Weight (empty) 46 kg 54 kg 62 kg
Weight (full) 255 kg 300 kg 349 kg
Common specifications (all capacities)
Diameter 579 mm
Insulation thickness 60 mm CFC/HCFC-free PU foam
Lower immersion heater 3 kW
Upper immersion (boost) 3 kW
Boost immersion heat-up time 67 minutes
Secondary return Yes — supported on all sizes
Connections 3/4" BSP male flow & return · 22 mm / 3/4" BSP cold inlet · 22 mm / 3/4" BSP DHW outlet
Inner vessel Duplex stainless steel — by Megaflo / Heatrae Sadia
Included accessories Immersion Heater Relay, Relay Box, DHW probe extension
Coil heat-up times measured at 15 L/min. Cylinders tested in conformance to Annex A, BS EN 12897:2016. Energy efficiency class measured per European Commission regulations 814/2013 and 812/2013. Manufacturer warranty applies — see baxi.co.uk/terms for details.
Installation requirements
Installation, commissioning and annual servicing must be carried out by a competent person — typically G3 unvented hot water qualified, and MCS registered for grant-funded heat pump installations. The unit must be installed in accordance with Building Regulations Part G, BS EN 12897:2016 and the Baxi Installation & Service Manual. An external expansion vessel (18 L on the 210L; 24 L on the 250L / 300L) and a 3.5 bar pressure-reducing valve are required and must be sized and fitted by the installer.
Frequently asked questions
Should I choose the standard ASHP cylinder or the pre-plumbed version?
Choose this standard cylinder if you want flexibility to design and assemble your own heating manifold, diverter valve scheme and controls — common on bespoke or larger installations. Choose the Baxi ASHP Pre-plumbed Cylinder if you want a faster, lower-labour install with the diverter valve, expansion vessel and integrated controller already factory-fitted. Both share the same duplex stainless steel inner vessel build.
What does the 'coil-in-coil' heat exchanger do?
A coil-in-coil arrangement places a finer secondary coil inside a primary coil, which significantly increases heat-transfer surface area within a compact footprint. This matters for heat pumps because they deliver heat at lower flow temperatures than a gas boiler, so a large, efficient coil is essential to recover hot water in a reasonable time — the design here delivers recovery times of 21–28 minutes depending on capacity.
Why does this cylinder have two immersion heaters?
The lower 3 kW immersion is positioned to heat the bulk of the cylinder volume, providing primary backup if the heat pump is unavailable. The upper 3 kW boost immersion sits higher in the vessel and only heats the top portion — useful for a fast hot-water top-up and for running the high-temperature Legionella cycle without consuming as much energy as heating the whole tank. Together they offer flexible backup heating tailored to the situation.
What size cylinder do I need?
As a guide: 210L matches a typical 3-bed, 1-bathroom home; 250L is a strong fit for a 3–4 bed property with two bathrooms; and 300L serves a 4-bed home with two or more bathrooms or an en-suite. For ASHP systems we recommend sizing slightly larger than you would for a gas boiler installation, because heat pumps reheat more slowly and benefit from drawing on a larger reservoir of pre-heated water.
What's included with the cylinder, and what do I need to source separately?
The cylinder ships with the Immersion Heater Relay Kit (relay, relay box and DHW temperature probe extension) and both factory-fitted immersion heaters. You'll typically need to supply separately: an external expansion vessel (18L for 210L cylinder; 24L for 250L/300L), a 3.5 bar pressure-reducing valve, a 6 bar pressure relief valve, a tundish, a 3-port motorised diverter valve, the heat pump's room/cylinder thermostat, and any zone valves or wiring centres needed for your control scheme.

Product Information

Shipping & Returns

Description

The Baxi ASHP Cylinder is a duplex stainless steel unvented hot water cylinder engineered specifically for air source heat pump systems. Available in 210, 250 and 300 litre capacities, it features a high-surface-area 'coil-in-coil' heat exchanger for exceptionally fast recovery and includes two 3 kW immersion heaters — a lower main element and an upper boost — to support the heat pump for continuous hot water and Legionella protection. Manufactured by Megaflo and Heatrae Sadia, with 60 mm of high-efficiency PU insulation and supplied with a complete Immersion Heater Relay Kit for straightforward integration.

2 + 3 m²
Coil-in-coil surface area
2 × 3 kW
Dual immersion heaters
60 mm
PU foam insulation
Sec. return
Available on all sizes
Cylinder only — designed for installer-built control schemes
This is the cylinder vessel and immersion heater relay kit, intended for installations where the heating manifold, diverter valve and controls are designed and assembled separately by the installer. If you'd prefer an all-in-one solution with the diverter valve, expansion vessel and integrated controller already factory-fitted, look at the Baxi ASHP Pre-plumbed Cylinder instead.
Engineered for heat pumps
Coil-in-coil heat exchanger
A high-performance 'coil-in-coil' design delivers a large heat-transfer surface area in a compact footprint. The 250L and 300L models use a 3 m² coil; the 210L uses a 2 m² coil — both delivering exceptionally fast recovery times from heat pump flow temperatures.
Dual immersion heaters
A 3 kW lower immersion works with the heat pump for primary backup heating; a 3 kW upper immersion provides a high-temperature boost for fast top-up and the Legionella cycle. Total backup heating capacity is 6 kW when both elements run.
Immersion Heater Relay Kit included
Each cylinder ships with the Baxi Immersion Heater Relay Kit — relay, relay box and DHW temperature probe extension — so the immersion control logic and sensor wiring required to interface with the Baxi heat pump is included in the box.
Duplex stainless steel build
Inner vessel manufactured by Megaflo and Heatrae Sadia from corrosion-resistant duplex stainless steel for long, low-maintenance service life — the same proven build quality used across Baxi's residential cylinder range.
Secondary return and 60 mm insulation
All three capacities support a secondary return circuit for properties with long pipe runs to outlets. 60 mm of PU foam keeps standing losses low — from 1.5 kWh/24h on the 210L up to 2.05 kWh/24h on the 300L.
Air source heat pump compatibility

Designed to pair with the full Baxi air source heat pump portfolio — the HP40 standard-temperature monobloc, the Baxi Assure HP50, the HP55 high-temperature monobloc and the HP60 high-temperature monobloc. The cylinder is also suitable for use with other heat pump brands when sized correctly: refer to the heat pump's specification sheet for required coil rating and flow rates. Flow rates of up to 76 L/min at 3 bar can be achieved when the cylinder is supplied with a 3.5 bar pressure-reducing valve and adequate cold mains supply.

Specifications by capacity
Specification 210L 250L 300L
Product code 7837416 7837491 7837562
Nominal capacity 210 L 250 L 300 L
Storage volume 209 L 246 L 287 L
Hot water at >40 °C 204 L 242 L 289 L
Coil heat-up time 21.5 min 22.8 min 27.8 min
Coil surface area 2 m² 3 m² 3 m²
Coil rating 33.2 kW 36.8 kW 35.5 kW
Standing heat loss 1.5 kWh/24h 1.78 kWh/24h 2.05 kWh/24h
Energy efficiency class B C C
External expansion vessel 18 L 24 L 24 L
Height 1516 mm 1767 mm 2082 mm
Weight (empty) 46 kg 54 kg 62 kg
Weight (full) 255 kg 300 kg 349 kg
Common specifications (all capacities)
Diameter 579 mm
Insulation thickness 60 mm CFC/HCFC-free PU foam
Lower immersion heater 3 kW
Upper immersion (boost) 3 kW
Boost immersion heat-up time 67 minutes
Secondary return Yes — supported on all sizes
Connections 3/4" BSP male flow & return · 22 mm / 3/4" BSP cold inlet · 22 mm / 3/4" BSP DHW outlet
Inner vessel Duplex stainless steel — by Megaflo / Heatrae Sadia
Included accessories Immersion Heater Relay, Relay Box, DHW probe extension
Coil heat-up times measured at 15 L/min. Cylinders tested in conformance to Annex A, BS EN 12897:2016. Energy efficiency class measured per European Commission regulations 814/2013 and 812/2013. Manufacturer warranty applies — see baxi.co.uk/terms for details.
Installation requirements
Installation, commissioning and annual servicing must be carried out by a competent person — typically G3 unvented hot water qualified, and MCS registered for grant-funded heat pump installations. The unit must be installed in accordance with Building Regulations Part G, BS EN 12897:2016 and the Baxi Installation & Service Manual. An external expansion vessel (18 L on the 210L; 24 L on the 250L / 300L) and a 3.5 bar pressure-reducing valve are required and must be sized and fitted by the installer.
Frequently asked questions
Should I choose the standard ASHP cylinder or the pre-plumbed version?
Choose this standard cylinder if you want flexibility to design and assemble your own heating manifold, diverter valve scheme and controls — common on bespoke or larger installations. Choose the Baxi ASHP Pre-plumbed Cylinder if you want a faster, lower-labour install with the diverter valve, expansion vessel and integrated controller already factory-fitted. Both share the same duplex stainless steel inner vessel build.
What does the 'coil-in-coil' heat exchanger do?
A coil-in-coil arrangement places a finer secondary coil inside a primary coil, which significantly increases heat-transfer surface area within a compact footprint. This matters for heat pumps because they deliver heat at lower flow temperatures than a gas boiler, so a large, efficient coil is essential to recover hot water in a reasonable time — the design here delivers recovery times of 21–28 minutes depending on capacity.
Why does this cylinder have two immersion heaters?
The lower 3 kW immersion is positioned to heat the bulk of the cylinder volume, providing primary backup if the heat pump is unavailable. The upper 3 kW boost immersion sits higher in the vessel and only heats the top portion — useful for a fast hot-water top-up and for running the high-temperature Legionella cycle without consuming as much energy as heating the whole tank. Together they offer flexible backup heating tailored to the situation.
What size cylinder do I need?
As a guide: 210L matches a typical 3-bed, 1-bathroom home; 250L is a strong fit for a 3–4 bed property with two bathrooms; and 300L serves a 4-bed home with two or more bathrooms or an en-suite. For ASHP systems we recommend sizing slightly larger than you would for a gas boiler installation, because heat pumps reheat more slowly and benefit from drawing on a larger reservoir of pre-heated water.
What's included with the cylinder, and what do I need to source separately?
The cylinder ships with the Immersion Heater Relay Kit (relay, relay box and DHW temperature probe extension) and both factory-fitted immersion heaters. You'll typically need to supply separately: an external expansion vessel (18L for 210L cylinder; 24L for 250L/300L), a 3.5 bar pressure-reducing valve, a 6 bar pressure relief valve, a tundish, a 3-port motorised diverter valve, the heat pump's room/cylinder thermostat, and any zone valves or wiring centres needed for your control scheme.