Name: Telford Tempest 400L Indirect Unvented Heat Pump Cylinder TSMI400/HP – Duplex Stainless Steel Hot Water Cylinder
Brand: AIZO Quality Heating
SKU: telford-tempest-400l-indirect-unvented-heat-pump-cylinder-tsmi400-hp-duplex-stainless-steel-hot-water-cylinder
Availability: InStock

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Telford Tempest 400L Indirect Unvented Heat Pump Cylinder TSMI400/HP – Duplex Stainless Steel Hot Water Cylinder

400 Litre Capacity Unvented Mains Pressure Duplex Stainless Steel 3 m² Heat Exchanger Coil 3 kW Immersion Included ErP Class C Full Unvented Kit Supplied

The Telford Tempest TSMI400/HP is a 400-litre indirect unvented hot water cylinder engineered specifically for use with air source and ground source heat pumps. Manufactured in the UK from Duplex 2304 stainless steel — a grade that delivers superior corrosion resistance and greater strength compared to standard austenitic stainless — the Tempest provides mains-pressure hot water with high flow rates throughout the property. Its purpose-designed 3 m² low-temperature heat exchanger coil maximises heat transfer efficiency from the heat pump's lower flow temperatures, while the factory-fitted 3 kW immersion heater provides reliable back-up heat when required. Supplied complete with a full unvented component kit, remote expansion vessel and wall-mounting bracket, the TSMI400/HP is ready to commission straight from the pallet.

Supplied Complete With:

✓ Acetal tundish

✓ Inlet control group

✓ Two-port motorised valve

✓ Remote expansion vessel

✓ Factory-fitted wall-mounting bracket

✓ Dual-probe thermostat

✓ Temperature & pressure relief valve (factory-fitted)

✓ 3 kW immersion heater (factory-fitted)

✓ 1" Female BSP connections & unvented kit

Key Features & Benefits

Duplex 2304 Stainless Steel Construction

Superior corrosion resistance and greater strength at lower weight compared to standard 316L or duplex 2101 grades. The cylinder ends are precision-manufactured and welded to the body, producing a strong, reliable pressure vessel rated for a 3 bar operating pressure and up to 12 bar incoming supply pressure.

3 m² Purpose-Designed Heat Pump Coil

Large-area low-temperature heat exchanger coil optimised for the lower flow temperatures of air and ground source heat pumps. The 316L grade stainless-steel 22mm compression fittings and coil ensure maximum heat transfer into stored water, improving system efficiency and reducing heat pump run time.

Factory-Fitted 3 kW Immersion Heater Backup

Pre-installed 3 kW immersion heater provides reliable electric backup heat, ensuring hot water availability during heat pump servicing, periods of peak demand or extreme weather. The 400 L size can optionally be supplied with an additional off-peak immersion heater to enable economy tariff charging.

Mains-Pressure Unvented Operation

Supplies hot water at full mains pressure, delivering powerful showers and high flow rates to multiple outlets simultaneously — without the need for a cold-water header tank or pump. Ideal for larger households, holiday lets, small commercial properties or any installation where high hot water demand is expected.

Fully Insulated — Low Standing Heat Loss

High-performance foam insulation minimises heat loss from stored water, reducing the frequency of heat pump re-heat cycles and cutting running costs. The TSMI400/HP achieves a standing heat loss of just 2.42 kWh/24h (101 W) — contributing to its ErP Class C energy efficiency rating.

Remote Expansion Vessel with Wall Bracket

External expansion vessel supplied factory-fitted with a wall-mounting bracket, keeping the cylinder footprint compact and making installation cleaner and more flexible. The remote vessel arrangement also simplifies future servicing and replacement.

Lifetime Warranty on the Inner Vessel

Telford backs the stainless steel inner container with a lifetime manufacturing warranty, with a 2-year warranty on all components. Made in Britain for over 30 years, Telford cylinders are a proven and trusted choice for heat pump installations across the UK. Terms & conditions apply.

⚠ Professional Installation Required: Unvented hot water cylinders must be installed by a qualified G3-competent person in compliance with Building Regulations Part G3 and the manufacturer's installation instructions. A 3-port valve (not included) must be purchased separately for heat pump system connections.

Technical Specifications — TSMI400/HP

Storage & Capacity
Nominal Capacity	400 L
Actual Usable Capacity	391 L
Thermostat Set Temperature	60°C
Heat Exchanger
Coil Surface Area	3 m²
Coil Material	316L grade stainless steel, 22mm compression wound
Heat Pump Flow & Return Connection	1" Female BSP
Immersion Heater
Immersion Heater	1 × 3 kW (factory-fitted); optional additional off-peak immersion available
Pressure
Operating Pressure	3 bar
Max. Incoming Supply Pressure	12 bar
T&P Relief Valve	½", 7 bar, factory-fitted
Energy Performance
ErP Energy Efficiency Class	C
Standing Heat Loss	2.42 kWh/24h (101 W)
Physical Dimensions
Height	1590 mm
Diameter	710 mm
Empty Weight	53 kg
Full Weight (water-filled)	444 kg
Insulation Thickness	40 mm (adds 40 mm to stated diameter)
Port Positions (from base)
A — Cold Feed Centreline	233 mm
B — Lower Thermostat Pocket	270 mm
C — HP Return	645 mm
D — HP Flow	740 mm
E — Upper Thermostat / Immersion	810 mm
F — Secondary Return	1240 mm
G — Hot Draw-Off	1340 mm
Connections
Heat Pump Flow & Return	1" Female BSP
Cold Feed	22mm
Hot Draw-Off	22mm
Secondary Return	22mm
Thermostat Pockets	22mm (× 2)
T&P Relief Valve	½"
Warranty
Inner Vessel (stainless steel)	Lifetime manufacturing warranty*
Components	2 years* *Terms & Conditions apply

ℹ Installation & Siting: Cylinder dimensions stated are for the vessel only and do not include the 40 mm foam insulation, which adds to the overall diameter. Ensure adequate floor-loading capacity is confirmed before installation — the TSMI400/HP weighs 444 kg when full. The cylinder can be sited virtually anywhere in the property; allow sufficient clearance for maintenance access and immersion heater removal.

Frequently Asked Questions

Is this cylinder compatible with any air source heat pump?

Yes. The TSMI400/HP is designed for use with any air source or ground source heat pump as the primary heat source. The 1" Female BSP flow and return connections are compatible with the majority of heat pump manufacturers. Note that a 3-port valve (sold separately) is required for most heat pump system configurations.

Why is a 400 L cylinder recommended for heat pump systems?

Heat pumps operate most efficiently when they run for longer, continuous cycles at lower temperatures rather than short bursts. A larger cylinder stores more thermal energy per heat pump run, reducing cycling frequency and improving the overall coefficient of performance (CoP). The 400 L size is particularly suited to larger households (4–6 bedrooms), holiday lets and small commercial premises with higher hot water demand.

What does the full unvented kit include, and what else do I need?

The cylinder is supplied with an acetal tundish, inlet control group, two-port motorised valve, remote expansion vessel with wall bracket, dual-probe thermostat, factory-fitted T&P relief valve and factory-fitted 3 kW immersion heater. You will additionally need a 3-port zone valve (not included) for the heat pump primary circuit, plus appropriate pipework, electrical supply for the immersion and any required programmer or controls. Always confirm requirements with your heat pump manufacturer's installation guidance.

Can a second immersion heater be added for off-peak electricity tariffs?

Yes. The 400–500 L Tempest heat pump cylinders can be supplied with an additional off-peak immersion heater, enabling the cylinder to be topped up or fully heated overnight using a cheaper Economy 7 or Agile-type electricity tariff. This is particularly useful when combined with solar PV or a smart energy tariff to further reduce running costs. Please specify the off-peak option at time of ordering.

What is the difference between Duplex 2304 and standard stainless steel?

Duplex 2304 stainless steel has a two-phase microstructure (austenitic + ferritic) that gives it approximately twice the yield strength of standard 316L austenitic stainless. This means thinner walls can achieve the same pressure rating, reducing the cylinder's empty weight. It also provides superior resistance to stress corrosion cracking and pitting, which is particularly important for unvented cylinders in areas with aggressive or chlorinated water supplies. Telford uses 2304, a step up from the 2101 duplex grade used by some other manufacturers.

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