Indirect hot water cylinder for heat pump:
what to look for when buying

Choosing the wrong indirect hot water cylinder for a heat pump is one of the most common specification mistakes on renewable heating projects. A cylinder designed for a gas boiler will underperform on a heat pump — not because of capacity, but because the coil surface area is too small to transfer heat at the lower flow temperatures a heat pump delivers. This guide covers the five specifications that matter most, how to size the cylinder correctly for MCS compliance, and where the Heatlyt HC series fits in the market.

Indirect vs direct cylinder: the distinction that matters for heat pumps

An indirect hot water cylinder heats domestic water via an internal heat exchanger coil. The primary heating circuit — the heat pump, solar thermal array, or boiler — circulates through the coil. Heat crosses from the primary circuit to the stored domestic water without the two water streams ever mixing. This is the correct configuration for heat pumps, solar thermal, and any system where the primary fluid may contain glycol inhibitors or pressurised circuit water that cannot enter the potable supply.

A direct cylinder uses an immersion heater element that sits inside the stored water and heats it electrically. There is no coil and no primary circuit connection. Direct cylinders cannot be connected to a heat pump and are only suitable as standalone electric DHW storage.

The term indirect hot water cylinder is the standard UK and Irish description. The same product is called a DHW tank in German and Scandinavian markets, a zasobnik ciepłej wody in Polish specifications, and an indirekt varmvattenberedare in Swedish. If you are sourcing for cross-border projects, confirm the product type rather than relying on the translated name alone.

Why heat pumps need a different cylinder to gas boilers

The critical difference between a gas boiler and a heat pump as a heat source is the primary flow temperature. A gas boiler typically delivers 70–80 °C to the cylinder coil. A heat pump in DHW mode typically delivers 45–55 °C — sometimes as low as 45 °C on a low-temperature ASHP. This matters because heat transfer through a coil is governed by the Log Mean Temperature Difference (LMTD) between the primary fluid and the stored water. At lower primary temperatures, the LMTD is smaller, which means a much larger coil surface area is required to transfer the same quantity of heat in the same time.

A standard enamel cylinder designed for a gas boiler typically has a coil of 1.0–2.0 m². Connect that cylinder to a heat pump running at 50 °C primary and you will find that charging a full tank takes three to four hours rather than one — because the coil area is insufficient for the lower driving temperature differential. In an MCS-assessed installation, the system will likely fail the performance check.

Heat pump-compatible indirect cylinders are therefore specified with coil surface areas of 3.0–6.0 m² depending on cylinder volume. This is not a premium feature — it is a fundamental requirement for the cylinder to perform correctly with a heat pump primary circuit.

The five specifications to check before buying

1. Coil surface area

This is the most important single specification. As a practical rule, the coil should provide at least 0.3 m² per kW of heat pump DHW output. For a heat pump with 10 kW heating capacity in DHW mode, a minimum of 3.0 m² of coil surface area is required. For larger or higher-output systems, aim for 0.4–0.5 m² per kW to keep charging times reasonable and to ensure the heat pump does not short-cycle while trying to raise a full cold tank.

The coil area figure should appear on the manufacturer's technical data sheet. If it is not stated, request it explicitly — any reputable manufacturer will provide it. A cylinder that lists only capacity and working pressure without coil area is not specified for heat pump use.

2. Coil material

Heat pump primary circuits frequently contain glycol-based antifreeze (particularly ASHP in climates where the evaporator can freeze). Copper coils can be used with inhibited glycol, but the inhibitor must be compatible with the coil material and must be monitored regularly. Stainless steel coils — particularly SUS316 grade — are chemically inert to all standard antifreeze formulations, require no compatibility monitoring, and resist the mild acidic conditions that develop in ageing primary circuits. They are the preferred choice for heat pump installations where long-term reliability matters more than initial cost.

3. Tank body material

The two main options for indirect hot water cylinders are enamel-lined mild steel and stainless steel. For heat pump applications specifically, stainless steel carries a meaningful advantage: because heat pump DHW systems operate at 45–55 °C rather than the 60–65 °C a gas boiler maintains, the stored water temperature spends more time in the Legionella growth range (20–45 °C) during partial-charge periods. Enamel-lined cylinders require a sacrificial magnesium anode that must be inspected and replaced every three to five years; if the anode is depleted, the enamel can pit and the cylinder will fail. Stainless steel cylinders contain no anode, require no sacrificial protection, and have a design life of 40–50 years rather than the 10–15 years typical of enamel-lined steel. See our detailed comparison in stainless steel vs enamel water tank for the full breakdown.

4. Cylinder capacity

Heat pump DHW charging is slower than gas boiler charging — even with a correctly sized coil. Because of this, heat pump cylinders are typically specified one size larger than the equivalent gas boiler installation would require. UK guidance is broadly 50–60 litres per occupant as a starting point, but this increases where the household has high simultaneous demand (multiple bathrooms, power showers). The standard sizes — 200 L, 300 L, 400 L — map well to households of 2–3, 3–5, and 5+ occupants respectively. If in doubt, size up: the cost of a larger cylinder is small relative to the cost of an undersized system that cannot meet peak demand.

5. Working pressure rating

Indirect cylinders are available in vented (open-vented, atmospheric) and unvented configurations. Unvented cylinders must meet the requirements of Building Regulations Part G and BS EN 12897 and must be installed by a G3-qualified engineer. The cylinder working pressure must match the system design — unvented cylinders are typically rated for 3.0–10.0 bar working pressure. Verify the cylinder's pressure and temperature relief valve (PRV/TPRV) settings against the system working conditions before specifying.

Coil surface area sizing reference table

The table below applies the 0.35 m² per kW guideline (a practical mid-range value between the minimum 0.3 m² and the comfortable 0.4 m² figure) across the common domestic heat pump output range, and maps each result to the appropriate Heatlyt HC model.

Stainless steel vs enamel: why the material choice matters more for heat pumps

The argument for stainless steel is stronger on heat pump installations than on gas boiler installations, for two reasons specific to how heat pumps operate.

Legionella risk at lower temperatures. A gas boiler DHW system routinely reaches 60–65 °C, which pasteurises the stored water and suppresses Legionella bacteria growth. A heat pump DHW system may operate at 50–55 °C, and if the occupant reduces the set point to improve heat pump efficiency, stored water can spend extended periods at 45–50 °C — the upper end of the Legionella growth range. Stainless steel is inherently smooth and non-porous, giving bacteria fewer anchor sites than the micro-pitting surface found in aged enamel. Combined with a weekly thermal disinfection cycle (a brief boost to 60 °C), a stainless steel cylinder provides a more hygienic long-term solution for lower-temperature DHW systems.

Anode-free maintenance. Enamel cylinders require a sacrificial magnesium anode to protect the steel body from corrosion where the enamel has pinholes — and all enamel linings develop pinholes over time. The anode must be inspected every three to five years and replaced when consumed. If this maintenance is missed on a heat pump installation (which may be in an out-of-sight plant room), the cylinder will fail within the sacrificial protection period. Stainless steel cylinders require no anode and no corrosion monitoring. The body material itself provides lifelong resistance to the water chemistry inside the cylinder.

Heatlyt HC series: indirect hot water cylinders for heat pump use

The Heatlyt HC range is manufactured at our facility in Hangzhou, China, using SUS304 stainless steel for the tank body and SUS316 stainless steel for the heat exchanger coil. SUS316 — the marine-grade alloy — is specified for the coil because it provides superior resistance to chloride stress corrosion, which can occur in primary circuits where the inhibitor regime is not perfectly maintained or where mains water is used as the primary fluid. The higher molybdenum content of SUS316 (2–3% Mo vs. 0% in SUS304) provides this protection at a modest material cost premium.

HC-200 — 200 L, 3.1 m² coil

The HC-200 is sized for households of two to three occupants connected to an ASHP with up to 8 kW of DHW heating capacity. The 3.1 m² SUS316 coil comfortably exceeds the 0.3 m²/kW minimum for an 8 kW primary circuit and allows full charging within a two-hour off-peak window in most UK climates. It is also widely used in continental Europe as a standard DHW tank on smaller residential ASHP projects.

HC-300 — 300 L, 4.7 m² coil

The HC-300 is the most commonly specified size for UK domestic heat pump installations. The 4.7 m² coil handles ASHP output up to 12 kW with significant margin — the higher coil area reduces charging time and allows the heat pump to operate at lower, more efficient flow temperatures without compromising hot water recovery. Suitable for households of three to five occupants with standard domestic demand.

HC-400 — 400 L, 5.0 m² coil

The HC-400 targets larger households (five or more occupants), light-commercial applications (small hotels, B&Bs, offices), and any project where simultaneous hot water demand from multiple bathrooms requires a larger stored volume. The 5.0 m² coil supports heat pump output up to 14 kW DHW capacity. Custom configurations with larger coil surface areas are available under the Heatlyt OEM programme for commercial projects above 14 kW.

OEM and wholesale supply from the manufacturer

Heatlyt supplies its HC range on an OEM and wholesale basis directly from the factory in Hangzhou. There is no intermediate distributor in the supply chain between the manufacturing floor and the distributor or installer receiving the product. This structure provides three practical advantages for European distributors evaluating a new cylinder range:

• Full manufacturer technical documentation: Dimensional drawings, material certifications (SUS304 body, SUS316 coil), pressure test records, and coil surface area verification are provided as standard with every order. This documentation is what MCS-registered installers and their certifying bodies require.
• OEM branding and custom specification: Cylinders can be supplied under a distributor's own brand label. Custom capacities (100 L to 500 L), coil configurations, and secondary coil additions for solar thermal are available on minimum order quantities. Contact the team for OEM specification sheets and lead times.
• Wholesale pricing without minimum order friction: Initial trial orders of single units or small batches are accepted for product evaluation. Pricing scales with volume — contact us for a tiered wholesale price list.

For background on manufacturing standards and material choices, see our articles on SUS304 vs SUS316 stainless steel for water tanks and weld quality and pitting corrosion prevention.

Key takeaways

• An indirect cylinder is essential for heat pump DHW — it separates the primary circuit fluid from the potable water supply via an internal coil.
• Heat pumps require a coil surface area of at least 0.3 m² per kW of DHW output — standard gas boiler cylinders (1.0–2.0 m² coil) will underperform significantly.
• Specify SUS316 stainless steel coil for compatibility with glycol-based primary circuits and long-term reliability.
• Stainless steel tank body eliminates the sacrificial anode maintenance requirement and extends design life to 40–50 years.
• Size the cylinder one step up from the gas boiler equivalent — heat pump DHW recovery is slower, and the stored volume is the buffer against peak demand.
• Request a coil surface area figure from any supplier before specifying — if it is not on the data sheet, ask for it in writing.
• The Heatlyt HC-200 (3.1 m²), HC-300 (4.7 m²), and HC-400 (5.0 m²) all exceed the minimum coil area requirement across their respective heat pump output ranges and come with full manufacturer technical documentation for MCS compliance.