A heat pump ready cylinder carries an indirect coil of 2.0–3.0 m² — three to five times the 0.5–0.8 m² coil in a boiler cylinder — so it can absorb full heat pump output at a 45–55 °C primary temperature. It also needs low standing loss, full-height coil geometry, a high-mounted immersion heater and 28 mm connections.
What actually makes a cylinder heat pump ready?
Four measurable things, none of which is the word on the label. A cylinder is heat pump ready when its coil is large enough, positioned correctly, its standing loss is low and its controls allow a legionella cycle without wrecking seasonal efficiency.
| Feature | Boiler cylinder | Heat pump ready cylinder | Why it changes |
|---|---|---|---|
| Indirect coil area | 0.5 – 0.8 m² | 2.0 – 3.0 m² (twin-coil models to 6 m²) | Temperature difference halves |
| Design primary flow | 70 – 80 °C | 45 – 55 °C | Sets achievable water temperature |
| Coil geometry | Upper half of the vessel | Full height, bottom-fed | Cools the return, protects COP |
| Standing loss class | C – D | B or better | Store sits at a lower temperature |
| Immersion heater | Mid-height | Upper third | Legionella cycle without heating the whole store |
| Primary connections | 22 mm | 28 mm | Heat pumps need higher flow rates |
Why does coil surface area matter so much?
Because heat transfer is area multiplied by the temperature difference. A boiler pushing 75 °C water into a store at 30 °C works with a difference of about 45 K. A heat pump delivering 50 °C into the same store works with about 20 K. To move the same kilowatts through the wall of the coil you therefore need roughly twice the area — and because a heat pump must not be throttled by the cylinder, real specifications land at 2.0–3.0 m² for domestic sizes, or roughly 0.25–0.30 m² of coil per kW of heat pump output.
Fit a modern 10 kW heat pump to a 0.6 m² boiler coil and the physics answers immediately: the cylinder cannot accept the output, the primary return stays hot, the compressor lifts its condensing temperature, efficiency collapses and reheat takes hours instead of forty minutes. See what coil surface area does a heat pump cylinder need? for the sizing arithmetic in full.
What else separates a good cylinder from an adequate one?
Stratification. A heat pump reheats a store slowly and at a modest temperature, so it depends on the hot layer staying at the top. Good inlet diffusers, a tall narrow form factor and a full-height coil preserve stratification; a badly designed inlet destroys it and turns a 300 litre cylinder into 200 usable litres.
Standing loss. Under the EU energy labelling framework, water heaters and storage tanks have carried A+ to F labels since 26 September 2017, with classes set by standing loss in watts. A class B 300 litre cylinder loses meaningfully less per day than a class D one, and with a heat pump every one of those kilowatt hours costs COP-divided electricity.
Corrosion protection. Enamelled steel to DIN 4753 with a magnesium anode is the mainstream answer; stainless steel removes the anode requirement but is sensitive to chloride levels. Solimpeks builds both, along with hygienic combination stores.
Hygiene strategy. A hygienic tank heats domestic water instantaneously through a corrugated stainless coil, so there is no stored potable volume to sterilise. That sidesteps the legionella cycle entirely, which matters because raising a whole store to 60 °C with a heat pump is the single most expensive thing it does each week.
Does the cylinder always have to be replaced?
Usually, though not always — a large twin-coil solar cylinder with a generous lower coil sometimes qualifies. The test is coil area and connection size, not age or condition. That decision is worked through in can I reuse my existing hot water cylinder with a heat pump?
Which standards should appear on the datasheet?
EN 12897 for unvented indirectly heated storage water heaters up to 2,000 litres, including the standing-loss test; EN 12977-3 where the store is part of a solar system; DIN 4753 for construction and enamelling, with Part 3 covering water-side corrosion protection by enamel and anode; and the ErP energy label class. A datasheet that states litres and nothing else is not a specification — ask for coil area in m², standing loss in watts and the connection size before comparing prices.
Frequently asked questions
How big should the coil be in a heat pump cylinder?
Between 2.0 and 3.0 m² for a domestic system, which is roughly 0.25 to 0.30 m² per kW of heat pump output. Large or twin-coil cylinders reach 6 m² by combining two 3 m² coils for higher-output systems.
Can I use a standard cylinder with a heat pump?
Only at a cost. A 0.5 to 0.8 m² boiler coil cannot absorb heat pump output, so reheat takes hours, the primary return stays hot and seasonal efficiency drops sharply. Most installers treat cylinder replacement as unavoidable.
What size heat pump cylinder does a family need?
Around 180 to 250 litres for two or three people and 250 to 300 litres for four or five, larger than a boiler equivalent because the store is held at a lower temperature and reheats more slowly.
Does a heat pump cylinder need an immersion heater?
Yes, mounted in the upper third. It provides the periodic anti-legionella cycle and backup, while its high position means only the top of the store is heated electrically rather than the whole volume.
Sources & further reading
- EN 12897 — Unvented indirectly heated storage water heaters
- Commission Regulation (EU) 812/2013 — energy labelling of water heaters and hot water storage tanks
- DIN 4753 — Water heaters and heating installations: construction and corrosion protection
- EHPA — domestic hot water and heat pump system guidance