Usually not. A boiler cylinder carries a 0.5–0.8 m² coil designed for 70–80 °C flow, while a heat pump runs at 45–55 °C and needs 2.5–3.0 m² to reheat the tank in a sensible time. Keeping the old cylinder typically triples the reheat time and hands the work to the immersion heater at three times the running cost.
Can an existing cylinder work with a heat pump?
Technically yes, practically almost never. The cylinder will heat up — but slowly, and at a fraction of the efficiency the heat pump was bought for. The obstacle is the coil heat exchanger inside it. A cylinder built for a gas or oil boiler carries roughly 0.5–0.8 m² of coil, sized for a primary flow temperature of 70–80 °C. A heat pump delivers 45–55 °C. Less than half the temperature difference across the same surface means less than half the heat transfer, and the tank takes two to three times as long to recover.
That is why most certified installers treat cylinder replacement as part of the job rather than an upsell, and why many scheme rules effectively require it.
What actually goes wrong if you keep the old cylinder?
Four things, in a predictable sequence:
- Reheat time stretches. A 200 litre tank that recovered in 45 minutes on a boiler takes two to three hours on a heat pump through an undersized coil.
- The heat pump is forced to run hotter. To push heat through a small coil, the control raises the flow temperature — and every degree above 45 °C costs roughly 2–2.5% of COP.
- The immersion heater takes over. When the tank cannot make temperature in the scheduled window, the backup element runs at COP 1.0 — around three times the cost per kilowatt-hour of the heat pump it is standing in for.
- Legionella cycles become expensive. A weekly 60 °C pasteurisation through an undersized coil is nearly always completed by the immersion element rather than the compressor.
The result is a bill that looks nothing like the quotation, and an owner who blames the heat pump for a cylinder problem.
When is reuse actually acceptable?
There are genuine exceptions. Check yours against this table before assuming replacement:
| Situation | Reuse the cylinder? | Why |
|---|---|---|
| Standard indirect cylinder, 0.5–0.8 m² coil | No | Reheat time and efficiency both collapse |
| Cylinder already specified for solar or heat pump duty (2.5 m²+) | Yes | It was built for low-temperature primaries |
| Twin-coil solar cylinder with a large lower coil | Often yes | Feed the heat pump into the large solar coil |
| Thermal store or open-vented cylinder heated directly | Case by case | Needs a hydraulic review, not a rule of thumb |
| Direct electric cylinder (immersion only, no coil) | No | There is no coil for the heat pump to use |
| Cylinder over 15 years old, any type | No | Reusing it risks the whole installation on a worn tank |
If the cylinder is a twin-coil solar model, you are in the best position of all: connect the heat pump to the lower, larger coil and keep the upper coil for the boiler or immersion. For new installations, Solimpeks' Enamel TSE-VS HP tanks carry dedicated heat pump coils of 2.1–4.0 m² in the 200–500 L sizes.
How do I check my cylinder's coil area?
Three routes, in order of reliability:
- The data plate or manufacturer's datasheet. Look for "coil surface area" in m², or a coil rating in kW quoted at a stated primary temperature. If the rating is given at 80 °C primary, it tells you nothing about performance at 50 °C.
- The recovery time. A cylinder that reheats fully in under an hour from a boiler probably has a coil under 1 m².
- The label. Cylinders marketed as "heat pump", "solar" or "renewable ready" generally carry 2.5 m² or more; anything marked simply "indirect" generally does not.
Full figures for each cylinder size are in what coil surface area a heat pump cylinder needs.
Is there an alternative to full replacement?
Yes — an external plate heat exchanger with its own pump, which bypasses the internal coil altogether and charges the cylinder through its connections. It works, and it is a reasonable answer where the existing cylinder is nearly new. It also adds a pump, a control strategy, a second circulation loss and a hygiene consideration, and in most cases the installed cost lands close to a purpose-built cylinder that will last another 20 years.
Do the arithmetic honestly: if reusing the cylinder costs an extra 400–700 kWh a year in immersion heating, a properly sized replacement pays for itself well inside the life of the heat pump. Sizing guidance for the replacement is in what size hot water tank do I need.
Frequently asked questions
Do I have to replace my hot water cylinder for a heat pump?
In most retrofits, yes. Standard boiler cylinders carry 0.5–0.8 m² coils and heat pumps need 2.5–3.0 m². Cylinders already built for solar or heat pump duty, and twin-coil solar cylinders with a large lower coil, are the main exceptions.
What size coil does my existing cylinder have?
Check the data plate or the manufacturer's datasheet for coil surface area in m². Standard indirect cylinders are typically 0.5–0.8 m²; anything labelled solar, heat pump or renewable-ready is usually 2.5 m² or more.
Can I add a plate heat exchanger instead of a new cylinder?
Yes. An external plate heat exchanger with its own pump charges the cylinder through its connections and bypasses the undersized coil. It adds a pump, controls and circulation losses, and usually costs close to a purpose-built cylinder.
How long should a heat pump take to reheat a cylinder?
Sixty to ninety minutes for a 200 litre tank from cold to 50 °C with a correctly sized coil. If yours takes two to three hours, the coil is almost certainly too small and the immersion heater is quietly making up the difference.
Sources & further reading
- EN 12897:2016+A1:2020 — Water supply: specification for indirectly heated unvented (closed) storage water heaters
- EN 12977-3:2018 — Thermal solar systems and components: performance test methods for solar water heater stores
- EN 12831-3 — Heating systems in buildings: domestic hot water heat load and characterisation of needs