The short answer
Yes — in almost all cases an air source heat pump needs a hot water cylinder. Unlike a combi boiler, which heats water instantly at a high temperature whenever you turn on a tap, a heat pump heats water more gently over a longer period. So it heats and stores hot water in an insulated cylinder, ready for when you need it. The cylinder is usually a heat-pump-specific design with a large internal coil that transfers heat efficiently at the lower temperatures a heat pump produces. This means a home moving from a combi boiler will typically need to find space for a cylinder — often in an airing cupboard, loft or utility area. The cylinder also runs a periodic higher-temperature cycle to keep the stored water safe from bacteria. Sizing the cylinder to your household's hot water use is part of the system design.
The cylinder is one of the practical differences between a heat pump and a combi boiler, and the one most likely to need new space in the home. Understanding why it is needed makes the requirement clear.
Cylinder facts
- Cylinder needed?Yes, in almost all cases
- WhyHeat pump heats water gently, then stores it
- Cylinder typeHeat-pump-specific, large coil
- Space neededAiring cupboard, loft or utility
- Anti-bacteria measurePeriodic higher-temperature cycle
The combi habit a heat pump changes
Many UK homes run on a combi boiler, which has quietly shaped how people think about hot water. A combi makes hot water the instant a tap opens, with no tank anywhere in the house, so a generation of homeowners has never needed to think about storage. That is exactly why the cylinder question comes up so often when switching to a heat pump.
A heat pump works differently because of how it makes heat. It is efficient when it warms water gently and steadily, and inefficient if asked to deliver a sudden blast of very hot water on demand. Storing pre-heated water in an insulated cylinder resolves this: the pump heats the tank quietly in the background, and the household draws on it whenever needed. In effect, a heat pump home returns to the stored-hot-water model that system and regular boilers always used — the combi was the exception, not the rule. Understanding this makes the cylinder feel less like an inconvenience and more like a sensible match to how the technology works. It also opens up the option of heating the water during cheaper off-peak electricity windows, which a combi can never do.
Why a heat pump stores hot water instead of heating on demand
A combi boiler produces hot water instantly by burning gas at a high temperature the moment you open a tap. A heat pump cannot do this efficiently — it produces heat steadily at a lower temperature, not in a sudden high-temperature burst.
The solution is to heat water in advance and store it. The heat pump warms a tank of water — the hot water cylinder — over a period, and that stored hot water is then available on demand. Storing it means the pump can heat the water efficiently, including during cheaper off-peak electricity periods if you have a suitable tariff. This is the same principle as the traditional hot water cylinders used with older system and regular boilers.
What kind of cylinder, and where it goes
Heat pumps usually use a heat-pump-specific cylinder. The key feature is a large internal heat-exchange coil, which transfers heat into the stored water efficiently even at the lower temperatures a heat pump supplies. A standard cylinder designed for a hot boiler may have too small a coil to work well with a heat pump.
The cylinder needs space. In a home replacing a combi boiler — which has no cylinder — this often means finding room in an airing cupboard, loft, garage or utility area. The cylinder is sized to the household's hot water demand: too small and you run short on busy mornings, too large and you waste energy keeping excess water warm. The installer sizes it as part of the system design.
The large coil is the detail that most often catches people out, and it is worth understanding why it matters. A combi or hot boiler can heat a small coil quickly because it runs at a high temperature, so cylinder makers historically used modest coils. A heat pump supplies water at a much lower temperature, so to transfer the same amount of heat into the tank it needs far more coil surface area in contact with the stored water. A heat-pump-specific cylinder has a coil several times larger than a standard one, which lets the tank reheat in a reasonable time even at the heat pump's gentle flow temperature. Fitting a heat pump to an existing standard cylinder usually means the water heats too slowly and the pump is forced to run at a higher, less efficient temperature — which is why the cylinder is almost always replaced rather than reused. Where space is genuinely tight, slimline and stainless-steel designs can help, but the coil requirement is non-negotiable for the system to work efficiently.
| Feature | Combi boiler | Heat pump with cylinder |
|---|---|---|
| Hot water | Instant, on demand | Heated and stored in advance |
| Storage tank | None | Insulated cylinder required |
| Space needed | Minimal | Room for a cylinder |
| Off-peak heating | Not applicable | Possible with a suitable tariff |
Indicative comparison for guidance only. Sources: Energy Saving Trust; MCS.
Keeping stored water safe and efficient
Because water is stored rather than used instantly, the system manages it to keep it both safe and efficient. A heat pump heats the cylinder to a comfortable everyday temperature, but periodically runs a higher-temperature cycle to guard against bacteria such as Legionella, which can grow in water held at lukewarm temperatures. This cycle is built into the controls and runs automatically.
Good cylinder insulation keeps heat losses low between heating cycles, so the stored water stays warm with minimal waste. Sizing the cylinder correctly is important: it should comfortably meet the household's peak hot water demand — showers, baths, washing up — without being so large that the pump wastes energy keeping unused water hot. Done well, the cylinder gives a heat pump home plentiful hot water that is heated efficiently.
Frequently asked questions
Can I have a heat pump without a hot water cylinder?
In almost all cases, no. A heat pump heats water gently rather than instantly, so it needs a cylinder to heat and store hot water in advance. There are limited specialist arrangements, but for a normal home providing showers and baths, a hot water cylinder is a standard and necessary part of a heat pump system.
How much space does a heat pump cylinder need?
Enough room for an insulated tank, typically housed in an airing cupboard, loft, garage or utility space. The exact size depends on your household's hot water demand. Homes replacing a combi boiler, which has no cylinder, most often need to find new space — something the installer identifies during the survey.
Why does my heat pump cylinder heat to a high temperature sometimes?
That is the anti-bacteria cycle. Because hot water is stored rather than used immediately, the system periodically raises the cylinder temperature to guard against bacteria such as Legionella that can grow in lukewarm water. The cycle runs automatically through the controls and is a normal, important safety function.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific home. They are guidance, not a quotation or guaranteed saving.