Why a cylinder
A combi boiler heats water the moment a tap opens, by burning gas fast enough to keep up with the flow. A heat pump works differently: it delivers heat steadily and at a moderate rate, and it cannot raise a flowing stream of cold water to shower temperature instantly. So the usual arrangement is to heat a store of water in advance, in an insulated cylinder, and draw from that store when a tap or shower runs.
The government's explainer is direct about it: a home with a heat pump needs space for the heat pump and a water cylinder. For many homes that switched to a combi, finding a place for a cylinder is one of the main jobs in fitting a heat pump.
How the cylinder works with the heat pump
The cylinder contains a coil, a large one in cylinders designed for heat pumps, through which the heat pump sends hot water. Heat passes from the coil into the stored water. Because a heat pump runs at lower temperatures than a boiler, the coil needs a bigger surface to pass heat across quickly enough, which is why a cylinder made for a heat pump differs from one made for a boiler.
When the stored water cools below a set point, the heat pump switches from heating the house to heating the cylinder, then back again. That switching is managed by the controls.
Why hot water costs more per kWh than space heating
Space heating can run at a low flow temperature for long periods. Hot water has to reach a higher temperature to be useful at the tap. The bigger lift means the heat pump delivers fewer units of heat per unit of electricity while it heats the cylinder than it does while it heats the house. That is one reason why measured seasonal figures, which include hot water, are lower than heating-only figures on a data sheet. It does not make hot water from a heat pump expensive, but it is worth knowing when reading efficiency claims.
Sizing the cylinder
- People and bathrooms. More people and more showers in a row need a bigger store.
- Reheat time. How quickly the heat pump can heat the cylinder back up after a busy morning.
- Space. Cylinders come in different shapes; tall slim ones fit some cupboards that a squat one would not.
- Standing heat loss. A well-insulated cylinder loses little heat while it waits, which matters for running cost.
A design should state the cylinder size and explain the choice. Too small and the household runs short; too large and it costs more to fit and keeps more water hot than needed.
The hygiene cycle
Stored hot water is periodically heated to a higher temperature as a precaution against bacteria growing in it. Systems often do this on a schedule, sometimes using the immersion heater for the final step. It uses a little extra electricity, which is part of the real running cost. The installer sets the schedule; it is worth knowing it exists if you see a short burst of higher consumption once a week.
Alternatives to a large cylinder
Where space is tight, there are options with trade-offs. Slimline and smaller cylinders fit tighter cupboards but store less. Heat batteries store heat in a material that changes phase, packing more heat into a small box; they can deliver hot water on demand from that store. Air-to-air heat pumps do not make hot water at all, so a home with one keeps a separate water heater. Each option changes how much hot water is available at once and what it costs to make.
Hot water and your bill
Hot water is a year-round load, so it matters in summer when no space heating runs. On the price cap for 1 October to 31 December 2026, electricity costs 26.32p a kWh; at the lower efficiency a heat pump achieves while heating water, the cost of a unit of hot water is higher than the cost of a unit of space heating, but still well below heating water with an immersion heater alone. The cost calculator lets you try lower efficiencies to see the effect. The government's rule of thumb of around 3 units of heat per unit of electricity is an average across both jobs.