Because a heat pump cannot make hot water on demand. A combi delivers 10–15 litres a minute by burning 24–35 kW instantly; an air source heat pump produces 5–12 kW at 50 °C, so it must charge a cylinder over an hour or two and store the result. Solar thermal has the same requirement — sunshine and showers do not coincide.
Why can't a heat pump work like a combi?
Power. A combi boiler is a burner sized for instantaneous heating: 24–35 kW of gas produces 10–15 litres a minute at shower temperature, and it does it the moment the tap opens. A domestic air source heat pump produces 5–12 kW, at 35–55 °C rather than 70–80 °C, and cannot be scaled up to combi output without an absurdly large machine and an electrical supply to match.
Storage solves the mismatch. The heat pump runs for an hour or two at its efficient output, the cylinder holds the result, and the household draws it at whatever rate it likes. The same logic applies to solar thermal: collectors produce heat at midday, and the tank is what carries it to the evening shower.
| Combi boiler | Heat pump + cylinder | |
|---|---|---|
| Instantaneous output | 24–35 kW | 5–12 kW |
| Hot water flow rate | 10–15 L/min, limited by burner | Limited only by mains flow, 20 L/min+ |
| Two showers at once | Rarely | Yes, until the tank is drawn down |
| Heating and hot water together | Yes | One at a time, hot water takes priority |
| Off-peak or solar charging | Not possible | Straightforward |
| Standing loss | None | 0.8–1.0 kWh/day, well-insulated 200 L |
How much space does a cylinder actually need?
Less than most people expect, but it is real. A 180–210 litre unvented cylinder is roughly 580–600 mm in diameter and 1.2–1.7 m tall, plus about 300 mm of clearance above for the immersion heater and anode. Slimline versions of 450–500 mm diameter exist at the cost of extra height and slightly worse stratification.
Airing cupboards, understairs voids and utility rooms take them comfortably. Lofts are possible but poor practice: heat losses go straight into an unheated space, and a leak has the whole house below it.
What do you gain by having a cylinder?
Five things a combi cannot do:
- Cheap heat. The tank can be charged on an off-peak tariff, from surplus PV, or by solar thermal collectors. A 200 litre cylinder holds about 11.6 kWh when heated from 10 °C to 60 °C — a usable thermal battery.
- Full mains flow. Output is limited by the plumbing rather than the burner, so 20 L/min showers and simultaneous outlets become possible.
- No cold sandwich. Stored water arrives at a constant temperature instead of the temperature swings a modulating combi produces at low flow.
- Solar thermal compatibility. A combi has nowhere to put solar heat; a twin coil cylinder is designed for it.
- Resilience. An immersion heater keeps hot water available if the main heat source fails.
What do you lose?
Space, and about a kilowatt-hour a day. A well-insulated 200 litre cylinder has a standing loss of 0.8–1.0 kWh per day — the Ecodesign ceiling is 16.66 + 8.33 × V^0.4 watts, and good tanks sit near 40% of it. You also accept a finite volume: once the tank is drawn down it needs an hour or more to recover, so sizing matters in a way it never did with a combi. Work it out with what size hot water tank you need, and remember that stored litres become roughly 1.5 times as many litres at shower temperature.
Can I keep the combi and add a cylinder?
Yes. A combi can charge an indirect cylinder through its heating circuit with a zone valve giving hot water priority, which is the standard route when a household outgrows a combi's flow rate or wants to add solar. It is also the natural first step towards a heat pump, since the cylinder is the expensive part of that conversion and a boiler is perfectly happy feeding an oversized coil.
If the motive is solar rather than flow rate, the practicalities are set out in combining solar thermal with a combi boiler. If a heat pump is likely later, buy a heat pump ready cylinder now — a large coil costs a boiler nothing and saves replacing the tank twice.
Frequently asked questions
Can a heat pump work without a hot water cylinder?
Only for space heating. Heat pumps produce 5–12 kW at 35–55 °C and cannot deliver the instantaneous 24–35 kW a combi uses for hot water, so domestic hot water has to be stored. The only exceptions are small point-of-use electric heaters or a district heat interface unit.
How much space does a hot water cylinder need?
A 180–210 litre unvented cylinder is about 580–600 mm across and 1.2–1.7 m tall, plus roughly 300 mm of clearance above for the immersion heater and anode. Slimline models of 450–500 mm diameter trade floor area for height.
Will a cylinder give me worse water pressure than my combi?
No — usually better. An unvented cylinder delivers at mains pressure and its flow rate is limited by the pipework rather than a burner, so it typically outperforms a combi at 20 litres a minute and supports two outlets at once.
Is a cylinder wasteful compared with heating on demand?
It costs 0.8–1.0 kWh a day in standing loss for a well-insulated 200 litre tank. Against that, it lets you heat with a heat pump at a COP above 3, or from solar or off-peak electricity, which more than repays the loss in almost every case.
Sources & further reading
- Commission Regulation (EU) No 814/2013 — Ecodesign requirements for water heaters and hot water storage tanks
- EN 12897:2016+A1:2020 — Water supply: specification for indirectly heated unvented (closed) storage water heaters
- Approved Document G — Sanitation, hot water safety and water efficiency (UK Government)