Heat pump hot water in the UK works very differently from what you’re used to with a gas combi boiler — and the differences matter. A combi boiler heats water on demand to 60–70°C in seconds. A heat pump works far more slowly, at lower temperatures, and typically stores hot water in a cylinder rather than heating it on demand. Get the cylinder size right, schedule production intelligently, and manage the Legionella requirement sensibly, and you’ll have reliable, cost-effective hot water. Get any of those wrong and you’ll either run out of hot water or spend more on electricity than necessary. This guide covers all of it in plain English.
Gas combi boilers heat water to 65°C or higher on demand — fast enough to run hot taps and showers without any stored water. Heat pumps prefer to operate at lower temperatures (45–55°C for domestic hot water is typical) and take much longer to heat a cylinder because their output is steady and moderate rather than a powerful gas burner. A heat pump providing domestic hot water must:
This is more complex than a combi boiler, but it’s entirely manageable. Heat pump systems from reputable manufacturers handle most of this automatically — you just need the right cylinder size and sensible scheduling settings.
If you currently have a gas combi boiler, you almost certainly don’t have a hot water storage cylinder. Installing a heat pump means installing one. The cylinder is the buffer between the heat pump’s slow, steady heat production and your household’s variable, unpredictable demand. Size it too small and you run out of hot water during a busy morning; size it too large and you’re heating water you don’t use, wasting energy.
Heat pump cylinders are purpose-built for heat pump operation. Unlike standard cylinders designed for a boiler’s high-temperature coil, heat pump cylinders have a larger internal coil — giving more surface area to transfer heat from the heat pump circuit at lower temperatures. Using a standard indirect cylinder with a heat pump is possible but compromises efficiency. A heat pump-specific cylinder (sometimes called a thermal store or buffer cylinder) is strongly recommended and is what all reputable installers will specify.
| Household size | Recommended cylinder size | Approximate cost (2026) | Notes |
|---|---|---|---|
| 1–2 people | 150–180 litres | £500 – £800 | Minimum for a heat pump system |
| 2–3 people | 200–250 litres | £700 – £1,100 | Most common size for small families |
| 3–4 people | 250–300 litres | £900 – £1,300 | Recommended for typical UK household |
| 4–5+ people | 300–400 litres | £1,100 – £1,800 | For high demand — longer showers, multiple bathrooms |
These costs cover supply only. Installation adds £300–£600 for labour, pipework connections, and the immersion heater element (required for Legionella cycling and emergency backup). The total cylinder and installation budget is typically £800–£2,400 depending on size and access.
Legionella bacteria can colonise stored water between 20–45°C — which unfortunately overlaps with the temperatures a heat pump might leave water sitting at if not managed carefully. To prevent growth, stored hot water must be periodically heated to 60°C or above for a sustained period (typically 1 hour), which kills Legionella at all stages of its lifecycle. This is called a pasteurisation cycle or Legionella protection cycle.
The Health and Safety Executive (HSE) recommends a minimum pasteurisation temperature of 60°C and advises this be carried out at least once a week for domestic hot water systems. In practice, most heat pump controllers have this built in — the system automatically runs a pasteurisation cycle weekly, typically in the early morning when electricity demand is lower.
Most heat pumps can reach 55–60°C in hot water mode, though their efficiency (CoP) at these temperatures is noticeably lower than at 50°C. Some systems use an electric immersion heater element (built into the cylinder) to boost temperature during the pasteurisation cycle rather than pushing the heat pump to its limits. This is efficient in aggregate — the immersion heater runs for a short time once a week, consuming perhaps 1–3 kWh, rather than the heat pump running at poor efficiency for longer.
The cost of a weekly pasteurisation cycle via immersion heater is approximately £0.25–£0.75 per week at current electricity rates — around £15–£40 per year. That’s a manageable cost. The alternative — ignoring Legionella management — is a genuine health risk, not a theoretical one. Don’t disable the pasteurisation cycle.
Heat pumps produce domestic hot water at higher flow temperatures than they use for space heating. Where space heating might run at 35–50°C flow temperature, hot water needs the cylinder to reach at least 50°C (with 55°C preferred for a comfortable buffer above the incoming mains cold water temperature). Running the heat pump at 55°C to heat water reduces its CoP to perhaps 2.0–2.5, compared to 3.5–4.0 for space heating at 45°C. This is unavoidable — the physics of heating water to a usable temperature are simply more demanding — but it can be mitigated.
The key mitigation is scheduling. By heating the hot water cylinder during the times of day when outdoor temperatures are highest (typically mid-morning), the heat pump operates at its best efficiency. At 10°C outdoor air temperature, CoP for water heating might be 2.8–3.2 — significantly better than the 1.8–2.2 you’d get at 0°C. Understanding this relationship is covered in more detail in our heat pump efficiency guide.
A well-configured heat pump system separates hot water heating from space heating and schedules them at different times of day. A sensible schedule looks something like this:
If you’re on a time-of-use electricity tariff (such as Octopus Go, Economy 7, or a dedicated heat pump tariff), scheduling hot water heating during cheap-rate hours can significantly reduce its effective cost. Heating a 250-litre cylinder from 40°C to 55°C requires roughly 4–5 kWh. At off-peak rates of 8–10p/kWh, that’s 32–50p per reheat versus 100–125p at standard rates. Over a year, smart scheduling on a time-of-use tariff can save £150–£250 on hot water electricity costs alone.
Yes — by a significant margin, in most circumstances. An immersion heater is 100% efficient: every 1 kWh of electricity produces 1 kWh of heat. A heat pump heating water typically achieves CoP of 2.0–3.0, meaning it produces 2–3 kWh of heat per kWh of electricity. At standard electricity rates (approximately 25p/kWh), heating water costs:
| Heat source | CoP/efficiency | Cost per kWh of hot water | Annual cost (4,000 kWh demand) |
|---|---|---|---|
| Immersion heater | 1.0 (100%) | 25p | £1,000 |
| Heat pump (average) | 2.5 | 10p | £400 |
| Heat pump (good conditions) | 3.0 | 8.3p | £333 |
| Gas boiler (for comparison) | 0.92 (92%) | 6.5p (at 6p/kWh gas) | £260 |
These figures use Ofgem’s current price cap rates as of late 2026. Even at a modest CoP of 2.5 for water heating, a heat pump is 60% cheaper than a pure immersion heater. It’s not quite as cheap as gas for hot water at current tariff rates, but this gap narrows significantly on a time-of-use tariff, with solar panels contributing, or as gas prices increase. The Energy Saving Trust estimates that heat pump hot water can cut water heating bills by 50–70% versus immersion-only electric heating.
Solar thermal panels heat water using solar energy and can provide 40–60% of a UK household’s annual hot water demand. Combined with a heat pump, they can further reduce hot water energy costs and reduce the frequency with which the heat pump needs to heat the cylinder. The combination works particularly well in spring and summer, when the heat pump is doing little or no space heating but hot water demand continues.
The practical challenge is system design. Solar thermal requires a twin-coil cylinder — one coil connected to the solar circuit, one to the heat pump. These are readily available. The controls must be set so the solar circuit has priority and the heat pump only activates when solar input is insufficient. A well-designed system with 3–4 m² of solar thermal panels and a 250-litre twin-coil cylinder can result in the heat pump providing hot water for only 4–5 months of the year — the rest covered by solar. Over a 20-year system life, that’s a material reduction in electricity costs.
Solar photovoltaic (PV) panels can also contribute indirectly by generating free electricity during the day that can be used to run the heat pump for hot water. This is increasingly popular with smart controls that divert surplus PV generation to the heat pump or cylinder immersion heater before exporting to the grid. The Energy Saving Trust’s solar panel guide explains the economics of solar PV in the UK context.
If you’re switching from a combi boiler, you need to budget for a cylinder as part of your heat pump installation cost. This is one of the less visible line items that sometimes catches homeowners out when comparing quotes. Key costs:
If you already have a hot water cylinder (common with older systems that had a separate boiler and cylinder), it may be reusable — but only if it has a suitable large coil for heat pump operation and is in good condition. Many installers recommend replacing older cylinders as part of the heat pump installation, as a failed cylinder 2 years post-installation would be costly and disruptive to fix. Check whether cylinder replacement is included in your quotes or listed separately.
For the full picture on installation costs — including the cylinder — read our heat pump costs guide, which breaks down every line item in a typical quote. And if you’re considering whether your home is ready for a heat pump in the first place, our suitability checklist covers cylinder space as one of the key criteria. Use our savings calculator to model the total financial impact for your home, including hot water costs.
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