Air Source Heat Pump Hot Water: Can It Heat Your Water?

One of the most common questions about air source heat pumps is whether they can heat your domestic hot water as well as your home. The answer is a clear yes — air-to-water heat pumps are designed to do both. But the way they heat water is different from a gas boiler, and understanding those differences helps you get the best performance from your system.

This guide covers everything you need to know about hot water from a heat pump: how it works, what temperature to expect, what size cylinder you need, the legionella question, and what happens if you are currently on a combi boiler.

How a Heat Pump Heats Your Hot Water

An air-to-water heat pump heats water in a cylinder — not on demand like a combi boiler. The heat pump circulates warm water through a coil inside the hot water cylinder, gradually raising the stored water to the target temperature. Once the cylinder reaches its set temperature, the heat pump switches back to space heating (or standby).

The Process

The heat pump controller detects that the hot water cylinder temperature has dropped below the set point (typically 48-52°C)
A motorised valve diverts the heat pump's output from the heating circuit to the hot water cylinder's coil
The heat pump runs, heating the water in the cylinder. This typically takes 1-3 hours from cold, depending on cylinder size and heat pump output
Once the cylinder reaches the target temperature, the valve switches back to space heating

During the hot water heating cycle, space heating is usually paused. In a well-designed system with adequate cylinder size, this pause is brief and does not noticeably affect room temperatures — the thermal mass of the building maintains comfort during the switchover.

Efficiency During Hot Water Production

Heating hot water to 50°C is less efficient for the heat pump than heating radiator water to 40°C, because the higher temperature reduces the COP. During hot water production, expect a COP of 2.0-2.8, compared to 3.0-4.0 during space heating. This is still far more efficient than a direct electric immersion heater (COP 1.0) and comparable to or better than a gas boiler.

Hot water typically accounts for 15-25% of a household's total heat pump electricity consumption. For a detailed breakdown, see our heat pump running costs guide.

What Temperature Does a Heat Pump Heat Water To?

This is where things differ from a gas boiler, and it is important to understand.

Standard Operating Temperature: 48-52°C

Most heat pump systems store hot water at 48-52°C. This is hot enough for showers, baths, washing up, and all normal domestic uses. It is lower than the 60-65°C you may have been used to with a gas boiler, but in practice the difference is barely noticeable — water at 50°C is still very hot to the touch.

The lower storage temperature is a deliberate efficiency choice. Heating water to 50°C instead of 60°C significantly improves the heat pump's COP during hot water production, reducing electricity consumption.

Legionella Protection: 60°C Weekly Cycle

Legionella bacteria can grow in water stored between 20°C and 45°C. At 50°C, they are largely suppressed but not eliminated. To provide full protection, heat pump systems include a weekly (or twice-weekly) pasteurisation cycle that raises the cylinder temperature to 60°C for at least one hour.

Most heat pumps cannot reach 60°C efficiently on their own (the COP would be very low), so the built-in immersion heater kicks in to boost the final 8-10°C. This uses direct electricity (COP 1.0) for a short period, but since it only runs once or twice per week, the overall energy impact is small — typically 3-5 kWh per cycle.

The legionella cycle is usually scheduled for a time when you are least likely to need hot water — early morning or late evening. Your installer should set this up during commissioning.

What Size Hot Water Cylinder Do You Need?

Cylinder size is one of the most important decisions in a heat pump installation. Too small and you run out of hot water regularly. Too large and the heat pump spends excessive time and energy reheating a larger volume.

Recommended Sizes

1-2 people: 150 litres
3-4 people: 200 litres (the most common size)
4-5 people: 250 litres
5+ people or high hot water demand: 300 litres

These are larger than the cylinders traditionally used with gas boilers (which were typically 120-180 litres). The larger size compensates for the heat pump's slower reheat time compared to a gas boiler — you store more water so you are less likely to run out before the heat pump can reheat it.

Types of Cylinder

Unvented (pressurised): The most common type used with heat pumps. Delivers hot water at mains pressure, giving strong flow rates to showers and taps. Requires an annual safety check.
Vented (gravity-fed): Relies on a cold water tank in the loft for pressure. Lower flow rates than unvented but simpler to install. Some older homes already have vented cylinders that can be reused or replaced.
Heat pump-specific cylinders: Some manufacturers offer cylinders designed specifically for heat pump use, with larger coils for better heat transfer at lower flow temperatures. These are more efficient but cost more than standard cylinders.

Cylinder Placement

The cylinder needs to be inside the house, ideally in an airing cupboard, utility room, or dedicated cupboard. Standard cylinders are approximately 500-600mm in diameter and 1,000-1,800mm tall (depending on capacity). Slimline models are available for tighter spaces but may compromise on coil size.

If you currently have a combi boiler and no cylinder, finding space for one is often the main practical challenge — more on this below.

Switching from a Combi Boiler: What You Need to Know

If you currently have a combi boiler, switching to a heat pump means adding a hot water cylinder to your home. Combi boilers heat water on demand with no storage, but heat pumps need stored hot water. This is a fundamental change and often the most discussed aspect of the switch.

Finding Space for the Cylinder

This is the practical challenge. Options include:

Reinstating the old airing cupboard: Many homes that switched to a combi boiler still have the original airing cupboard space, now used for storage. Converting it back is usually straightforward.
Utility room or garage: If you have an indoor utility room or an attached garage, a cylinder can often fit in the corner. The garage must be insulated around the cylinder to prevent heat loss.
Under the stairs: A common location for smaller cylinders (150-200 litres). The space needs to be accessible for maintenance.
Bedroom cupboard: In some homes, a built-in wardrobe or cupboard in a bedroom can accommodate a slimline cylinder. Not ideal aesthetically, but practical.
New cupboard: Purpose-built cupboard in a hallway, landing, or kitchen. Costs £300-£800 to construct depending on finish.

Pipework Changes

Switching from a combi boiler to a cylinder system may require some pipework modifications. The cold water feed, hot water distribution, and heating circuit connections all need to be reconfigured. This is standard work for a heat pump installer and is included in the installation cost.

Will I Notice a Difference in Hot Water?

There are some differences compared to a combi boiler:

Flow rate: With an unvented cylinder at mains pressure, flow rates are typically equal to or better than a combi boiler. Two showers can often run simultaneously — something many combi boilers struggle with.
Temperature: Hot water is stored at 48-52°C rather than the 55-60°C a combi typically delivers. In practice, you are unlikely to notice the difference — both are hot enough for comfortable showers and baths.
Running out: Unlike a combi boiler (which never runs out because it heats on demand), a cylinder can be emptied if several people shower and run a bath in quick succession. Correct cylinder sizing prevents this from being a regular issue.
Immediate availability: A combi boiler takes a few seconds to fire up and deliver hot water. A cylinder system delivers hot water instantly — no waiting for the boiler to light. This is actually an improvement.

Maximising Hot Water Efficiency

There are several ways to get the most efficient hot water production from your heat pump.

1. Set the Right Storage Temperature

Store water at 48-50°C rather than 55°C or higher. The lower the target temperature, the more efficient the heat pump is at reaching it. The weekly legionella cycle to 60°C provides the necessary safety margin.

2. Schedule Hot Water Heating Wisely

If you are on a time-of-use electricity tariff, schedule the main hot water heating cycle during off-peak hours (typically overnight). The heat pump reheats the cylinder at cheap rates, and you have a full tank of hot water for the morning rush.

3. Insulate the Cylinder

Modern unvented cylinders come with factory-fitted insulation (typically 50mm of polyurethane foam). If you are reusing an older cylinder, ensure it has adequate insulation — heat loss from a poorly insulated cylinder wastes energy 24 hours a day.

4. Consider Solar Pre-Heating

Solar thermal panels or a solar PV diverter can pre-heat the hot water cylinder using free solar energy, reducing the amount of work the heat pump has to do. This is particularly effective in spring and summer when solar generation is highest and hot water demand is a larger proportion of total energy use.

5. Use Mixer Showers and Taps

Thermostatic mixer showers and taps blend stored hot water with cold to deliver water at the desired temperature. This means the stored water can be at 50°C while the delivered shower temperature is 38-42°C — comfortable and efficient, with the hot water lasting longer because less is drawn from the cylinder per minute.

Hot Water Cylinder vs Combi: A Fair Comparison

Many homeowners see the shift from combi to cylinder as a downgrade. In reality, it is more of a side-step — with some genuine advantages:

Better flow rates: An unvented cylinder at mains pressure can supply multiple outlets simultaneously. A combi boiler often struggles with two taps running at once.
Instant hot water: No waiting for the boiler to fire up. The hot water is already stored and ready.
More consistent temperature: A cylinder delivers a steady temperature. Combi boilers can fluctuate, especially when another tap is opened.
Space trade-off: Yes, you lose some space for the cylinder. But you gain back the wall space where the combi boiler was mounted (the heat pump's indoor components are typically smaller than a combi boiler).

Heat Pump Hot Water in Summer

In summer, when space heating is not needed, the heat pump only runs for hot water production. This is actually very efficient — the heat pump operates in warm ambient conditions (high outdoor temperatures mean high COP) and only runs for short periods each day.

Typical summer hot water electricity consumption is 3-5 kWh per day — roughly 70p to £1.25 at current electricity rates. This is comparable to or slightly less than the gas cost of heating water with a combi boiler in summer.

For an overview of the full installation process and what to expect, see our heat pump installation guide, or start with our complete guide to air source heat pumps.

Frequently Asked Questions

Can a heat pump heat hot water?

Yes. Air-to-water heat pumps heat both your home and your domestic hot water. The heat pump heats water in a storage cylinder, typically to 48-52°C, which is plenty hot enough for showers, baths, and washing up.

What temperature does a heat pump heat water to?

Standard operating temperature is 48-52°C, with a weekly legionella pasteurisation cycle to 60°C. The immersion heater usually assists with the legionella cycle. Some high-temperature heat pumps can reach 60°C without immersion assistance.

Do I need a hot water cylinder with a heat pump?

Yes, for an air-to-water heat pump. Unlike a combi boiler, heat pumps do not heat water on demand — they heat a stored volume in a cylinder. You will need a cylinder of at least 150 litres, ideally 200-250 litres for a family home.

Will I run out of hot water with a heat pump?

With the right cylinder size, it should be rare. A 200-litre cylinder provides enough hot water for 2-3 consecutive showers and a washing-up session. If the household uses hot water faster than the heat pump can reheat the cylinder, there may be occasional waits — but correct sizing prevents this from being a regular issue.

Is the hot water as hot as with a gas boiler?

It is slightly cooler — 50°C stored temperature vs 55-60°C from a typical combi boiler. In practical use, most people do not notice the difference. Water at 50°C is still hot enough to cause scalding, so it is blended with cold water at the tap or shower for comfortable use.

How much does a hot water cylinder cost?

A quality unvented cylinder suitable for heat pump use costs £600-£1,200 for the cylinder itself, plus £300-£500 for installation. Heat pump-specific cylinders with oversized coils cost slightly more. The cylinder cost is typically included in the overall heat pump installation quote.

Can I use my existing hot water cylinder?

Possibly, if it is in good condition and has a coil suitable for heat pump use. Heat pumps work best with cylinders that have a large coil (for efficient heat transfer at lower flow temperatures). Many older cylinders have smaller coils designed for gas boiler temperatures. Your installer will assess whether your existing cylinder is suitable or whether replacement is recommended.