Air Source Heat Pump Hot Water: UK Guide (2026)

How Does an Air Source Heat Pump Heat Hot Water?

The heat pump uses the same refrigeration cycle for hot water as for space heating. Outside air warms refrigerant in the evaporator. A compressor raises its pressure and temperature, and the condenser transfers that heat into the home’s water circuit. An expansion valve then resets the cycle.

When the cylinder needs heat, a valve normally diverts output towards its coil or heat exchanger. Once the target temperature is reached, the system returns to space heating.

For a more detailed explanation, see Get Free Quote’s guide to how air source heat pumps work. Energy Saving Trust’s current air source heat pump guidance also confirms that air-to-water systems can heat stored water for taps, showers and baths.

Does a Heat Pump Heat the House and Hot Water at the Same Time?

Many systems use hot-water priority. When the cylinder calls for heat, output temporarily goes to domestic hot water rather than the radiators or underfloor heating.

A correctly designed home should tolerate this short hot-water cycle without a noticeable fall in room temperature. An unsuitable cylinder or high target temperature can extend recovery time.

This is one reason reheat time deserves attention when comparing quotations. Two systems with similar cylinder capacities may perform differently because of their heat-pump output, heat-exchanger design and control strategy.

Do You Need a Hot-Water Cylinder?

For a standard air-to-water heat pump supplying domestic hot water, usually yes. Heat pumps deliver heat steadily rather than producing the high instantaneous output expected from a combi boiler.

The current MCS Heat Pump Design Standard requires designers to calculate average daily domestic hot-water demand using 45 litres × N, where N is the greater of:

• the number of bedrooms plus one; or
• the number of known occupants.

The designer then uses that demand and the proposed heat-pump heating capacity to specify an appropriate cylinder. The desired cylinder reheat time must also be agreed with the customer.

That formula is a design basis, not a substitute for understanding how the home is used. Baths, shower flow rates, simultaneous use and household routines still matter.

How Big Should the Hot-Water Cylinder Be?

A good cylinder specification starts with demand, not simply the number of bedrooms.

The designer should consider occupancy, back-to-back showers or baths and the required recovery time.

Volume is only part of the answer. Heat-pump cylinders often need a larger heat-exchanger surface area because heat pumps generally use lower flow temperatures than boilers. MCS requires the cylinder heat exchanger to follow the heat-pump or cylinder manufacturer’s recommendations.

This explains why an existing cylinder can hold enough litres yet still be poorly suited to a heat pump.

Ask for the proposed cylinder capacity and expected reheat time in writing. A vague assurance that the cylinder is “big enough” gives you much less useful information.

What Temperature Does Heat-Pump Hot Water Reach?

Domestic hot water and radiator flow temperature are separate settings.

Many systems use an everyday hot-water setting around 48–55°C, although the correct value depends on the equipment, cylinder and controls. MCS requires a heat pump serving a domestic hot-water cylinder to be capable of producing at least a 55°C flow temperature at its specified design conditions.

Stored water also needs protection against bacterial growth, including Legionella. MCS requires domestic hot-water systems to include a means of preventing bacterial growth. Energy Saving Trust explains that many installations use a periodic disinfection or sterilisation cycle, sometimes with an immersion heater helping raise the stored water to a higher temperature.

Do not disable that cycle simply to reduce electricity consumption. Follow the settings specified by the manufacturer and system designer.

For more detail on space-heating and domestic hot-water temperatures, see Get Free Quote’s air source heat pump temperature guide.

Will an Air Source Heat Pump Provide Enough Hot Water for Showers and Baths?

Yes, if the cylinder and heat pump are sized for the household’s actual demand.

Stored water is normally hotter than the water used at a shower or bath, so it mixes with cold mains water at the outlet. This means a cylinder can deliver more litres of mixed shower-temperature water than its nominal stored volume.

Usable hot water depends on cylinder temperature, incoming cold-water temperature and outlet flow rate. Households with long showers, large baths or several bathrooms should make that usage pattern clear during the survey.

If hot water runs out regularly, cylinder capacity, reheat time or control settings should be checked. The fact that water is stored does not mean a heat-pump home should routinely suffer cold showers.

Can You Keep an Existing Hot-Water Cylinder?

Sometimes.

MCS says a retained cylinder must meet relevant insulation requirements, remain mechanically fit for purpose and satisfy the heat-pump manufacturer’s sizing requirements.

The heat exchanger is often the important detail. A cylinder originally designed for a boiler may use a smaller coil that cannot transfer heat quickly enough when supplied at lower heat-pump temperatures.

So do not assume your existing cylinder can automatically stay. Equally, a replacement should not be specified without explaining why the existing unit is unsuitable.

What If You Do Not Have Much Cylinder Space?

Space can be one of the biggest practical issues when replacing a combi boiler.

Energy Saving Trust says the appropriate cylinder size depends on household hot-water demand and notes that a cylinder can often fit into a cupboard with roughly an 80cm × 80cm footprint. It also describes other approaches in some circumstances, such as hybrid systems, heat batteries and local instantaneous water heaters.

If you want one air-to-water system to supply both central heating and normal whole-home hot water, however, suitable hot-water storage will usually remain part of the design.

A survey should therefore establish the proposed cylinder location early. Get Free Quote’s heat pump survey guide explains the checks that should happen before installation.

Is Heat-Pump Hot Water Efficient?

It can be, but domestic hot water normally requires a higher temperature than space heating.

The greater the temperature difference the heat pump must overcome, the harder the compressor works. As a result, its coefficient of performance, or COP, can be lower while heating the cylinder than while supplying low-temperature underfloor heating or suitable radiators.

Scheduling may help. Some households favour periods when outdoor air is warmer, while others use cheaper time-of-use electricity. The most economical approach depends on the tariff, controls and household routine.

For the wider cost picture, see Get Free Quote’s air source heat pump running-cost guide.

Does a Heat Pump Provide Hot Water in Winter?

Yes. Air source heat pumps continue working when outdoor temperatures fall below freezing, although their efficiency generally decreases as the temperature drops.

In cold, damp weather, frost can form on the outdoor heat exchanger. The unit then runs an automatic defrost cycle, briefly interrupting normal heat delivery while it clears the coil. This is normal heat-pump operation rather than evidence of a fault.

A correctly selected system should account for the property’s winter design conditions. If hot-water recovery becomes unacceptably slow during cold weather, the system design or settings should be investigated.

What Should Your Survey Confirm?

Before accepting a quotation, ask the designer to confirm your calculated daily hot-water demand, proposed cylinder model and capacity, expected reheat time, normal hot-water setting, bacterial-control strategy, immersion-heater role and how hot-water priority affects the heating schedule.

If you already have a cylinder, ask whether it can be retained and why. If you currently have a combi boiler, confirm the exact location and dimensions of the proposed cylinder before installation starts.

Current MCS requirements specifically make domestic hot-water demand, cylinder selection and agreed reheat time part of the heat-pump design process.