Do you need a hot-water cylinder with an air source heat pump?

What to know first

These points explain when a cylinder is usually needed, how it is sized and what to consider if space is tight.

Usually, yes: an air-to-water heat pump that supplies domestic hot water needs storage, most often in a cylinder. This matters for homes replacing a combi boiler, which heats water on demand and has no storage tank.

It is not an absolute rule. A heating-only air-to-water system can leave hot water to another appliance, while most air-to-air installations need a separate water-heating solution. The choice depends on space, demand, cost and funding.

Why does an air-to-water heat pump usually need a cylinder?

An air-to-water heat pump transfers heat from outside air into water for radiators or underfloor heating. It can also serve taps, but a standard unit does not work like an instantaneous combi boiler, so heated water is normally stored.

The cylinder is usually an indirect cylinder. This means heating water from the heat pump passes through an internal coil and transfers heat to the stored tap water. MCS says heat-pump cylinders tend to need a larger coil surface than boiler cylinders because the heating-water temperature is generally lower.

What size hot-water cylinder might you need?

There is no reliable one-size-fits-all answer. Under the MCS heat-pump design standard, domestic daily hot-water demand is calculated as 45 litres multiplied by N. N is the greater of the number of bedrooms plus one or the known number of occupants.

These figures are design demand, not a prescribed cylinder capacity. The installer must combine demand with the heat pump’s heating capacity and the required recovery time. Bath size, shower flow, overlapping use, storage temperature and manufacturer requirements can all affect the final choice.

For scale, one current manufacturer range lists domestic heat-pump cylinders from 150 to 300 litres, including a slimline 150-litre model. This is a product example, not an MCS sizing rule.

Expert insight

“Bedroom count starts the calculation; it does not finish the design. Ask the installer to show how the proposed cylinder will cover the household’s busiest period and how quickly it will recover afterwards.”

Giles Crosse

Energy Editor

Experienced editor, journalist and communications consultant specialising in consumer energy and low carbon technologies.

• Editor and campaign author for Shell, EDF Energy and Good Energy.
• Consultant to the United Nations and contributor to the World Economic Forum.
• Journalist for Reuters, the BBC, The Economist and The Guardian.

More about Giles Crosse

Where can a heat-pump cylinder go?

Energy Saving Trust says a cylinder can usually fit in a cupboard with at least 80cm by 80cm of floor space. A former airing cupboard or utility area may work, while slimline or integrated products can reduce the footprint.

The survey must consider more than diameter. A 200-litre cylinder holds about 200kg of water before the vessel and fittings are added. The installer must also check access, pipe routes, safety discharge, insulation and clearances. A garage or loft needs a structural and technical assessment rather than an assumption that it will be suitable. MCS requires the proposed equipment to be installable in line with its manufacturer’s instructions.

Good to know

Installing or replacing a heating or hot-water system may involve building-regulations requirements. In England, a suitably registered competent person can self-certify relevant work; rules differ in Scotland, Wales and Northern Ireland.

Can you keep an existing hot-water cylinder?

Potentially, but an ordinary boiler cylinder is not automatically heat-pump-ready. MCS requires an existing cylinder and its pipes to meet the relevant insulation standard. The installer must also consider corrosion, scale, mechanical condition and whether its size meets the heat-pump manufacturer’s requirements.

The coil may decide the issue. A boiler cylinder can have too little heat-exchanger surface for operation at heat-pump temperatures. Reuse should therefore be a documented design decision, not an assumption made simply to reduce the quotation.

How long will the hot water take to reheat?

There is no universal recovery time. MCS requires the desired reheat time to be agreed with the customer. Before installation starts, the customer must also receive the specification and recovery rate of a proposed new cylinder.

A rough illustration shows why the answer varies. Raising 200 litres of water by 35°C needs about 8.1kWh of heat. If 5kW reached the cylinder continuously, the theoretical minimum would be about 1 hour 38 minutes. A real system may take longer because output changes, heat is lost and the full rated output may not reach the cylinder continuously.

The design must also include a means of controlling bacterial growth, including legionella. MCS says the frequency of any periodic high-temperature pasteurisation cycle can be informed by a risk assessment. The installer should therefore explain the system’s hygiene controls rather than applying an unexplained generic schedule.

What if there is no room for a standard cylinder?

The alternatives solve different problems, so they should be compared as complete heating-and-hot-water systems. Energy Saving Trust identifies hybrids, heat batteries, point-of-use heaters and air-to-air systems as possibilities when conventional storage is difficult, while current scheme rules exclude fossil-fuel hybrids from the Boiler Upgrade Scheme.

Air-to-air may suit some smaller homes or flats where a wet system is impractical. It is not usually a cylinder-free way to obtain both heating and hot water: another appliance must normally serve taps, baths and showers. Some specialist ducted or model-specific systems can also serve hot water, so the exact product design matters.

A hybrid can preserve combi-style hot water, but funding may change the economics. In England and Wales, hybrids combining a heat pump with fossil-fuel heating are not eligible for the Boiler Upgrade Scheme. Scotland’s main grant-and-loan scheme also excludes hybrids and requires a funded heat pump to provide all the property’s heating and hot water.

How do cost and funding affect the decision?

Energy Saving Trust put a typical air source heat-pump installation at around £12,000 in July 2026. A new cylinder, radiator upgrades, electrical work, pipework or building work can change the quotation, so these costs should be itemised.

Checked on 27 July 2026, the Boiler Upgrade Scheme in England and Wales offers:

• £7,500 for an air-to-water heat pump;

• £2,500 for a residential air-to-air heat pump;

• an additional £1,500 for an eligible air-to-water or ground-source installation where the property uses oil or liquefied petroleum gas and has no mains-gas connection.

That temporary uplift produces a maximum grant of £9,000 and is available from 21 July 2026 until 31 March 2027. Zero-rate VAT on qualifying heat-pump installations also runs until 31 March 2027.

In Scotland, the Home Energy Scotland Grant and Loan offers up to £7,500 in grant funding and an optional interest-free loan of up to £7,500 for eligible air-, ground- or water-to-water heat pumps. A £1,500 rural and island uplift may also apply. Energy Saving Trust said Northern Ireland did not have a specific heat-pump grant as at 16 July 2026.

Good to know

An Energy Saving Trust page updated on 16 July still described air-to-air funding as forthcoming. GOV.UK and Ofgem pages checked on 27 July list the £2,500 grant as available from 21 July 2026, so this article follows the later formal scheme position.

A grant is not proof that a particular cylinder or system suits the property. The installer-led scheme application and the customer’s private installation contract are separate decisions. Ofgem also says the grant should appear as an upfront discount on the installer’s quotation.

What should you ask for before signing a contract?

1. The hot-water calculation. Ask for the assumed occupants, outlets and daily demand.

2. The exact cylinder specification. Check nominal capacity, dimensions, recovery rate, heat loss and whether it is heat-pump-compatible.

3. A location plan. Confirm access, structural support, pipe routes and safety discharge.

4. The operating explanation. Ask what happens after heavy use, how reheating is scheduled and how the hygiene cycle works.

5. An itemised quotation. Separate the heat pump, cylinder, radiators, electrical work, pipework and any building work.

MCS requires the proposed cylinder’s manufacturer datasheet to be supplied before the contract is awarded, where a cylinder applies. Before installation, the customer must receive the calculated hot-water demand, the new cylinder specification and its recovery rate.

For government funding, use a Microgeneration Certification Scheme-certified installer. MCS-certified installers must also belong to a recognised consumer-protection scheme, and GOV.UK recommends obtaining more than one quotation.

Key takeaways

• An air-to-water heat pump normally needs a cylinder when it supplies domestic hot water.

• A cylinder is not required for every air source system, but another hot-water source will then normally be needed.

• MCS sizing starts with demand, then accounts for heat-pump output and the agreed reheat time.

• Published domestic product examples span 150 to 300 litres, but the property survey determines the final specification.

• Existing cylinders can be reused only after compatibility, condition and insulation checks.

• Space-saving alternatives have trade-offs and can affect grant eligibility.

• Funding figures and tax treatment should be checked again immediately before publication or contracting.

Frequently asked questions