Most homes with a heat pump need a cylinder sized at about 45 litres per person, with the number of people taken as the number of bedrooms plus one. That works out at around 180 litres for a 3-bedroom house and 225 litres for a 4-bedroom house, which installers usually round up to the nearest stock size, such as 200, 210 or 250 litres.
That figure comes from the MCS heat pump design standard and is a starting point. Extra bathrooms, big baths, a lower stored temperature and the space available can all push the choice up or down.
Why Does a Heat Pump Need a Hot Water Cylinder?
A heat pump needs a hot water cylinder because it cannot heat water instantly the way a combi boiler does. It heats a store of water in advance, and you draw from that store when you run a tap or shower.
The reason is power. As an illustration, a shower running at 10 litres a minute, with cold water heated from 10°C to 40°C, needs roughly 21 kW of heat the moment you turn it on. A combi boiler is built for that kind of burst. A heat pump is sized to your home’s heat loss, often a fraction of that figure, so it builds up a store over time instead.
The Energy Saving Trust says all heat pumps need space for a hot water cylinder, ideally inside the house. If you are replacing a combi, finding that space is one of the first things our surveyors look at.
How Is a Heat Pump Cylinder Different From a Standard Cylinder?
A heat pump cylinder has a much larger internal heating coil than a standard cylinder, so it can transfer heat from the lower-temperature water a heat pump produces. A boiler sends water to the coil at 70–80°C. A heat pump typically works at around 55°C or below, so the coil needs more surface area to heat the stored water in a reasonable time.
The MCS design standard (MIS 3005-D) notes that heat pump cylinders need a greater coil surface area than boiler cylinders, and it accepts cylinders that meet the Hot Water Association specification HWA 002:2020 as meeting the requirement. Many engineers recommend a coil area of around 3 m², according to trade magazine PHAM News, although coil design and material also affect performance. As one example of a real range, Vaillant lists 1.4 m² of coil in its 200 litre uniSTOR heat pump cylinder and 2.8 m² in the 300 litre version.
Hot Water Association test results show a heat pump cylinder reaching 50°C in about 20 minutes, against more than 40 minutes for a conventional cylinder. A boiler cylinder on a heat pump reheats slowly and forces the heat pump to run hotter, so reusing one is rarely a good idea.
How Do Installers Work Out Cylinder Size for a Heat Pump?
MCS installers work out cylinder size from the daily hot water demand in MIS 3005-D, which is 45 litres multiplied by the greater of the number of bedrooms plus one or the number of known occupants. The installer then picks a cylinder that suits that demand and the output of the heat pump.
So a 3-bedroom house is assumed to have four people, giving 180 litres a day. If five people live there, the calculation uses five, giving 225 litres. The standard accepts that available space may limit the size.
The table below shows the MIS 3005-D daily demand alongside the cylinder sizes Heat Geek suggests in its heat pump cylinder guide, which also takes bathrooms into account.
| Bedrooms | Assumed occupants (bedrooms + 1) | MIS 3005-D daily demand | Bathrooms | Typical cylinder suggested |
|---|---|---|---|---|
| 1 | 2 | 90 litres | 1 | 180 litres |
| 2 | 3 | 135 litres | 1 | 180 litres |
| 3 | 4 | 180 litres | 1 | 180 litres |
| 3 | 4 | 180 litres | 2 | 210 litres |
| 4 | 5 | 225 litres | 2 | 250 litres |
| 5 | 6 | 270 litres | 2 | 300 litres |
| 6 | 7 | 315 litres | 3 | 300–400 litres |
Should I Size Up for Extra Bathrooms or Heavy Use?
Yes, you should size up if you have more than one bathroom used at the same time, a large bath or a household that uses more hot water than average. The per-person figure assumes typical use, and real households vary a lot.
The Hot Water Association suggests allowing 35–45 litres per occupant for a typical home, and gives these bands for daily use per person:
| Usage level | Litres per person per day | 3 people | 4 people | 5 people |
|---|---|---|---|---|
| Low | 20–30 | 60–90 litres | 80–120 litres | 100–150 litres |
| Average | 30–50 | 90–150 litres | 120–200 litres | 150–250 litres |
| High | 50–70 | 150–210 litres | 200–280 litres | 250–350 litres |
Baths are the big variable. The HWA puts a bath at about 100 litres of water at 40°C. Two baths back to back can empty a small cylinder, and the heat pump then needs time to recover. If your household runs a bath and two showers in the same hour, tell your surveyor. Adding litres “just in case” has a cost, since a larger cylinder loses more heat and takes more space.
What Temperature Should a Heat Pump Cylinder Be Stored At?
Most heat pump cylinders are stored at around 48–55°C, lower than the 60–65°C typical of boiler systems, because the heat pump runs more efficiently the less it has to lift the water temperature. Vaillant’s handover guide for its aroTHERM Plus suggests 50–55°C, while Heat Geek recommends setting it as low as practical, often around 48°C.
Kensa, a UK heat pump manufacturer, puts the efficiency penalty at around 2% for every 1°C rise in flow temperature, and hot water already runs at the top of a heat pump’s range.
A lower stored temperature costs less usable water than you might expect, because you rarely use water above about 40°C and stored water is mixed with cold. As an illustration, with mains water at 10°C, a 200 litre cylinder at 50°C gives roughly 267 litres of water at 40°C. The same cylinder stored at 45°C gives about 233 litres. Installers often go up one size when the stored temperature is kept low. MCS also requires the heat pump to reach a flow temperature of at least 55°C at design conditions so it can heat the cylinder properly.
How Does a Heat Pump Cylinder Deal With Legionella?
A heat pump cylinder deals with legionella by running a periodic pasteurisation cycle that raises the whole store above 60°C, usually weekly and usually with help from the immersion heater. The Health and Safety Executive advises that hot water should be stored at 60°C or more to kill legionella bacteria, and heat pumps normally store below that.
MIS 3005-D requires every heat pump hot water system to include a means of preventing bacterial growth, and allows periodic heat treatment with a frequency set by a risk assessment. The Energy Saving Trust says weekly is safe for most homes, and daily is unnecessary if you use hot water regularly. The HWA notes that the immersion heater usually does the lift above 60°C.
Check how your cycle is set at handover. A daily cycle on an immersion heater adds cost for little benefit in a normal family home, so weekly, timed to a cheaper tariff period, usually makes more sense.
Should I Choose an Unvented or Vented Cylinder?
Most heat pump installs use an unvented cylinder, which runs at mains pressure and needs no cold water tank in the loft. A vented cylinder is fed by gravity from a loft tank and can still work with a heat pump, but it gives lower pressure at showers and taps unless you add a pump.
| Feature | Unvented cylinder | Vented cylinder |
|---|---|---|
| Water supply | Direct from the mains | Gravity-fed from a cold tank, usually in the loft |
| Pressure at taps and showers | Mains pressure, good for upstairs showers | Depends on the height of the tank; often low upstairs |
| Loft tank needed | No | Yes |
| Safety devices | Energy cut-out, temperature and pressure relief valve, expansion vessel, discharge pipe | Open vent pipe to the tank |
| Building Regulations | G3 applies above 15 litres; installer must be qualified and notify or self-certify | No G3 unvented requirements |
| Servicing | Annual check recommended by manufacturers | Less routine attention |
An unvented cylinder depends on your incoming mains, so your surveyor should measure mains pressure and flow before recommending one.
What Rules Apply to Unvented Cylinders in the UK?
Unvented cylinders holding more than 15 litres fall under Part G3 of the Building Regulations in England and Wales, which requires safety devices, safe discharge pipework and installation by a competent person. Scotland and Northern Ireland have their own equivalent rules.
Approved Document G sets out the key safety requirements. The stored water must never exceed 100°C, and there must be a non self-resetting energy cut-out plus a temperature relief valve or combined temperature and pressure relief valve. Any discharge must go to a visible, safe point through a tundish and discharge pipe, and bath hot water should be limited to 48°C. The work is notifiable, although an installer in a competent person scheme can self-certify instead.
In Scotland, standard 4.9 of the domestic Technical Handbook covers unvented systems. It calls for at least two independent safety devices, installation by someone with appropriate training and experience, and storage at not less than 60°C for legionella control. Northern Ireland covers unvented systems in Technical Booklet P under regulation 88, and Wales broadly follows the same framework as England.
Does an Unvented Cylinder Need an Annual Check?
Yes, an unvented cylinder should be checked once a year, and in many cases this is a condition of the warranty. Most manufacturers and HWA members recommend servicing at least annually.
A typical check includes briefly opening the expansion relief valve and the temperature and pressure relief valve to confirm they are not seized, checking the pressure reducing valve, recharging the expansion vessel if needed and making sure the discharge pipe is clear. The engineer then updates the Benchmark service record, ideally at the same visit as your heat pump service.
How Much Space Does a Heat Pump Cylinder Need?
A typical 200 litre heat pump cylinder needs a floor area of about 60 cm by 60 cm and around 1.3 metres of height, plus room around it for pipework, the expansion vessel and access. Larger cylinders are the same width but taller.
Vaillant’s uniSTOR range gives a useful real-world guide. The 200 litre model is 1,265 mm tall and 595 mm in diameter, and weighs about 237 kg when full. The 300 litre model has the same 595 mm diameter but stands 1,745 mm tall and weighs about 357 kg full. That weight matters if the cylinder is going in a loft or on a suspended timber floor, so the surveyor should check the structure.
Common locations include the old airing cupboard, a utility room or a garage, where pipe runs and heat losses need keeping down.
What Are the Options If There Is No Room for a Cylinder?
If there is no room for a standard cylinder, the main options are a slimline cylinder, a heat battery such as the Sunamp Thermino, or an indoor unit with a built-in cylinder. Each trades space against cost or capacity.
- Slimline cylinders: taller and narrower, for tight cupboards. Vaillant’s 150 litre slimline pre-plumbed uniSTOR is 435 mm across but 1,667 mm tall, and its standing heat loss (1.8 kWh per 24 hours) is higher than the full-width 200 litre model (1.2 kWh).
- Heat batteries: Sunamp’s Thermino units store heat in a phase change material rather than water. Sunamp says they can be up to four times smaller than an equivalent cylinder. The xPlus model, which can work with a heat pump using Sunamp’s compatible Optimino key, comes in 150, 210 and 300 litre equivalents, and retailer listings give heights of roughly 640–1,050 mm.
- Integrated indoor units: several heat pump brands sell an indoor unit that combines the cylinder, pump, valves and controls in one cabinet. It still needs floor space, but it saves the separate wall space those parts would otherwise take.
- Smaller cylinder with more frequent reheats: a smaller store can work for a flat or a couple, provided the controls allow reheating during the day.
Heat batteries cost more than cylinders, so confirm the heat pump and battery combination with both manufacturers before committing.
How Much Does a Heat Pump Hot Water Cylinder Cost?
A heat pump cylinder typically costs around £700 to £1,800 to buy, depending on size and specification, with installation labour on top if it is fitted as a separate job. On a full heat pump installation the cylinder is normally part of the overall quote.
| Item | Typical price | Notes |
|---|---|---|
| Basic heat pump cylinder (supply only) | £700–£900 | Smaller or entry-level models |
| Mid-range cylinder (supply only) | £1,000–£1,400 | Common in family homes |
| Larger or high-spec cylinder (supply only) | £1,800 or more | Larger capacities or higher specification |
| Labour to replace an existing cylinder | £800–£1,500 | Standalone job; varies with pipework and location |
| Sunamp Thermino xPlus heat battery (supply only) | From about £2,340 | Retailer price; larger sizes cost more |
These figures come from UK merchant and retailer guides and will vary by brand, region and installer. When a cylinder is fitted as part of an MCS heat pump installation, it falls within the project that the Boiler Upgrade Scheme grant of £7,500 goes towards in England and Wales, so the cost you see on the quote is usually the whole system less the grant.
If you want a cylinder size checked against your household and the space you have, we can look at it as part of a heat pump survey.
