Energy Saving Trust says a standard air source heat pump does not provide hot water on demand like a combi boiler. So you need a way of storing hot water for when you need it. Clause 5.6.2 of the MCS heat pump design standard sets daily hot water demand at 45 x N. On space, Energy Saving Trust says a cylinder will usually fit inside any cupboard of at least 80cm by 80cm. Every figure here comes from MCS, the Hot Water Association, the Health and Safety Executive, gov.uk or Energy Saving Trust. Each one is named and linked where it is used.
The formula your installer sizes it with
The MCS heat pump design standard, MIS 3005-D, requires in clause 5.6.2 that the daily hot water demand for a home is calculated as 45 litres times N, which it writes as Vd,average = 45 x N. N is the greater of two numbers: the number of bedrooms plus one, or the number of known occupants. The standard gives the formula but no worked examples.
So on a three-bedroom semi with two people in it, bedrooms plus one is four, known occupants is two, and N is four. The design figure is 45 litres times four. A four-bedroom house with five people gives an N of five instead.
Clause 5.6.3 then uses that demand and the proposed heat pump heating capacity, and requires an appropriately sized cylinder to be specified. Its note adds that the size of cylinder may be limited by the available space. Clause 5.6.4 requires that the desired cylinder reheat time shall be agreed with the customer. So ask to agree that number rather than have it handed to you.
The Hot Water Association's rule of thumb is to allow between 35 and 45 litres for every person. It puts a bath at 100 litres of hot water at 40°C, which it gives as 60 litres at 60°C. It puts a shower at about 18 litres a minute at 40°C, or 11 litres at 60°C. It puts a typical four person household at around 200 litres of hot water a day.
A smaller heat pump needs a bigger cylinder
MCS's reference document MGD 007 bases its cylinder selection guide on an existing British Standard, BS 6700, and the sizes get larger as the heat pump gets smaller. Take a home with one bathroom. MGD 007 gives minimum storage of 122 litres on a 3 kW heat pump. It drops to 88 litres on a 6 kW, then 73 litres on an 8 kW. At 10 kW it is 70 litres, and at 15 kW and above it is 70 litres again. For a home with two bathrooms the same run is 260 litres, then 200 litres, 131 litres, 130 litres and 130 litres.
MGD 007 applies the same pattern to its per-person figure. For storage without rapid reheat, it says an allowance should be made of 45 litres per occupant. That can be reduced to 35 litres per person for 10 to 15+ kW heat pumps, on suitably sized heat exchangers.
Read the heat pump output and cylinder volume on your quote together, not apart. If the survey comes back with a smaller heat pump than you expected, check what happened to the cylinder beside it. See our guide to what size heat pump you need.
The heat pump has to reach 55°C for it
The same standard covers cylinders used with heat pumps, in clause 5.6.6. It says the heat pump must be able to send water out at 55°C or more at design conditions. Its note defines design conditions as the lowest temperature at which the heat pump can meet the building's heat demand, with no input from a supplementary heater.
See our guide to radiators and heat pumps for the heating side of the same system, and our page on underfloor heating with a heat pump.
Where the cylinder goes, and what to do if you have no space
Energy Saving Trust says the size of hot water cylinder depends on the amount of hot water your household typically uses. Its in-depth heat pump guide adds that for all heat pumps you need space to put a hot water cylinder, ideally inside the house.
Look at the airing cupboard, the space under the stairs, a corner of the utility room, and any bedroom cupboard. Measure the depth as well as the floor space before the survey.
Where there is no space, Energy Saving Trust lists other options. Some hybrid heat pump systems use the heat pump for heating and a boiler for hot water on demand. Instantaneous hot water heaters can be fitted under a kitchen sink to give a smaller amount of hot water. For flats, Energy Saving Trust says compact heat pumps only need a cupboard with around 80cm by 80cm of floor space, and that the cupboard should be near an exterior wall.
Picking the hybrid option costs you the grant. Gov.uk says you cannot get a Boiler Upgrade Scheme grant for a hybrid heat pump system. Its example is a gas boiler combined with an air source heat pump. Raise the space question at the survey, not after you sign. Clause 5.6.3's note treats available space as a limit on the design.
Ask what heats the water in the cylinder, not just the volume
Clause 5.6.5 of MIS 3005-D says the specification of the coil in the hot water cylinder shall follow the heat pump manufacturer's or the cylinder manufacturer's recommendations. Its note says heat pump hot water heat exchangers tend to require a greater surface area than those for traditional combustion-based heat sources, such as boilers. The reason it gives is that the flow temperature is typically lower.
The same note gives a quicker way to check that specification. The Hot Water Association has produced a specification for cylinders suitable for use with heat pumps, HWA 002:2020. The note says using a cylinder labelled in accordance with that document would also satisfy the requirement. So one question covers it. Is the cylinder you are quoting labelled to HWA 002:2020, or named by the heat pump maker?
MGD 007 says its own sizing results are based on having a suitably sized heat exchanger, which with heat pumps needs a greater surface area and the right shape. It then says whose job that is. Whoever specifies the system has to confirm with the cylinder supplier that the coil has the right shape and surface area to meet the necessary performance.
Legionella and the 60°C rule
The standard also requires, in clause 5.6.8, that a home's hot water system must include a way to prevent bacterial growth. The clause names legionella bacteria as one of those bacteria. The Health and Safety Executive describes Legionnaires' disease as a potentially fatal type of pneumonia, caught by inhaling airborne water droplets that carry viable legionella bacteria. The clause's note says that where that prevention works by periodically heating the system through, a bacterial risk assessment can help determine how often it should happen.
The temperatures come from the Health and Safety Executive. Its legionella guidance is that hot water should be stored at 60°C or higher, and that hot water distribution should be above 50°C.
Ask your installer which means of prevention the design uses, and how often it runs. Ask for both answers before you sign.
Can you keep the cylinder you already have?
Clause 5.6.7 of MIS 3005-D sets out what has to be true where an existing domestic hot water cylinder is used. The insulation on the cylinder, and on every pipe connected to it, has to be upgraded to the standard a new installation would need.
If the cylinder has factory-applied insulation, the standard says that condition is met where the heat it loses standing idle is certified to comply with Section 12 of BS 1566-1:2002 + A1:2011 or equivalent. If that certificate is not apparent, the MCS contractor shall install extra insulation certified to comply with BS 5615:1985.
The same clause requires proper care to check the old cylinder is still sound in itself. It names three things to look at: scale build-up affecting overall system efficiency, damage, and deterioration caused by corrosion. The standard says these shall be considered in the context of any extra stress the heat pump connection places on the cylinder, such as thermal stress or added system pressure. It also requires that the size of the cylinder shall comply with the heat pump manufacturer's requirements.
Ask to see the insulation certificate named in that clause.
What the installation costs, and what to get in writing
That is the specification. So what does the whole installation cost?
Energy Saving Trust puts the typical cost of installing an air source heat pump at around £12,000, and says that if you live in England and Wales, you can get £7,500 towards one with the Boiler Upgrade Scheme. Gov.uk gives the same £7,500 figure for an air source heat pump, sometimes called an air-to-water heat pump. It says you can get one grant per property. For Scotland, Energy Saving Trust points to an interest-free loan or a grant through Home Energy Scotland. It says that in Northern Ireland there is not a specific grant currently available. See our heat pump cost guide for the whole-system figures.
MIS 3005-D gives you paperwork at two stages. Clause 4.2.1 covers the datasheet. The manufacturer's datasheet for the proposed cylinder shall be communicated in writing to the customer before the contract is awarded. Clause 5.9.2 covers what you must get before the installation starts. By then the customer shall be provided in writing with the calculated daily hot water demand. The same clause covers the specification of any new cylinder and how fast it reheats.
Four questions get you everything on this page. What is N, so what daily demand have you designed to? What volume is the cylinder, and how fast does it reheat? Is it labelled to HWA 002:2020, or named by the heat pump maker? And what reheat time have we agreed?
You can ask MCS-certified installers covering your postcode for quotes. Ask each one to give the cylinder as its own line, with the volume, the model and the reheat time in writing.
Get a Heat Pump Quote →Frequently asked questions
MCS clause 5.6.2 sets the basis: 45 x N, where N is the greater of bedrooms plus one or known occupants. MGD 007 then picks the volume against the heat pump's output.
Not a standard air source one. Energy Saving Trust names a hybrid system with a boiler doing the hot water, or instantaneous heaters under a sink. Gov.uk says there is no Boiler Upgrade Scheme grant for a hybrid heat pump system.
A cupboard of at least 80cm by 80cm, on Energy Saving Trust's figure, ideally inside the house. For flats it says a compact heat pump needs around 80cm by 80cm of floor space near an exterior wall. Measure the depth, not just the floor space.
MGD 007 allows 45 litres per occupant for storage without rapid reheat, dropping to 35 litres per person for 10 to 15+ kW heat pumps on suitably sized heat exchangers. Its two-bathroom row runs 260 litres on a 3 kW heat pump down to 130 litres at 15 kW and above.
Only on clause 5.6.7's terms. The insulation has to be certified to Section 12 of BS 1566-1:2002 + A1:2011 or equivalent, or topped up to BS 5615:1985. The cylinder's size has to comply with the heat pump manufacturer's requirements, and how sound the cylinder itself is has to be assessed.
The Health and Safety Executive says store hot water at 60°C or higher and distribute above 50°C. MCS clause 5.6.8 leaves the means of preventing legionella to the design, so ask which one yours uses.
MCS clause 5.6.5 notes that the Hot Water Association has produced a specification for cylinders suitable for use with heat pumps, HWA 002:2020. The same note treats a cylinder labelled to it as satisfying the heat exchanger requirement, so the label is the short way to check the coil.
The cylinder datasheet before the contract is awarded, under clause 4.2.1. The calculated daily hot water demand and the new cylinder's specification and how fast it reheats before installation commences, under clause 5.9.2.