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Heat pumpsSizing a heat pump: what a proper survey covers
The size of your heat pump is not a judgement call. It is the output of a room by room heat loss calculation that MCS makes mandatory, and skipping it is the most expensive shortcut in the whole project.
Updated 4 August 2026 · written by the Clearline guidance team
The short answer
A heat pump is sized by a room by room heat loss calculation carried out to BS EN 12831-1:2017. Every room is measured, every wall, window, floor and roof it touches is given a heat loss figure, and the totals are added up. That result, at the coldest outside temperature your area is designed against, is the number the heat pump has to meet. MCS makes the calculation mandatory under its heat pump design standard, MIS 3005-D, before any certified installation goes ahead. Nothing substitutes for it: not your old boiler size, not your floor area, not your EPC, and not a figure quoted over the phone before anyone has been inside the house.
Your old boiler size proves nothing
Gas boilers are routinely fitted far larger than the house actually needs, because oversizing a boiler costs very little to run. Copy that number across to a heat pump and you have bought a machine that spends the winter switching itself on and off instead of running steadily. The efficiency falls, and the bill follows it.
What the survey measures, room by room
- Every room measured properly: floor area, ceiling height, and the area of each external wall, window, door, floor and roof surface it touches.
- The construction of each of those elements and how much insulation is in it, so a real U-value can be assigned rather than assumed.
- The ventilation rate for each room, because air changes carry heat out of a house just as walls do.
- A design temperature for each room, since a bathroom, a hallway and a bedroom are not held at the same temperature.
- The external design temperature for your location, which is the cold snap the system has to cope with rather than an average winter day.
- Every existing radiator, its type and panel depth, and the pipework feeding it.
- Your hot water demand, where a cylinder can physically go, and the pipe route to it.
- The electrical supply, the position available for the outdoor unit, and the noise assessment for that position.
Three numbers come out of it
The heat pump capacity
The design heat loss of the whole house in kilowatts, at the external design temperature. Under the MCS design standard the heat pump has to provide at least 100 percent of that calculated heat loss. There is no adding a margin for luck and no rounding up to the next model in the range.
The design flow temperature
The single water temperature the system is built around. Heat pumps are most efficient at 35 to 45C, against 70 to 80C for a gas boiler, and MCS design practice commonly lands at 45 to 55C on a radiator retrofit. Lower is cheaper to run, and the emitters are what make lower possible.
The emitter size for every room
Each room's calculated heat loss set against what its existing radiator or underfloor circuit actually delivers at the design flow temperature. That comparison, room by room, is what decides which radiators change and which stay exactly where they are.
| Survey output | What it sets | What guessing it costs |
|---|---|---|
| Design heat loss in kW | The capacity of the heat pump | Too big and it short cycles, switching on and off instead of running steadily, and seasonal efficiency drops. Too small and you are running on backup immersion heat in February. |
| Design flow temperature | How hot the water runs, and the efficiency that follows from it | Set high to avoid emitter work and the penalty is paid on every unit of electricity, every day, for the life of the system. |
| Emitter output per room | Which radiators change and which stay | Either a blanket replace-them-all line in the quote, or a north facing bedroom that never gets warm. |
The three are linked. Change the flow temperature and every emitter verdict changes with it, which is why the calculation has to come before the quote rather than be reverse engineered from it.
Want to know what your house actually needs before anyone quotes you a model? The free remote design call walks through the heat loss picture with you, with no obligation.
Get your free quote →Why a guessed size is expensive
Electricity costs 26.11p a unit under the current price cap and gas costs 7.33p, a ratio of 3.56 to one. Against a gas boiler running at 90 percent efficiency, a heat pump has to hold a seasonal performance factor above 3.21 just to draw level with gas on unit rates. That is a comfortable bar for a well designed system and an impossible one for a badly sized system, and the survey is the entire difference between the two.
| System | Efficiency | Fuel used per year | Cost per year |
|---|---|---|---|
| Gas boiler | 90 percent | 11,111 kWh of gas | £814 |
| Heat pump | SPF 2.8, the trial median | 3,571 kWh of electricity | £933 |
| Heat pump | SPF 3.5, a well designed install | 2,857 kWh of electricity | £746 |
| Heat pump | SPF 4.0, low flow temperature and generous emitters | 2,500 kWh of electricity | £653 |
Assumes 10,000 kWh of useful heat a year, which is an illustration rather than your house. Unit rates are the Ofgem price cap averages for 1 July to 30 September 2026: electricity 26.11p per kWh, gas 7.33p per kWh. Standing charges are excluded from the table. Cap rates change every quarter.
The gap between the trial median and a well designed install is £280 a year on the same house, with the same heat demand and the same weather, before any heat pump tariff enters the picture. A home that removes gas entirely also stops paying the gas standing charge, which at 29.04p a day is a further £106 a year. None of that is decided by which badge goes on the wall outside. It is decided by the calculation that came first.
Old house, awkward layout, thin loft insulation
The government funded Electrification of Heat project installed 742 heat pumps across Victorian terraces, pre-war semis, 1960s flats and most things in between, and concluded that there is no property type or architectural era unsuitable for a heat pump. The same project found something more useful for anyone choosing an installer: performance varied enormously between individual installations, with a median in-situ seasonal performance factor of 2.80 for air source systems and a wide spread around it. An older house usually means higher heat loss, a larger unit and more emitter work, which is a cost question rather than a feasibility one. The age of the building is not the variable that decides your bill. The quality of the design is.
What to ask for before you sign
- The room by room heat loss calculation itself, as a document, showing each room's calculated loss in watts. If it does not exist, the quote is a guess with a price on it.
- The external design temperature used, and the design temperature assumed for each room.
- The single design flow temperature the system is built around, with the reasoning behind that choice.
- Each room's existing emitter output at that flow temperature, printed next to the room's calculated loss, so you can see the pass or fail for yourself.
- The heat pump model and its rated output at the design conditions, rather than the headline figure on the brochure, since output is quoted at a stated air and water temperature.
- Confirmation that the company holds current MCS certification, because that is what forces the design standard and what unlocks the grant.
The grant only exists through a certified installation team
The Boiler Upgrade Scheme pays £7,500 towards an air source heat pump in England and Wales, rising to £9,000 where the property is off the gas grid and currently heated by oil or LPG, a window Ofgem lists as running to 31 March 2027. You do not apply for it yourself. An MCS certified installer applies for and redeems the voucher on your behalf, which is another reason certification is not a box-ticking detail. Hybrid heat pump systems are not eligible.
Where to go next
Sizing is the first decision and the one every later decision leans on. Flow temperature explained covers the single number that sets your running cost, and do you need new radiators works through the emitter arithmetic room by room. The full heat pumps guide covers the rest of the system, and the Boiler Upgrade Scheme explained covers the grant in detail. Every county page on this site carries a local calculator and the current grant position, so check your county for the local picture. If you are weighing a heat pump alongside solar panels or battery storage, the interaction between them runs through your electricity tariff rather than through the plumbing.
Already holding a quote? Bring it to the free design call and we will read the heat loss calculation with you, line by line, and tell you what is missing.
Get your free quote →Clearline gives independent guidance and never installs. Work is carried out by an MCS-certified installation team for your area.
Survey and sizing questions, answered
Can an installer size a heat pump without visiting?
Not to MCS standards. The design standard requires a room by room heat loss calculation to BS EN 12831-1:2017, and that needs the real dimensions, construction and glazing of every room in the building. A remote conversation is a useful first filter and will tell you whether the project is worth pursuing, but the number that ends up on the order has to come from a survey of the property itself.
What size heat pump do I need for a 3 bed house?
There is no honest answer to that without a survey, which is rather the point. Two three bed semis on the same street can differ by several kilowatts depending on insulation, glazing and how the extension was built. What can be said is that the MCS design standard requires the heat pump to provide at least 100 percent of the calculated heat loss at design conditions, so the figure comes out of the calculation rather than out of a lookup table.
What actually happens if the heat pump is oversized?
It short cycles. Instead of running steadily at part load, which is where a heat pump is at its most efficient, it repeatedly reaches temperature, shuts down and restarts. Seasonal efficiency falls, and because electricity costs 3.56 times what gas costs per unit this quarter, any drop in efficiency shows up on the bill quickly. The Electrification of Heat project recorded a median in-situ seasonal performance factor of 2.80 across 742 installations with wide variation around it, and design quality is the main reason for that spread.
Is my house too old for a heat pump?
The 742-home Electrification of Heat project covered Victorian terraces, pre-war semis and 1960s flats, and concluded that no property type or architectural era is unsuitable. Older properties usually carry higher heat loss, which means a larger unit and more emitter work. That is a cost question rather than a feasibility one, and the survey is what turns it into a number you can actually judge.
Does a heat pump cost more to run than gas?
It depends entirely on the seasonal efficiency the design achieves. At the current cap rates a heat pump needs a seasonal performance factor above 3.21 to beat a 90 percent efficient gas boiler on unit rates alone. A median-performing system at 2.80 costs slightly more than gas, while a well designed one at 3.5 or above costs less. A dedicated heat pump tariff shifts the picture further, and a home that removes gas entirely also drops the gas standing charge of 29.04p a day, worth £106 a year.
Why does the grant require an MCS certified installer?
Because the Boiler Upgrade Scheme voucher is applied for and redeemed by the installer rather than by you. Certification also binds that company to the MCS heat pump design standard, which is what forces the room by room heat loss calculation in the first place. The money and the survey requirement arrive together, which is a reasonable argument for not going outside the scheme.
Are hybrid systems eligible for the grant?
No. Hybrid heat pump systems are excluded from the Boiler Upgrade Scheme, and the trial data supports the call. Hybrid installations in the Electrification of Heat project returned a median heat pump efficiency of 2.37, against 2.80 for full heat pump systems.
See the numbers next: Tesla Powerwall 3, installed, or what a battery costs.
Figures and specifications in this guide are sourced below and were checked on the date shown. Rates and product specifications change; we confirm the current picture on your free design call.
Sources
- MCS, MIS 3005-D The Heat Pump Design Standard: room by room heat loss calculation and the requirement to provide at least 100 percent of calculated heat loss mcscertified.com
- MCS Heat Load Calculator documentation: design heat load calculated to BS EN 12831-1:2017, room by room mcscertified.com
- Energy Systems Catapult, Electrification of Heat demonstration project: 742 installations, all housing types suitable, median in-situ SPF 2.80 for air source and 2.37 for hybrid systems es.catapult.org.uk
- Ofgem, energy price cap 1 July to 30 September 2026: electricity 26.11p per kWh, gas 7.33p per kWh, gas standing charge 29.04p per day ofgem.gov.uk
- GOV.UK, Boiler Upgrade Scheme: £7,500 for an air source heat pump, £9,000 off gas grid uplift for oil and LPG homes, hybrid systems excluded gov.uk
- Ofgem, Boiler Upgrade Scheme: MCS certified installers apply for and redeem vouchers on the owner's behalf, £9,000 window running to 31 March 2027 ofgem.gov.uk
- Reonic, heat loss survey guide: MCS design flow temperatures of 45 to 55C on radiator retrofits and the three outputs of the survey reonic.com
- APMI, low temperature system design: heat pump flow temperatures of 35 to 45C against 70 to 80C for a conventional boiler apmi.uk
Read next
Do you need to replace your radiators for a heat pump?
Some change, most do not. The room by room arithmetic that decides which.
Read the guide →Flow temperature: the one number that decides your running cost
The water temperature your system produces. It sets efficiency, radiator cost and bills.
Read the guide →Why your inverter is smaller than your panel array, and why that is correct
More panel capacity than inverter capacity is a deliberate design choice.
Read the guide →