If you are planning a heat pump, a new boiler or underfloor heating, you will soon hear the phrase “heat loss survey”. In this guide you will learn what the survey measures, how the room-by-room calculation works, why MCS installers must carry one out and how the results decide the size of your heat pump and radiators. Our MCS certified engineers carry out these surveys every week in homes across Surrey, Sussex and Kent, so we know exactly what a good one looks like.
“We never size a heat pump from the number of bedrooms. We measure every room, because the survey decides whether your home ends up warm and cheap to run or cold and expensive.”
1. Understand what a heat loss survey is — the foundation of good heating design
Every home loses heat all the time. It escapes through walls, roofs, floors, windows and doors, and leaks out through draughts and ventilation. On a mild day the losses are small. On a freezing January night they are at their highest.
A heat loss survey works out how much heat each room loses on a very cold day, so the heating system can be designed to replace it. The answer is given in watts for each room and in kilowatts for the whole house. That total tells the designer how powerful the heat source needs to be, and the room figures tell them what each radiator or underfloor circuit must deliver.
It can sound like paperwork. It is actually the single most important step in getting a system that keeps you warm without wasting energy. Our dedicated heat loss surveys for local homes are the starting point for every heat pump we design.
2. Know what the surveyor measures — room by room, not house by house
A proper survey looks at each room in turn rather than estimating from the number of bedrooms. Expect the surveyor to spend time in every room with a tape measure or laser, and to look in the loft and under floors where they can.
- Room dimensions. Floor area, ceiling height and the size of every external wall, window and door.
- Construction. Solid or cavity walls, insulated or not, the floor type, the roof and loft insulation, and the type of glazing.
- Ventilation and air tightness. Open chimneys, extractor fans, trickle vents and general draughtiness.
- Target temperatures. Living rooms are usually designed warmer than bedrooms, halls and landings.
- Local design temperature. A standard cold outdoor temperature for your area, used as the worst-case condition.
A figure based only on floor area or bedroom count is not a heat loss survey. If a heat pump quote does not mention a room-by-room calculation, ask for one before you sign anything.
3. See how the calculation works — U-values, temperatures and air changes
Each building material has a U-value, which describes how easily heat passes through it. A solid brick wall has a much higher U-value than an insulated cavity wall, so it lets far more heat out. Single glazing is worse again.
The calculation follows a recognised method so results are consistent from one installer to the next:
- Measure the area of each wall, window, door, floor and ceiling in the room.
- Multiply each area by its U-value and by the temperature difference across it.
- Add the heat lost through ventilation and air leakage.
- Account for heat moving between rooms held at different temperatures.
- Total the figures to give the room's heat loss in watts, then add every room to get the whole-house figure.
The calculation is then done at your local design temperature, so the system is sized for a genuinely cold day rather than an average one.
4. Recognise the link with MCS — why certified installers must do it
The Microgeneration Certification Scheme (MCS) sets the standards for installing heat pumps and other renewable technology. For an MCS certified heat pump installation, the installer must carry out a room-by-room heat loss calculation and use it to design the system. That covers the heat pump size, the flow temperature and the emitters in each room.
MCS certification also unlocks funding. Under the Boiler Upgrade Scheme in England and Wales, you can get £7,500 towards an air or ground source heat pump, or £9,000 for an air or ground source heat pump in eligible homes currently heated by oil or LPG between 21 July 2026 and 31 March 2027. The application is made by your MCS installer and the grant comes off the quote. Read the official gov.uk Boiler Upgrade Scheme guidance or check an installer on the MCS find an installer search.
Installing a heat pump in a home currently has 0% VAT until 31 March 2027, after which it rises to 5%. Our guide to heating grants and funding explains what you may be entitled to.
5. Size the heat pump correctly — not too big, not too small
Heat pumps work best when they are sized closely to the home's real heat loss. An oversized unit tends to cycle on and off in milder weather, which reduces efficiency and adds wear. An undersized one struggles on the coldest days and leaves rooms cool.
The survey gives the designer the total to work from, so they can choose a unit with the right output at your design temperature. It also shapes the hot water cylinder size and the way the system is controlled. According to the Energy Saving Trust's air source heat pump advice, heat pumps are more than three times as efficient as a gas or oil boiler, but only a correctly sized system will get close to that in practice.
| Rule-of-thumb estimate | Room-by-room heat loss survey | |
|---|---|---|
| Based on | Floor area or bedroom count | Measurements, construction and ventilation in every room |
| Radiator sizing | Not possible room by room | Each emitter checked against the room's need |
| Meets MCS requirements | No | Yes |
| Typical outcome | Oversized unit or cold rooms | Right-sized system with lower flow temperatures |
6. Check every radiator and floor — matching emitters to each room
Heat pumps run most efficiently at lower water temperatures than a typical older boiler system. At those temperatures a radiator gives out less heat, so some existing radiators may be too small for their room on a cold day.
The room-by-room figures let the designer check each radiator against the heat that room needs at the chosen flow temperature. Some will be fine. Some may need replacing with larger or double-panel models. Some rooms, especially extensions, may suit underfloor heating designed for heat pumps instead. Our post on keeping your radiators with a heat pump goes into this in more depth.
A lower flow temperature generally means cheaper running but more radiator upgrades. The design is a balance between installation cost and long-term efficiency, and the same thinking applies to low-temperature heating with modern boilers.
7. Use the results to cut heat loss — fabric first, then the system
A good survey shows you exactly where your heat is going. That makes it a useful tool even before any equipment is chosen. A cold bedroom over a garage or a draughty hallway shows up clearly in the numbers.
Simple improvements such as topping up loft insulation, draught-proofing doors or blocking an unused chimney can reduce the heat loss. Sometimes that allows a smaller heat pump, fewer radiator changes and lower running costs.
The survey also helps you prioritise. Rather than guessing which upgrade matters most, you can see which rooms lose the most heat and why. A solid north-facing wall, a large patio door or a poorly insulated loft hatch will stand out straight away. That turns a vague plan to “make the house warmer” into a clear list of jobs, in order of the difference each one makes.
“If you are planning new windows, an extension or extra insulation, tell us before the survey. It changes the numbers, and it is far better to design for the house you will have than the one you have today.”
8. Prepare for your survey — a quick checklist
A survey for a typical home often takes a couple of hours on site, with the calculation and design done afterwards. You can make it quicker and more accurate with a little preparation.
- Gather any paperwork for cavity wall or loft insulation.
- Find your Energy Performance Certificate using the gov.uk find an energy certificate service.
- Note any rooms that feel cold or draughty, or that you would like warmer or cooler.
- Make sure the surveyor can reach every room, the loft and the current boiler or cylinder.
- Ask for a copy of the room-by-room results so you can see where the heat goes.
We carry out surveys throughout the area, from Crawley to homes in and around Horsham, and we always talk you through the results before recommending a system.
Frequently asked questions
How long does a heat loss survey take?
For a typical home the site visit often takes a couple of hours, while the surveyor measures each room and looks at the construction. The calculation and system design are then completed afterwards. Larger, older or more complex homes take longer, particularly where there are extensions, loft conversions or a mix of solid and cavity walls.
Do I need a heat loss survey for a new boiler?
It is not always required for a straightforward boiler swap, but it is good practice. It is especially useful if you are changing radiators, adding an extension or want the boiler to run at lower flow temperatures for better efficiency. The results also help if you are thinking about a heat pump in the future.
Is an EPC the same as a heat loss survey?
No. An Energy Performance Certificate gives a general efficiency rating for the whole property and suggests improvements. A heat loss survey calculates the heat each individual room needs on a cold day, to a recognised design standard. An EPC is useful background, but it cannot be used to size a heat pump or radiators.
Will I definitely need new radiators for a heat pump?
Not always. Many homes keep a good number of their existing radiators. The survey compares each radiator's output at the planned flow temperature with what the room needs, so you only replace the ones that fall short. Rooms with generous radiators or good insulation often need no change at all.
Written by the NW Heating Services team
NW Heating Services installs, services and repairs boilers, heat pumps and underfloor heating for homes and landlords across Surrey, Sussex and Kent, with more than ten years of local experience. Our guides are written from the jobs our engineers carry out every week. More about our team.