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A heat loss survey is the measured calculation that decides how big your heat pump needs to be and how hot it has to run. Every room is worked out separately, to a published British standard, and on an older Devon property with solid stone or cob walls the result rarely matches a guess.
Key Takeaways
- A heat loss survey is a room-by-room calculation to BS EN 12831-1:2017, and the MCS heat pump design standard requires one before any MCS installation.
- Devon postcodes are designed against minus 1.5°C, with another 0.6°C taken off for every 100 metres a property sits above the reference station.
- An oversized heat pump cycles on and off and costs more to run. An undersized one leaves rooms short in the coldest week.
- The survey settles four numbers: heat pump size in kW, design flow temperature, the radiator schedule and the cylinder specification.
- Your installer also has to hand you an MCS 031 performance estimate at the quoting stage, before you commit to anything.
- Ours costs £250 plus VAT and is waived in full on installation, which makes measuring the rooms the cheapest step in choosing a size.
James charges £250 plus VAT for the survey and waives the fee in full if you go ahead with the installation, so it is worth knowing exactly what you are buying. This guide covers what gets measured, why a Devon postcode changes the answer, what lands in your hands at the end, and what the fee pays for.
What a Heat Loss Survey Is, and How It Differs From the Free Visit
Two different visits get confused with each other, and a lot of the hesitation starts there. The free home assessment comes first and costs nothing. The heat loss survey is the technical job that follows it, and the two produce very different things.
The total for the house is the only honest basis for choosing a heat pump. MIS 3005-D, the MCS heat pump design standard, requires a room-by-room heat loss calculation to BS EN 12831-1:2017, so no MCS installation goes ahead without one, and without an MCS installation there is no Boiler Upgrade Scheme grant.
What the Survey Measures in Every Room
Survey day takes a while. One published installer guide puts a properly measured survey at two to four hours on site for a typical three or four bedroom house, plus the modelling afterwards in certified software. Anything much quicker is usually an estimate wearing a survey’s clothes.
Five inputs, worked out for each room
Illustration: one room, with each input pinned where it applies.
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Floor area and ceiling height Measured in the room itself, rather than read off a plan.
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A U-value for every surface Each external wall, window, door, floor and roof, judged from what it is built of.
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The air change rate How much air the room loses, set by its age, its use and how leaky it is.
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The design internal temperature The temperature it has to hold, from CIBSE tables. A bathroom is designed to 22°C.
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Thermal bridging Heat lost at junctions and window reveals. Left out, the loss is understated by 5 to 15%.
Two rooms of identical size can lose heat at very different rates, which is why floor area on its own tells you little. In the older East Devon stock, a cob or solid stone wall, a large north-facing window and an exposed gable all pull the number the same way.
Why a Devon Postcode Changes Your Design Temperature
The standard does not let an installer pick a winter temperature that suits the sale. It uses CIBSE Guide A external design data from 14 UK weather stations, and your postcode is matched to the nearest one by straight-line distance.
How your design winter is set
Illustration, not to scale.
Altitude then adjusts that figure, and the kilowatt figures follow the winter each home is designed for. Internal targets come from CIBSE tables as well, and they change with room type and with the age band of the building.
A solid stone or cob wall adds another variable, because its U-value depends on what it is built of. Our guide to heat pumps in old houses sets out how those designs are handled.
Why a Wrong Heat Loss Number Ruins a Good Heat Pump
The Energy Saving Trust’s installer guidance puts the consequences plainly in both directions. An oversized heat pump runs with excessive cycling, and an undersized one works overtime while still leaving rooms short.
Where the kilowatt figure should land
Works overtime and still leaves rooms short in the coldest week.
Sized to the design heat loss the survey measured, room by room.
Switches on and off far more than it should, which pushes up running cost and wear.
Older Devon stone and cob properties are where this guesswork does the most damage. Leaving thermal bridging out errs the other way and understates the loss.
James works as a freelance MCS assessor for NAPIT, auditing other installers’ finished heat pump work alongside running his own, so he sees how much of a system’s behaviour is decided at the design stage. Getting the heat loss figure right is the cheapest part of the whole job.
The Four Numbers a Heat Loss Survey Settles
Four things come out of the calculation, and they arrive as a written document instead of a conversation on the doorstep. Our post on which radiators change for a heat pump goes through the third of them in detail.
Four numbers, settled room by room
Also at the quoting stage: an MCS 031 performance estimate, handed to you before you commit to anything.
That estimate lets one certified quote be compared against another on the same basis. The written design is the starting point for every air source heat pump installation we do, whichever unit and cylinder the numbers point to.
What a Heat Loss Survey Costs, and Why Ours Comes Back to You
Ours is £250 plus VAT, waived in full if you go ahead with the installation and invoiced if you decide not to. The installer guide behind that two to four hour figure notes that many installers fold the fee into an accepted quote, which is what we do.
We charge because it is several hours of real work, and we waive it on install because a measured answer beats a persuasive estimate. Where the fee sits against everything else is covered in our guide to what an air source heat pump costs in Devon.
The grant is separate and does not touch the survey fee. The Boiler Upgrade Scheme pays £7,500, rising to £9,000 for an off-grid property heated by oil or LPG, with that higher figure running to 31 March 2027. Not every home qualifies, and our MCS certification lets us claim it and take it off your quote.
Frequently Asked Questions
A room-by-room measurement of how much heat your property loses at the design outdoor temperature, calculated to BS EN 12831-1:2017. It sets the heat pump size, the design flow temperature, the radiator schedule and the cylinder specification.
Book my free home assessmentOne published installer guide puts it at two to four hours on site for a typical three or four bedroom house, plus modelling time afterwards in certified software. James does the site visit himself.
Book my free home assessmentNo. The free home assessment is the first visit and produces a fixed estimate on the spot. The heat loss survey is the measured calculation that follows, and it is the one the system is designed from.
Book my free home assessmentIn practice, yes. The grant only covers MCS installations, and the MCS heat pump design standard requires a room-by-room heat loss calculation before the system is designed.
Book my free home assessmentDevon postcodes are matched to the Mountbatten weather station in Plymouth, where the CIBSE design figure is minus 1.5°C. Another 0.6°C comes off for every 100 metres of altitude above that station.
Book my free home assessmentOurs is £250 plus VAT and it is waived in full if you go ahead with the installation. If you decide not to, it is invoiced.
Book my free home assessmentFind Out What Your Own Rooms Actually Lose
Paying for a heat loss survey is a fair thing to hesitate over. What settles it is the alternative, a kilowatt figure someone arrived at without measuring your rooms. The free home assessment comes first, and James does it himself.
Book my free home assessmentPrefer to talk it through first? Ring 01395 202030. James answers, and he is the person who will price the job.
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“The heat pump is almost silent, which was something we were worried about, and the results have been fantastic. We’re lovely and warm and always have plenty of hot water.”
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