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HOMEOWNER GUIDE · BEFORE YOU INSTALL
How to Check an Installer’s Heat Loss Calculation
What you can check yourself, which assumptions matter, and when to get an independent review.
- Last updated: September 2026
- Heat Loss Hub · Homeowner Advice
If a heat pump installer tells you “your house heat loss is 8.6kW”, how do you know whether that figure is reasonable?
You probably cannot verify every engineering calculation yourself. But you can check something just as important: does the heat-loss calculation actually describe your home correctly?
Room dimensions, wall construction, insulation, glazing, ventilation and design temperatures all influence the final result. If those inputs are wrong, an impressive-looking calculation produced by specialist software can still produce the wrong answer.
You therefore do not need to start by checking the maths. Start by checking the house.
KEY TAKEAWAY
This guide explains what a homeowner can realistically check, which assumptions are worth questioning and when an independent review becomes useful.
Can I Check a Heat Loss Calculation Myself?
Yes — to a point.
You can usually check whether the rooms are correct, whether dimensions look reasonable, whether extensions have been included, whether wall, roof and floor construction broadly match the property, whether known insulation has been included, whether windows and doors are correctly described, whether ventilation systems and open chimneys have been recorded, whether the design room temperatures make sense, and whether any results look obviously inconsistent.
You are less likely to be able to independently verify uncertain U-values, infiltration methodology, complex ground-floor calculations, unusual thermal bridging, every ventilation calculation, or every detail of the underlying heat-loss standard.
That is fine. Your job is not to become the heating designer. Your job is to establish whether the calculation appears to be based on a realistic description of your property.
Step 1 — Ask for the Full Room-by-Room Calculation
A single final figure is not enough. If the only information you have been given is “total heat loss: 8.6kW”, there is very little you can sensibly check.
Ask for the detailed room-by-room calculation. Ideally, you want to see individual room heat losses, overall property heat loss, room dimensions, wall, floor and roof assumptions, window and door details, ventilation assumptions, internal design temperatures, external design temperature, and the U-values or construction categories used.
The room-by-room figures are particularly important because they are also normally used when assessing radiator or underfloor-heating requirements.
Step 2 — Check Every Heated Space Is Included
Compare the report with the actual property. Look for living room, dining room, kitchen, bedrooms, bathrooms, utility rooms, hallways, landings, extensions, loft conversions, studies, conservatories where treated as heated space, and any other areas inside the heated envelope.
Hallways and landings still matter even if they do not have their own radiator. If an area forms part of the heated building, omitting it can affect both the room calculations and the whole-property figure.
Also check the opposite. A garage, porch or other genuinely unheated space should not simply be treated as though it were another normally heated room.
Step 3 — Check the Room Dimensions
You do not need surveyor-level accuracy to identify an obvious problem. Check room length, room width, ceiling height, unusual shapes, vaulted ceilings, double-height areas, stairwells, and rooms within the roof.
If your lounge is approximately 4.8m × 3.9m but the calculation shows 6.0m × 5.0m, that deserves an explanation.
Small differences do not necessarily indicate a problem. Different software and surveying methods may use internal, external or other measurement conventions. The aim is to find material discrepancies, not to challenge a few centimetres.
Step 4 — Check How Walls, Windows and Doors Have Been Measured
This is an easy area to misunderstand.
Suppose a wall is 4m × 2.4m = 9.6m² and contains a 3m² window. You might expect the report to show 6.6m² wall plus 3m² window.
But some calculation systems allow the full wall area to be entered and automatically deduct the window internally. So a report appearing to show 9.6m² wall and 3m² window does not automatically mean the opening has been counted twice.
Before deciding a measurement is wrong, ask how the particular calculation software handles openings.
Step 5 — Check the Wall Construction
This is often more useful than trying to decide whether a particular U-value is correct.
Ask whether the description of the wall matches reality — for example solid brick, cavity wall, insulated cavity wall, timber frame, stone, internal wall insulation, external wall insulation, or modern insulated construction.
A homeowner may not know the precise U-value of a wall. But you may know that your 2018 extension is not an uninsulated solid-brick wall. That alone can be enough to identify an assumption that needs checking.
KEY TAKEAWAY
A good question is: “What wall construction have you assumed, and what evidence was that assumption based on?”
Step 6 — Check Extensions Separately
Extensions deserve particular attention because they often have completely different construction from the original building.
For example, a 1930s house might have original cavity walls, an older suspended floor, and traditional pitched roof construction. A modern extension might have highly insulated cavity walls, an insulated concrete floor, much better roof insulation, and modern glazing.
Applying one generic construction assumption to the entire property can distort the result. Check the extension’s walls, floor, roof, windows, doors, and insulation separately.
Building-control plans, construction drawings and invoices can be particularly useful evidence.
Step 7 — Check the Roof and Loft Assumptions
Look at what the calculation says about loft insulation, flat roofs, pitched roofs, sloping ceilings, dormers, rooms-in-roof, and unheated loft spaces.
If you know the loft has approximately 300mm of mineral wool but the calculation assumes little or no insulation, the result may be unnecessarily high.
Equally, do not assume the whole roof is well insulated simply because the main loft is. A converted loft, dormer cheeks or sloping ceiling may have a completely different construction.
Step 8 — Check the Floor Construction
Floors can be another major assumption.
Check whether the property has suspended timber floors, solid concrete floors, insulated floors, floors over garages, floors over other unheated spaces, or floors directly to the ground.
Again, the homeowner’s most useful role is usually checking the construction description. You do not necessarily need to prove the precise thermal value yourself.
Step 9 — Check the Windows and Doors
Look at window dimensions, single, double or triple glazing, approximate age, frame type where shown, patio doors, French doors, bifold doors, and rooflights.
A heavily glazed door may be treated more like a window than a traditional solid external door within some calculation systems.
The important question is whether the report broadly reflects what is actually installed. If the property has modern triple glazing and the report shows old single glazing, that obviously deserves correction.
Step 10 — Check What Is on the Other Side of Each Surface
Not every wall loses heat to outdoor air. A wall might adjoin another heated room, a neighbouring property, a garage, an unheated porch, a conservatory, a loft, an unheated stairwell, or outside air.
Likewise, a floor may be directly above the ground, above a heated room, above a garage, or above an external opening.
These conditions are not thermally equivalent. If the calculation describes a wall to a heated neighbouring property as though it were directly exposed outside, the room heat loss could be overstated.
Step 11 — Check Known Insulation Has Been Included
If you know insulation has been added, make sure that information has not been missed.
Examples include cavity-wall insulation, loft insulation, internal wall insulation, external wall insulation, insulated floors, upgraded roof construction, and extension insulation.
Useful evidence can include photographs, installation certificates, invoices, building-control documents, construction drawings, and previous surveys.
If there is no reliable evidence, the designer may reasonably need to make an assumption. The important thing is that you understand what was assumed and why.
Step 12 — Check the Airtightness or Air-Leakage Assumption
This is one of the most important checks and one that homeowners often miss.
A building loses heat not only through walls, roofs, floors and windows. It also loses heat when warm indoor air is replaced by colder outside air. That means assumptions about air leakage, infiltration, draughtiness, and airtightness can materially affect the result.
The answer might involve standard/default assumptions, a measured air-permeability result, a design air-change rate, or another recognised method.
A default assumption is not automatically wrong. But if the house has measured airtightness data or is a modern building constructed to a known standard, it is reasonable to ask whether better evidence could have been used.
KEY TAKEAWAY
Ask: “What airtightness or infiltration method has been used?”
Step 13 — Check Ventilation Equipment and Openings
Airtightness and deliberate ventilation are not the same thing.
Check whether the calculation reflects features such as MVHR, MEV, PIV, trickle vents, bathroom extract fans, kitchen extract fans, passive vents, open chimneys, wood-burning stoves, and flues.
A recirculating cooker hood is also different from a cooker hood that actually vents air outside. Likewise, an open fireplace is not the same as a sealed or blocked chimney or a closed stove with a flue.
You do not need to know the exact numerical effect. You simply need to check whether the report accurately describes what is installed.
Step 14 — Check the Internal Design Temperatures
Heat loss increases as the difference between indoor and outdoor temperature increases. So room temperature assumptions matter.
Check the design temperature used for each room. A bathroom may reasonably have a different design temperature from a bedroom. You may also have specific preferences. If you expect 22°C in the living room, that is different from designing it for 18°C.
Likewise, if every room has been entered at an unusually high temperature you did not request, ask why. A higher room temperature is not automatically wrong. It may simply be intentional. The important thing is that the assumption matches the intended use of the property.
Step 15 — Check the External Design Temperature
A heat-loss calculation is performed against a winter outdoor design condition. This should not simply be an arbitrary temperature used everywhere in Britain.
Do not assume an internet rule such as “every UK calculation should use -3°C.” Different locations can have different design conditions.
The important point is that the value is appropriate to the property location and the calculation methodology being used.
KEY TAKEAWAY
Find the external design temperature shown on the report and ask: “What source or methodology was used for this figure?”
Step 16 — Check Whether Any Additional Safety Margin Has Been Added
This is important because the calculated heat loss and the later heat-pump selection are not the same thing.
Suppose the room-by-room calculation produces 7.4kW. Then someone adds a 20% safety margin, giving 8.9kW. Then the next available heat-pump size is 10kW.
That is a different design process from simply selecting the next suitable product above a credible 7.4kW requirement.
Some design uncertainty may legitimately need consideration. The concern is unexplained or compounded margins.
KEY TAKEAWAY
Ask: “Is this number the actual calculated heat loss, or has any additional margin been added?”
Watch for Several Conservative Assumptions Stacking Together
One slightly conservative assumption may have relatively little effect. Several can accumulate.
For example: conservative wall U-values, conservative ventilation assumptions, colder outdoor design condition, higher indoor design temperatures, then an additional percentage safety margin.
Each might appear small in isolation. Together they can materially increase the final design figure.
This does not automatically make the calculation wrong. But the assumptions should be visible and explainable.
Step 17 — Look for Room Outliers
One of the easiest ways to find something worth investigating is to compare room results.
Imagine three similar bedrooms show: Bedroom 1: 620W, Bedroom 2: 690W, Bedroom 3: 1,650W. Bedroom 3 deserves a closer look.
There may be a perfectly good explanation — perhaps it has two external walls, a much larger window, an exposed roof, different construction, or substantially more ventilation.
But if it appears almost identical to the other two rooms, check the inputs. You do not need to calculate a single U-value to spot that something looks unusual.
Step 18 — Compare Similar Rooms
This is another useful homeowner check.
Compare rooms that are broadly similar in size, location, construction, glazing, and exposure. Their heat losses do not need to be identical. But substantial differences should usually have an identifiable cause.
This can quickly expose incorrect dimensions, wrong window area, different construction assumptions, ventilation entries, or incorrect room temperatures.
Step 19 — Look at What Is Driving Each Room’s Heat Loss
If the report breaks the room result into separate components, look at which element is responsible — walls, windows, roof, floor, or ventilation.
If one component appears surprisingly dominant, ask why. Do not use arbitrary rules such as “ventilation should never be more than 20%.” Buildings differ too much for simplistic percentage limits.
KEY TAKEAWAY
Instead ask: “What assumption is causing this part of the heat loss to be so high?”
Step 20 — Separate Measurements, Evidence and Assumptions
A good way to audit a report is to mentally divide its inputs into three categories.
Known or Measured
Examples: room length, room width, window size, ceiling height.
Evidenced
Examples: extension wall construction shown on building-control plans, insulation thickness shown on an installation certificate, measured airtightness test.
Assumed or Defaulted
Examples: unknown wall build-up, standard airtightness assumption, default thermal performance based on age.
Assumptions are unavoidable in many existing buildings. They are not automatically a problem. The question is: are they reasonable and transparent?
Things That Can Look Wrong but Might Be Fine
The Wall Area Appears to Include the Window
The software may automatically deduct openings.
An Internal Wall Shows a Heat Transfer
Room-by-room calculations sometimes need to consider heat transfer between spaces at different design temperatures. That does not necessarily mean the heat has been added incorrectly to the whole-building total.
The Outdoor Design Temperature Is Colder Than Today’s Weather
That is normal. The calculation is assessing a winter design condition, not today’s temperature.
The Calculation Uses Some Default Values
Existing buildings often contain information that cannot be measured directly. Defaults may be necessary where evidence is unavailable.
The Heat Loss Looks Higher Than Your Annual Energy Use Suggests
Annual energy consumption and peak heat demand are not the same measurement. Ask before assuming the calculation is wrong.
Red Flags Worth Questioning
The following do not automatically prove an error, but they deserve explanation: no room-by-room calculation available, obvious room-dimension errors, rooms missing from the calculation, modern extension treated exactly like the original building without explanation, known insulation omitted, incorrect glazing type, wrong floor or roof construction, garage or porch treated incorrectly, ventilation equipment omitted, open chimney shown where there is none, unusual room-temperature assumptions, unexplained outdoor design temperature, additional percentage safety margin with no explanation, significant room outliers that cannot be explained, and substantially different results from two installers where neither can explain the assumptions.
KEY TAKEAWAY
A red flag means: ask a question. It does not mean: the installer is definitely wrong.
Why Do Two Installers Get Different Heat-Loss Figures?
This is very common.
Installer A might calculate 7.2kW while Installer B calculates 9.4kW and Installer C calculates 11.0kW.
Do not automatically trust the lowest, trust the highest, or average the three.
Compare the assumptions. Differences often come from dimensions, construction, U-values, ventilation, airtightness, internal temperatures, external design temperature, and adjoining spaces.
The useful question is: what assumptions caused these figures to diverge?
Can I Compare the Heat Loss With My EPC?
Only as supporting information.
An EPC can provide useful clues about building age, wall construction, insulation, glazing, floor area, and previous energy estimates.
But an EPC is not the same as a detailed room-by-room design heat-loss calculation. Annual energy use and peak heating demand answer different questions.
Treat the EPC as evidence worth comparing with the survey — not as proof that the heat-loss figure is right or wrong.
Can I Compare It With My Previous Gas Usage?
Sometimes, as a rough sense check.
If you know how much gas the property consumed during a particularly cold period, it may help you judge whether a design heat-loss number looks wildly implausible.
But several adjustments are needed. Gas consumption can include domestic hot water, cooking, boiler losses, and intermittent heating. You would also need to know outdoor temperature, indoor temperature, boiler efficiency, and the length of the measurement period.
So: gas use can provide supporting evidence. It should not replace the room-by-room calculation.
What About Watts per Square Metre?
You may see homeowners comparing properties using W/m² — for example 60W/m² or 100W/m².
This can be useful as a rough sanity check between broadly similar properties. But it cannot validate the design.
Two houses with the same floor area can have very different external-wall area, glazing, insulation, airtightness, roof exposure, desired room temperatures, and locations.
Treat W/m² as a clue, not a verdict.
What Should I Do If I Find Something That Looks Wrong?
Ask a precise question. For example:
“The calculation shows the rear extension as solid brick. It was built in 2016 with insulated cavity walls and I have the building-control drawings. Can you confirm why solid-wall construction has been used and how correcting it would affect the room and whole-house heat loss?”
That is much more useful than: “Your heat-loss calculation is wrong.”
The installer may explain the assumption, provide evidence you did not have, correct the input, or issue a revised calculation. Any of those moves the design forward.
Keep a Simple Discrepancy Log
If you identify several questions, record them rather than trying to remember everything. A simple table works well:
| Item | Installer’s calculation | What I know | Evidence / question |
|---|---|---|---|
| Lounge dimensions | 5.2 × 4.0m | Approx. 5.2 × 4.0m | Looks reasonable |
| Rear extension wall | Solid brick | Insulated cavity | Building-control drawings |
| Loft insulation | 100mm | Approx. 300mm | Photograph available |
| Glazing | Double | Double | Correct |
| Airtightness | Default | Unknown | Ask what assumption was used |
| External design temp | -4.5°C | Unknown | Ask source |
| Bedroom design temp | 22°C | Prefer 18°C | Ask why 22°C used |
This turns a vague feeling that “the calculation doesn’t look right” into specific questions that can actually be answered.
When Should I Stop Checking It Myself?
A homeowner can reasonably check whether the report matches the property. That includes dimensions, room list, layout, construction descriptions, known insulation, glazing, ventilation equipment, desired room temperatures, and obvious inconsistencies.
An independent technical review becomes more useful when the disagreement involves uncertain U-values, disputed wall construction, infiltration methodology, unusual ventilation calculations, complex floors, thermal bridging, unusual geometry, competing technically plausible assumptions, or differences large enough to change the heat-pump or radiator selection.
You do not need to resolve every engineering question yourself.
Which Heat Loss Hub Service Fits My Situation?
I Haven’t Installed the Heat Pump Yet and the Calculation Looks Questionable
An Independent Design Review can review the heat-loss calculation alongside the proposed heat pump, radiators, flow temperature and wider design.
I Want My Own Independent Heat-Loss Calculation
A Heat Loss Calculation gives you an independent room-by-room assessment rather than reviewing the installer’s figures alone.
My Heat Pump Is Already Installed and I Now Think the Original Heat Loss Was Wrong
See: Was My Heat Pump Heat Loss Calculation Wrong? This is designed for retrospective investigation of an existing installation.
Frequently Asked Questions
You can check many of the inputs and assumptions. Dimensions, construction, insulation, glazing, ventilation equipment and room temperatures can often be checked without specialist knowledge. Verifying every technical calculation may require professional review.
A proper design calculation should establish the heat requirement of the individual rooms as well as the whole property. It should account for the building fabric, dimensions, ventilation and design temperatures.
For heat-pump design, room-by-room figures are extremely important because they help establish individual emitter requirements as well as the whole-building load. A single property total provides much less information.
They may have used different measurements, U-values, construction assumptions, ventilation assumptions, internal temperatures, or external design conditions. Compare the assumptions before deciding which calculation is more credible.
Their usefulness depends heavily on the quality of the information entered. A detailed methodology cannot compensate for incorrect measurements or unrealistic property assumptions. Older buildings also contain more uncertainty than properties with fully documented modern construction.
There is no single correct U-value for every wall, floor or roof. It depends on the actual construction. A homeowner is often better placed to check whether the construction description is correct and then ask how the associated U-value was chosen.
Ask what method has been used. It may be a standard/default assumption or based on measured data. If you have an airtightness test or reliable construction information, provide it to the designer.
Yes, spaces within the heated building still need to be represented appropriately even if they do not have their own dedicated radiator.
Its individual rooms should form part of the complete room-by-room calculation, but its construction may need different assumptions from the original property. Modern and historic parts of the same house should not automatically be treated identically.
Only as supporting evidence. An EPC is not a substitute for a room-by-room design heat-loss calculation.
Previous fuel use can sometimes provide a useful sense check, particularly during cold weather. But several adjustments are required, so it should not replace the detailed calculation.
It can provide a rough comparison, but it is too simplistic to validate a heat-loss calculation by itself.
The calculated building heat loss and the eventual heat-pump selection should be distinguishable. If an additional margin has been added, ask why and how it influenced the final equipment selection.
Check the inputs rather than trying to force the number lower. Look particularly at dimensions, construction, insulation, airtightness, ventilation, and design temperatures. Then ask the installer to explain any assumptions you cannot reconcile.
Carry out the same process. Look for missing rooms, omitted external surfaces, optimistic insulation assumptions, incorrect glazing, understated ventilation, or incorrect design temperatures. The goal is accuracy, not the lowest possible number.
Consider one when important assumptions remain unexplained, two installers produce materially different results, known property information appears to have been ignored, or the disputed heat-loss figure materially changes the proposed heat-pump or radiator sizing.
Not Sure Whether the Calculation Deserves Your Confidence?
Do not start by trying to prove the final kW figure right or wrong. Start with a simpler question: does this calculation accurately describe my house?
Check the rooms, the dimensions, the construction, the insulation, the glazing, the ventilation, and the design temperatures. Then identify which remaining values are assumptions and ask how they were justified.
If the calculation still contains material questions that you cannot resolve, Heat Loss Hub can independently review the design before you commit to the installation.
GET AN INDEPENDENT REVIEW
Request an Independent Design Review before you commit to the installation.