If you’ve got two or three heat pump quotes and the heat loss figures behind them don’t agree, you’re not imagining a problem. This happens often, and it isn’t because one installer is bad at maths. It’s because a heat loss calculation isn’t really one number, it’s a chain of assumptions, and every assumption in that chain shifts the final figure.

The calculation is only as good as what went into it

A proper heat loss calculation measures how much heat your property loses through walls, windows, roof, floor, and ventilation, room by room, at your local design outdoor temperature. That sounds precise, and mathematically it is, but the inputs feeding that maths are rarely measured with total certainty. Wall construction is often assumed rather than opened up and inspected. Cavity insulation is frequently unknown unless there’s paperwork proving it was installed. Air change rate, how draughty the property actually is, is usually estimated from age and general condition, not measured directly. Each of those is a judgement call, and different surveyors make different judgement calls.

Small assumption differences compound into big kW differences

This is the part that surprises most homeowners. Guess “unfilled cavity” on a wall that’s actually insulated, and that wall’s calculated heat loss can roughly double. Use a slightly more pessimistic air change assumption, and a well-sealed house gains heat loss it doesn’t actually have. Use a generic regional design temperature instead of checking the specific local figure, and the whole calculation shifts again. None of these are dishonest shortcuts, they’re often reasonable judgement calls made without opening up walls or spending hours testing airtightness, but they don’t cancel each other out. They stack. A calculation built on three or four slightly pessimistic assumptions can land significantly higher than a calculation built on realistic ones, for the exact same house. We go through the mechanics of how the number is actually built up in Heat Loss Calculation Explained.

Time on site is usually the real tell

There’s a rough rule that holds up well in practice: the less time someone spends actually measuring your property, the more the calculation leans on assumptions rather than data. A thorough room-by-room survey, checking every wall, window, and door, measuring existing radiators, and confirming insulation where it can genuinely be verified, takes real time on site. A calculation built from an EPC printout or a five-minute walkthrough is filling most of those gaps with generic defaults instead. Both can be called a “heat loss calculation.” Only one of them is actually measuring your house.

Why this isn’t just an academic difference

An inflated heat loss figure doesn’t just mean a slightly bigger number on paper, it directly changes what heat pump you’re sold, and an oversized system is one of the most common causes of poor real-world performance, higher running costs than expected, and short-cycling rather than running efficiently. Our article on what happens if a heat pump is oversized covers the practical symptoms. We’ve also seen this play out in a real case: in My Electricity Bills Doubled – The Heat Pump Was Running At 55°C In Mild Weather, the root cause traced back to a sizing and flow-temperature decision made at the design stage, not a fault with the equipment itself. A separate real example of getting this right before installation is covered in our Heat Loss Survey Case Study, where correcting the flow temperature and sizing assumptions changed the system design before work began.

So which figure do you trust?

Neither number is automatically right just because it exists, and neither is automatically wrong just because it’s lower or higher than the other. The honest answer is that you need to know which assumptions each figure is built on, not just what the final kW says. That’s a different question than “which installer do I believe,” and it’s one most homeowners have no practical way to check themselves, since it means comparing methodology, not just comparing quotes side by side.

What we actually check when the numbers disagree

This is exactly the situation our Pre-Installation Design Review exists for. Rather than producing a third competing number, we review the assumptions behind the calculation you’ve already been given, checking whether the insulation, airtightness, and design temperature figures used are reasonable for your specific property, not a generic default. If you don’t have any heat loss figures yet and want a genuinely independent starting point instead of comparing installer assumptions after the fact, our Heat Loss Survey calculates it properly from scratch, room by room.

FAQ

Is a higher heat loss figure always safer than a lower one?
No. An inflated figure usually leads to an oversized heat pump, which brings its own problems, short-cycling, higher costs, and reduced efficiency, rather than being a cautious, harmless overestimate.

Can I tell which installer’s number is more accurate just from the quote?
Not reliably. The quote shows you the result, not the assumptions behind it. A calculation that took two hours on-site and one that took twenty minutes can produce equally confident-looking numbers.

If my two quotes differ by only 1-2kW, does that matter?
It can. A couple of kW is a meaningful percentage difference on many UK homes and can affect whether radiators are adequate at the proposed flow temperature, not just the headline unit size.