HEATLOSSPROHeat loss calculation too high?
A heat loss report says your house needs 12 kW, but the boiler has managed on much less for years. Most of the gap usually comes from a few default assumptions in the report or spreadsheet, not from the house. Here they are, and a calculator that takes each one back out.
In short: the three defaults that add the most are a high air change rate, a flat thermal bridge allowance on every surface, and neighbours, cellars or lofts counted as if they were outside. Together they often add 20–40%. Check the result against your gas or oil bill before choosing the heat pump.
Take the defaults back out
Enter the total from the report and the assumptions it used, then what is realistic for your house. The calculator shows how much each change takes off.
The defaults that add the most
1. Air changes per hour
Ventilation loss is 0.34 W per m³ of air per kelvin, times the number of air changes per hour. Some tables give living rooms 1.5 air changes an hour and kitchens and bathrooms 2 to 3, whatever the house is like. A house without open chimneys or obvious draughts is usually nearer 0.5 across the house. In a 140 m² house at a 23 K difference, every 0.5 air changes an hour is about 1.4 kW. A blower-door test settles it: its n50 figure divided by about 20 gives the everyday air change rate.
With MVHR, count two parts: the air that leaks in around the unit, often 0.1–0.2 air changes an hour in an airtight house (an air test's n50 ÷ 20), and the unit's own air times what its heat exchanger lets through, 1 − its heat recovery. With 85% recovery, 0.5 air changes through the unit lose about as much as 0.08 through an open window. A report that puts 0.4–0.5 on an MVHR house without the recovery about doubles its ventilation loss.
2. A flat thermal bridge allowance
Where walls meet floors, roofs and window frames, a little more heat escapes. Without a detailed calculation, a flat 0.1 W/m²K is often added to every U-value. For ordinary construction details, half of that is closer. On 400 m² of outside surface that is about 0.5 kW at a 23 K difference.
3. Neighbours, cellars and lofts counted as outside
A party wall to a heated house, a floor above a cellar or a ceiling below a loft does not face the outdoor temperature. A house next door is usually at 10–15 °C or warmer, and a cellar at 5–10 °C. A spreadsheet that treats these as outdoor surfaces, or treats unheated rooms inside the house as outdoors, adds heat loss that never happens.
Smaller ones that add up
- A design outdoor temperature colder than the local one. Each degree colder adds about 4% at 20 °C inside. See design outdoor temperature.
- Every room at 22–24 °C. Bedrooms at 18 °C and halls at 18 °C lose noticeably less.
- U-values from the age of the house when the walls, loft or windows have since been insulated or replaced.
- Safety margins and heat-up allowances added on top, sometimes twice: once in the heat loss and again when choosing the heat pump.
Check it against your bills
Last year's gas or oil use shows how much heat the house really needed. Work out the heat pump size from your gas or oil usage and compare: if the report is 50% or more above the bill-based figure, its assumptions are worth a second look. The bill can come out low if rooms were left cold, so neither figure is the answer on its own.
In the HeatLoss Pro designer you trace the floor plan, set the air changes for the house or for each room, choose MVHR with its heat recovery, mark party walls to a heated neighbour and floors over an unheated space, and see each room's heat loss change as you go. Drawing is free; a full calculation with a PDF report costs €1.
Frequently asked questions
Why is my heat loss calculation so high?
Usually because of default assumptions rather than the house: a high air change rate for every room, a flat thermal bridge allowance on every U-value, neighbouring houses or a cellar counted as if they were outside, a very cold design temperature, and safety margins added on top. Each one is reasonable on its own, but together they can add a third or more.
How can I check whether a heat loss figure is realistic?
Compare it with last year's gas or oil use. The bill shows how much heat the house really needed, and the degree-day method turns that into a peak load. If a calculation comes out 50% or more above the bill-based figure, look at its assumptions before buying a bigger heat pump.
What air change rate should I use?
For a house that isn't noticeably draughty, about 0.5 air changes per hour across the house is typical, more in older houses with open chimneys or gaps. Some older tables use 1.5 to 3 per hour for living rooms, kitchens and bathrooms, which roughly doubles or triples the ventilation loss. A blower-door test measures it: divide its n50 result by about 20.
Is an oversized heat pump a problem?
Yes. It switches on and off on mild days, which wears it and lowers its efficiency, it costs more to buy, and it may need a bigger electricity connection. Size it on a realistic peak load, using the heat pump's datasheet output at the design outdoor temperature.
Do the whole house in 5 minutes
Trace your floor plan and see every room's heat loss, the heat pump size and the radiator check as you draw. Free to try, nothing to install, no sign-up.
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