The Heating Industry has Lost its Way
The heating industry has been left in a right old pickle.
Years of upgrading insulation in houses without upgrading the rules of thumb have left behind a wave of boilers and heat pumps that have their sizings way off.
Then when boilers are replaced they are replaced with the same power input as the old unit, forgetting that the old scaled unit was only 50% efficient yet worked fine and the new one is 90% efficient.
Then combination boilers came along, and people started believing that the bigger the better.
This concept was not helped by manufacturers displaying pictures of larger houses and stating that they would require a 40kW boiler, then medium houses a 30kW unit with smaller houses requiring a 25kW unit.
Ok enough of the ranting…
So What is my Heat Loss and how do I calculate it?
The reality is that the average property requirement is around 6 to 8 kW. But also that it really depends on the size of the property and how well insulated it is.
To help you as a homeowner or an installer get an idea of what your heat loss is, we've created a little cheat sheet table.
Please note that the cheat sheet below is not to be used instead of proper heat loss calculations. It’s intended only as a guide to show the breadth of heat losses and give an idea of where accurate calculations may come out.
If you're getting a heat pump installation it's imperative that you get a proper room-by-room heat loss calculation done.
Also remember that although engineers say they have completed heat loss calculations, they can vary greatly in accuracy and depth.
If your surveyor has not measured every window, wall and door, checked the depth of loft insulation, enquired about cavity insulation and measured every radiator, they have not performed a true heat loss survey.
The data collection alone should take 1–2 hours on site, excluding time talking through options. If you reach the installation stage and this information still hasn’t been taken, stop the installation and abandon the contract.
The Heat Geek Heat Loss Calculation Cheat Sheet
So, in reverse order, if you have an older Victorian or pre-war building with little upgrading — say, single glazing and minimal loft insulation — you should expect 95–110 W/m² (of total floor area, not just ground floor).
If the same property has upgraded to double glazing and some loft insulation, this drops to 65–85 W/m². Further upgrades such as cavity insulation can reduce it to 40–65 W/m².
Pre-2006 builds or recent full renovations typically range from 30–50 W/m².
Recent new builds fall around 20–40 W/m².
Low-carbon homes average 10–20 W/m², and Passive Haus standards come in below 10 W/m².
Please note that hot water demand and available storage space may also affect the size of unit required.
If you want another method — or are trying to design for a specific power limit — you can estimate the heat loss by placing a fixed-output heat source such as a fan heater, then measuring internal and external temperatures overnight.
Alternatively, you can use a specialist service like Veritherm to do this for you.
Bonus Tip
To estimate your total annual energy demand, multiply your heat loss (in kW) by 1,500 for heating and 500 for hot water.
Example: A 10 kW heat loss equates to about 20,000 kWh per year — 15,000 kWh for heating and 5,000 kWh for hot water.