For homeowners

What is heat loss? A plain-English guide for homeowners

Heat loss is the rate your home leaks warmth to the outside, measured in watts or kilowatts (kW, a thousand watts). Knowing that number tells you how much heat your heating system has to put back in to keep you comfortable on the coldest days.

That's really it. Everything else in this guide is about where the heat goes, why the number matters, and how you find out what yours is.

Installer, or want the formulas, design temperatures and limitations? Read the full guide to heat loss.

Where does the heat actually go?

Heat always moves from warm to cold, so in winter it is constantly trying to get out of your house. It escapes in two main ways.

Through the fabric of the building

Heat passes through solid things by conduction. Think of a pan on a cooker: once the pan gets hot, the heat moves through the metal to the colder food inside. The same thing happens in your home. Warmth in your rooms passes through the walls, roof, windows, doors and floors towards the colder air outside. Walls are usually the biggest area, so they matter most.

Some materials let heat through far more easily than others. An uninsulated solid stone or brick wall loses much more heat than a modern insulated cavity wall. Every wall, window, roof and floor type has a U-value, which says how easily heat passes through it. The lower the U-value, the better it keeps heat in.

With the air that leaves

Warm air inside your home is constantly being replaced by colder air from outside. Some of this is unintentional infiltration: small gaps and draughts around windows, chimneys, letterboxes and floorboards, and especially open fireplaces. How much air leaks through the building is its airtightness. The more airtight the home, the less uncontrolled leakage there is.

The rest is ventilation, where air is deliberately allowed in and out through trickle vents, air bricks and extractor fans. You need some of it, for fresh air and to keep damp away, but it still carries heat with it.

Every home is different. A 1900s stone terrace in Snowdonia and a 2015 new build in Cardiff can be the same size and lose wildly different amounts of heat.

Why should you care?

Because you want your heating to keep you comfortably warm without costing more to buy or run than it needs to, using as little energy as possible.

Knowing your home's heat loss is the starting point. If you know how much heat the house actually needs, you can choose a heating system that is properly sized for it. Too small and it may struggle on the coldest days. Too large and you pay more for equipment that does not run as efficiently as it could.

Boilers

Gas boilers have historically been oversized for the heat a house actually needs. That certainly gets the house warm, but it is not a good way to run one. Imagine doing a long drive by accelerating hard and braking, over and over, instead of cruising at a steady speed. You would still get there, but you would use more fuel and wear the car out faster.

An oversized boiler behaves the same way. It heats the house quickly, switches off, then fires up again a short while later because it cannot turn itself down far enough to match what the house needs. This stopping and starting is called short cycling, and it costs efficiency and wears the boiler.

Heat pumps

Getting the heat loss right matters even more with a heat pump. Heat pumps work best running steadily for long periods at lower temperatures, so they need to be closely matched to the house. Too small and it struggles on the coldest days. Too large and it short cycles too.

Radiators matter more as well. Heat pumps normally run with cooler water than a boiler, so radiators that worked perfectly well before may not give out enough heat at those lower temperatures. Knowing the heat loss of each room tells you how much heat its radiator has to provide.

Even if nothing changes

If you are keeping your existing boiler, knowing your heat loss still earns its keep. It shows you which rooms, and which parts of those rooms, are losing the most heat, so you can see where insulation, better glazing or draught-proofing would make the biggest difference.

How is it worked out?

Heat loss is worked out room by room. For each room there are four steps.

1. Work out the temperature difference. This is the gap between the temperature you want in the room and the outside temperature on a cold design day. For example, 21°C inside and −3°C outside is a difference of 24°C. Engineers write it as ΔT.
2. Work out the heat lost through the fabric. For every wall, window, door, floor and ceiling between the room and somewhere colder, multiply its area by its U-value and by the temperature difference across it, then add them up. The difference is smaller for a wall onto an unheated garage than for one onto the outside, so each surface uses its own.
3. Work out the heat lost with the air. This is the heat carried away as warm air leaves and colder air replaces it, through draughts and through ventilation. It depends mainly on the size of the room and how often its air is replaced.
4. Add the two together. Fabric loss plus air loss is the heat loss for the room, in watts. Add every room and you have the heat loss for the whole house.

In the UK, heat loss calculations follow BS EN 12831-1, with guidance from CIBSE for domestic heating design and additional MCS requirements for heat pump installations.

What happens in a heat loss survey?

If you are having a heat pump fitted by an MCS-certified installer, they have to do a room-by-room heat loss survey first. It is not as dramatic as it sounds. Expect them to:

• Walk round every room measuring walls, windows and ceiling heights.
• Ask about insulation: in the loft, the walls and under the floors.
• Look at your windows and doors, and note any open fireplaces or vents.
• Check what radiators you have and how big they are.

It usually takes an hour or two depending on the size of the house. Afterwards you should get a report showing the heat loss for each room, the total for the house, and whether your radiators can keep up.

It is worth asking to see it. If a quote comes with no heat loss figures at all, that is a fair question to put to the installer.

Can you work it out yourself?

Yes, roughly. You do not need to be an engineer to measure your rooms and answer some questions about your walls and windows. Doing it yourself is genuinely useful for:

• Getting a feel for your home before you talk to installers, so you are not going in blind.
• Checking a quote. If an installer suggests a 12 kW heat pump and your own numbers say 6 kW, it is worth asking why.
• Seeing which rooms are the problem, and whether a bigger radiator or some insulation would fix them.
• Planning improvements, like whether loft insulation or new windows are worth doing first.

What it will not do is replace the installer's own survey. If you are having a heat pump fitted under MCS, which you need for the Boiler Upgrade Scheme grant, the installer still carries out their own. Think of yours as homework that helps you ask better questions.

Try it on your own home

If you have an iPhone or iPad, you can do a room-by-room heat loss survey with Heatworx for free: one survey a month, no card needed. It walks you through each room with simple questions. If your phone has a LiDAR scanner it measures the walls for you, and if not you type the measurements in. At the end you get a summary showing each room, the total for the house, and whether your radiators are up to the job.

It is the same app heating engineers use on site, so the numbers are worked out properly, and you can see every assumption it makes.

Give it a go , and if anything in your results does not make sense, get in touch. We are happy to help.

Go deeper