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How much heating does my home need?

Enough to keep the house warm on the coldest days of a normal winter, and not much more. That amount is the house's heat loss, in kilowatts (kW). It depends on the building and where it stands, not just its floor area, so the same size of home can need anything from 5 to over 30 kW.

In short

Work out the heat the house loses on a cold "design day": through walls, roof, floor, windows and doors, and with the air that leaks or is let in.
Choose a heating system about that size, with a modest margin. Thermalix adds 20 %.
Rules of thumb per m² often give a unit two or three times too big, which costs more to buy and runs worse.
The size also tells you which kinds of heating will work well, and which emitters (radiators, underfloor, fan coils) they need.

What decides how much heating a house needs

A house keeps warm when the heating puts in as much heat as the house loses. The loss is largest on the coldest days, so that is what the heating must be able to cover. Engineers use a design outdoor temperature: a cold that a place only reaches for a few hours in a typical year. It is about 0 °C in Athens, −4 °C in Paris and −14 °C in Oslo.

From there, six things set the answer:

Where the house is. The colder the design day, the more heat leaves. Height above sea level matters too: it is colder up a hill than on the coast nearby.
Insulation of walls, roof and floor. Each is described by its U-value, the heat that passes through one square metre for each degree of difference inside to outside. Lower is better: an insulated wall might be 0.2–0.35 W/m²K, an old solid wall 1.5 or more.
Windows and doors. Even good double glazing loses several times more heat per square metre than an insulated wall, so their area counts as much as their type.
Air leaks and ventilation. Cold air that comes in has to be warmed. It is often a sixth to a quarter of the total; an airtight house with heat-recovery ventilation loses much less.
Size and shape. What counts is the outside surface (walls, roof, floor), not the floor area. A compact house or a flat between other flats has less surface to lose heat through.
How warm you keep it. 21 °C is the usual design figure for living spaces. Each extra degree adds about 3–5 %: less in a cold climate, more in a mild one, where the difference inside to outside is smaller.

The same wall on a −4 °C day with 21 °C inside. Without insulation, four and a half times as much heat goes through it, and its inside surface is about 4 degrees colder, which is why rooms with bare walls feel chilly even when the air is warm. The heat through each m² is the U-value times the difference in temperature: 1.6 × 25.1 = 40 W, against 0.35 × 25.1 = 9 W.

Add up the heat lost through every part of the building, plus the air, at the design temperature, and you have the design heat load. In Europe the method for this is set out in the standard EN 12831-1, which works it out room by room so that each room's radiator or loop can be sized as well.

One house, four climates: a worked example

Take a made-up two-storey house of 178 m² inside, built to a fairly good modern standard: walls U 0.35, roof 0.30, double glazing 1.9, reasonably airtight, kept at 21 °C. Here it is worked out by EN 12831-1 in four places, and then the same house with no insulation and single glazing:

The heating to fit the same house in each place, against a 100 W/m² rule of thumb.

The house	Design outdoor temperature	Heat loss	Heating to fit (+20 %)	Per m²
Nicosia, Cyprus	1.5 °C	5.0 kW	6.0 kW	28 W/m²
Athens, Greece	0 °C	5.4 kW	6.5 kW	30 W/m²
Paris, France	−4.1 °C	6.8 kW	8.2 kW	38 W/m²
Oslo, Norway	−13.6 °C	9.5 kW	11.4 kW	53 W/m²
Paris, no insulation, single glazing, draughty	−4.1 °C	23.5 kW	28.2 kW	132 W/m²

Three things stand out:

Same house, double the heating from Nicosia to Oslo, only because of the climate.
Same place, three and a half times the heating in Paris without insulation. The building matters even more than the climate.
The heat goes everywhere. In the insulated house in Paris, the walls take 28 % of it, windows 26 %, air 17 %, the roof 12 %, the ground floor 7 %, the joints between them (thermal bridges) 7 % and the doors 3 %. That's why one figure per m² can't capture it.

Where the heat leaves the insulated house in Paris on the design day, 6.8 kW in all. Arrow widths are to scale.

Why rules of thumb get it wrong

A common shortcut is a fixed figure per square metre, often around 100 W/m². For the house above that gives 17.8 kW, whatever it is built of and wherever it is. That is three times what it needs in Nicosia and twice what it needs in Paris, and yet still too small if the house were old and uninsulated in Paris.

The rule doesn't know about insulation, glazing, air leaks, shape or climate, which are exactly the things that set the answer. It was handed down from older, leakier buildings and from a time when oversizing a boiler cost little. For a modern home, and above all for a heat pump, it tends to oversize.

Too big or too small: what happens

Too big is the more common mistake, and it isn't harmless:

It costs more to buy, and a bigger heat pump also needs a bigger outdoor unit and sometimes a larger electricity supply.
Most of the year it runs far below its size. Heat pumps and boilers can only turn down so far; below that they switch on and off ("short cycling"), which wears them and lowers their efficiency.
Rooms can swing between too warm and too cool as it cycles.

An illustration of one heating season, its days put in order from the coldest. Most days need a fraction of the coldest day's heat. If a unit can turn down to about a third of its size, one sized from the heat loss runs steadily on nearly half the days; one sized by the rule of thumb is too big to run steadily on almost any of them.

Too small means that on the coldest days the house doesn't quite reach the set temperature, or a backup electric heater makes up the difference, which is expensive to run. A modest margin over the design load, sized properly, avoids both.

Hot water is separate. A heat pump or boiler that also heats a hot-water tank usually does it between heating runs, so it rarely needs to be bigger; a combination boiler heating water on demand is sized for the hot water instead.

What kind of heating?

Once you know the size, you can see which systems fit. A well-insulated house with a low heat loss has the most choice.

System	How it heats	Suits	Watch out for
Air-to-water heat pump	Takes heat from the outside air and heats water for radiators or underfloor heating, often hot water too	Most houses, from mild to cold climates; best with low heat loss	Works best at low water temperatures (35–45 °C), so radiators may need to be larger
Air-to-air heat pump (split units)	Indoor units blow warm air; the same units cool in summer	Mild and warm climates, homes that need cooling too, flats	No hot water; one indoor unit per room or zone
Ground-source heat pump	Takes heat from the ground through buried pipes or a borehole	Larger plots, colder climates, the highest efficiency	Higher cost of the ground works
Gas boiler	Burns natural gas to heat water	Homes on the gas grid	Fossil fuel; the EU no longer subsidises stand-alone fossil boilers from 2025, and several countries restrict them in new homes
Oil or LPG boiler	Burns oil or bottled gas to heat water	Homes off the gas grid	The highest CO₂; a tank and deliveries
Wood pellet boiler or stove	Burns pellets or logs	Rural homes with space for fuel	Storage, deliveries or loading, ash; local air quality
Electric heaters	Turn electricity straight into heat	Very low heat loss, or rooms used rarely	Cheap to fit, but two to four times the running cost of a heat pump for the same heat
District heating	Hot water piped from a central plant	Where a network exists	Depends on what's available in the street

Two systems can also share the work, for example a heat pump for most of the year with a boiler or stove for the coldest days. In warm climates, where the house needs cooling in summer, a heat pump that also cools is often the natural choice, and the cooling load may decide the unit's size rather than the heating.

Radiators, underfloor or fan coils?

The system makes the heat; the emitters put it into the rooms. Each room needs emitters able to give out its own share of the heat loss, at the water temperature the system runs at.

Underfloor heating covers a large area, so it works at low water temperatures (around 30–40 °C), ideal for heat pumps. It is slow to warm up and cool down, so it suits steady heating more than short bursts.
Radiators are quicker. Sized for an old boiler at 70 °C, they give out much less heat at a heat pump's 45 °C, so some rooms may need bigger ones. Working out each room's heat loss shows which.
Fan coils blow air over a coil of hot or cold water, so the same unit can heat in winter and cool in summer.

The lower the water temperature the emitters can work at, the more efficient a heat pump is through the year. That's another reason to work out the heat loss room by room before choosing.

How to work it out for your own house

Find the design outdoor temperature for where the house is, including its height above sea level.
List every part of the outside surface: walls, roof, floor, windows and doors, with their areas and U-values. Ask the architect or builder, or estimate them from the age and type of construction.
Allow for the air: how airtight the house is and how it is ventilated.
Add up the loss at the design temperature, room by room, by EN 12831-1, then add a modest margin to choose the unit.

By hand that's an afternoon of careful work for one house. Thermalix does it for you: pick the building on a map anywhere in Europe and it takes the design temperatures and a typical year of hourly weather from the nearest weather stations. Then describe the house with a few plain questions, or room by room from the drawings. It gives the heat loss and the size of unit, the cooling load if the house needs air conditioning, and the yearly running cost with the system you're considering.

Work out your own house. Thermalix is opening in stages to engineers, installers and homeowners.

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The figures above are for a made-up house, worked out by Thermalix with EN 12831-1 and each place's design temperature from NOAA weather-station records. They show the method; they are not advice for any particular building. Ask an engineer or installer to check the figures before you buy a system.