Find the right boiler size for your London home in four steps. A properly sized boiler runs more efficiently, lasts longer, and keeps every room at a comfortable temperature — even on the coldest London winter day.
A heat loss calculation tells you exactly how much heat your home loses on the coldest day of the year, measured in kilowatts (kW). Without it, installers guess — and most guess too high. An oversized boiler fires briefly then shuts off (short cycling), wasting 10–25% of your gas and wearing out components prematurely. An undersized boiler runs constantly and still cannot reach temperature. The correct size, backed by a room-by-room calculation using BS EN 12831, ensures your system runs at peak efficiency for its entire lifespan.
This sets the base heat-loss profile for your home.
Used to estimate your total floor area.
Older homes have higher base heat-loss factors per m².
Getting the right size boiler is the single most important factor in heating efficiency. Here is what goes wrong when the calculation is skipped.
An oversized boiler heats radiators too quickly and shuts off before the system reaches steady state. This is called short cycling — the boiler repeatedly fires for a few minutes then stops. Each cycle wastes gas during ignition and purging, increases component wear, and prevents the boiler from modulating down to its efficient range. Real-world efficiency can drop 10–25% below the advertised figure.
An undersized boiler cannot meet your home's heat loss on cold days. It runs at maximum output continuously and still cannot reach the target room temperature. Radiators stay lukewarm, the house feels cold, and the boiler works harder than it should — shortening its lifespan and increasing fuel consumption per degree of warmth achieved.
These are the shortcuts that lead to wrong boiler sizes in London homes.
Many installers still use "1 kW per 10 m²" or "1 kW per radiator" — outdated rules that ignore insulation quality and produce oversized systems that short cycle.
If you have added loft insulation, cavity wall fill, or double glazing since your last boiler was fitted, your heat loss has dropped. Replacing like-for-like leaves you with an oversized boiler.
Choosing a 35 kW combi because you want strong showers, when your heating load is only 12 kW, means the boiler modulates down constantly and never reaches its efficiency sweet spot.
Without measuring each room, radiator outputs, and window areas, there is no way to know whether your existing radiators can deliver the heat the new boiler produces.
Every RARE boiler installation starts with a full room-by-room heat loss survey using BS EN 12831 methodology. We measure each room, log radiator dimensions, assess insulation quality, and calculate the exact kW output your home needs — not a guess, not a rule of thumb.
1
We measure every room, window, and radiator to build a complete heat-loss profile.
2
Industry-standard methodology accounts for transmission and ventilation losses per room.
3
We size the boiler, cylinder, and radiators as a matched system — not just swap a box on the wall.
A heat loss calculation measures how much heat your home loses through walls, windows, roof, floors, and ventilation gaps, expressed in kilowatts (kW). It tells you the minimum boiler output needed to keep every room at 21°C on the coldest day of the year. Without it, boiler sizing is guesswork.
An oversized boiler fires up, heats the radiators quickly, then shuts off before the system reaches temperature — a problem called short cycling. This wastes gas, increases wear on components, and reduces real-world efficiency by 10–25%. An undersized boiler runs constantly on cold days and still cannot reach target temperature.
This calculator gives a useful ballpark figure based on property type, age, insulation, and hot water demand. A proper heat loss survey uses room-by-room measurements, exact radiator outputs, and BS EN 12831 methodology to give a precise figure. Always have a qualified engineer confirm sizing before purchase.
BS EN 12831 is the European standard for calculating heat losses in buildings. It accounts for transmission losses through each building element, ventilation losses, and internal/external design temperatures. RARE engineers use this methodology on every boiler installation to ensure correct sizing.
No. Radiators can only output a fixed amount of heat regardless of boiler size. An oversized boiler simply cycles off sooner — it does not heat rooms any faster. The radiators, not the boiler, determine how quickly a room reaches temperature.
Hot water demand is added to the heating load. A combi boiler must be large enough to heat water on demand (typically 15–30 kW just for hot water). A system boiler with an unvented cylinder stores hot water, so the boiler only needs to cover the heating load plus cylinder recovery time.