Room Heat Need Calculator
How much heat does your room lose on a cold winter night — and which Charnwood will replace it? Calculated with the standard U-value heat-loss method used by heating engineers.*
1 · Tell us about your space
These answers set the starting point for the whole calculation. You can fine-tune any individual wall, window, ceiling or floor below.
2 · Windows, glazed doors & openings
Windows can be grouped: enter how many share the same size, glazing and dressing. Use + to add another group (e.g. 3 windows with curtains, 1 without). It doesn’t matter which wall they’re on.
3 · Fine-tune individual walls, ceiling & floor (optional)
Each surface starts from your answers above. Adjust only what differs — e.g. one wall that borders a warm interior room.
Ceiling
Floor
Set automatically from your region: the ground a metre or two beneath a slab holds close to the annual average air temperature all year round — warmer than a winter night. You can override it.
Show the detailed breakdown
Recommended Charnwood stoves
What is a U-value?
A U-value measures how quickly heat escapes through one square metre of a building element — wall, window, ceiling or floor — for every degree of temperature difference between inside and outside (watts per m² per °C). Lower is better. A single-glazed steel window (U 5.7) leaks heat nearly thirty times faster than a well-insulated ceiling (U 0.2). Each element’s loss is simply its U-value × its area × the temperature difference across it — the calculator adds these up, plus the heat carried away by air leaving the room.
Why the stove’s air supply changes the answer — and how to fix it
A wood stove breathes as well as burns. Combusting 1 kg of dry wood needs about 4.4 m³ of air, but a closed stove actually draws four to five times that from the room — around 20 m³ per kg. Allowing for realistic transfer efficiency (~70%) and refuelling, a stove pulls about 7.5 m³ of room air per hour for every kW it delivers — over 35 m³/hr for a 5 kW stove. Every cubic metre it swallows is replaced by cold outside air leaking in.
Selecting “Charnwood external air intake” above removes that extra cold-air penalty, because the fire feeds directly from outside instead of diluting the warm air you’ve already paid to heat. The comparison line in the results shows exactly what the intake saves in your room. Ask your Charnwood SA dealer which models offer a direct external air connection.
▶ Watch: the external air intake explained on our YouTube channel
High ceiling or double volume? A ceiling fan genuinely helps
Warm air is lighter than cold air, so it pools at the ceiling — in tall spaces the air at roof level can be many degrees warmer than at your feet, heat that you paid for but never feel (it also drives faster losses through the roof). This layering is called thermal stratification, and studies of high-ceiling buildings attribute 20–40% of heating energy waste to it.
A ceiling fan run in reverse (winter mode) at low speed draws air up through the middle and pushes the warm ceiling layer down the walls without creating a draught. Field studies and industry guidance (including the UK Carbon Trust) put the heating energy saving from destratification at roughly 10–30%, biggest in double-volume spaces. If your room is taller than about 3.5 m, fit a fan — and select the ceiling-fan option in section 1 to include a conservative 15% saving in your result.