Identical floor area, identical fabric, identical weather. The only thing that changes is how many surfaces face outdoors — and it moves the answer by more than most insulation upgrades do.
The whole-house figure above sizes a heat pump. It cannot size a radiator, and radiators are what actually stop a heat pump working. Each room gets its own design temperature and ventilation rate, its own external walls, and its own emitter requirement at the flow temperature you chose.
| Room | Use | Floor m² | Ext wall m | Window m² | Roof | Floor | ΔT | Loss | W/m² | Rated emitter |
|---|
Fabric loss ΣU·A·ΔT plus ventilation 0.33·n·V·ΔT. Add them for the kW needed on the coldest design day; that sizes the plant.
Rated at 50 K above the room. Run cooler and output falls with roughly the 1.3 power of that difference, not linearly.
About 40% of the Carnot limit between the air it takes heat from and the water it delivers: a ratio that climbs steeply as flow temperature comes down.
A wall only loses heat if something colder is on the other side. A party wall, or the ceiling under a heated flat, carries essentially nothing.
Designed at 3 air changes an hour and held 4 K warmer than a bedroom, a small bathroom loses most of its heat as air rather than through walls: around three-quarters of the total on a modern fabric, so it needs more per square metre than any other room, and why towel rails that look oversized still struggle.
Heat pumps work fine at −10 °C; Scandinavia runs on them. What stops them working is emitters sized for 70 °C water: the same radiator gives about a third of its rating at 45 °C.
Raising the flow temperature to compensate for small radiators trades away the efficiency the heat pump was bought for. 55 °C instead of 45 °C is roughly a quarter of the running cost gone.
Fabric improvements reduce the plant size, the emitter size and the running cost at once. A bigger heat pump fixes only the first, and costs more to run forever.
At one air change an hour a normal house loses a kilowatt or two just exchanging air. Draught-proofing is the cheapest kW you will ever remove, but do not go below about 0.35 ACH without deliberate ventilation, or you trade heat for damp.
A pump much larger than the load cycles on and off instead of modulating, which lowers efficiency and wears the compressor. Bigger is not safer here, unlike a boiler.
A party wall between two heated homes has your neighbour’s living room behind it at much the same temperature as yours, so it carries essentially nothing; neither does the ceiling under a heated flat or the floor above one. This page used to cost every house as though it stood alone in a field: one 110 m² house came out at 5.8 kW whatever it was attached to, when the honest spread is 3.6 to 6.2 kW. For a mid terrace that is about a quarter too much, the difference between a 5 kW unit and a 7 kW one, and an oversized pump short-cycles at worse efficiency: the error costs money twice.
The ground floor loses heat to the earth, near 10 °C all winter, not to the outdoor air, so EN 12831 corrects it by roughly half; charging it the full outdoor difference nearly doubled the floor term. Against that, junctions where a wall meets a roof or a window meets its reveal conduct more than the flat areas either side, so they are added at 12% of the fabric loss, typical for ordinary construction and far higher on a badly detailed one.
A heat pump has an efficiency for a given lift, about 40% of the Carnot limit, and that limit climbs steeply as flow temperature falls. An earlier version fitted a straight line: right at 45 °C, but it said 3.7 at 35 °C where field data sits nearer 4.0 to 4.8, understating the entire argument for running cool.
A radiator responds to the mean of its flow and return. A system plumbed for a 5 K drop instead of a boiler’s 10 K keeps the radiator water 2.5 K hotter on average for the same flow temperature, giving roughly 7% more output for the cost of a faster pump: the cheapest output on the list, and the one most often left on the table.
The design heat loss, rounded up to the nearest available unit — not a rule of thumb per square metre. Oversizing costs efficiency because the unit cycles instead of modulating, and it costs money up front. A well-insulated 110 m² house is often 4–6 kW; a solid-walled Victorian one of the same size can be double that.
Rarely all of them. Radiators are frequently already oversized for their rooms, and the requirement is per room, not per house. Work out each room's loss and compare it with what the existing emitter gives at your intended flow temperature — this tool gives you that multiplier.
Twice, in opposite directions. It sets how much output your emitters give, and it sets the heat pump's efficiency — roughly 2.5% less COP per extra °C. Running hot to compensate for small radiators is exactly the trade that makes a heat pump expensive to run.
Because they are built from different information. The whole-house number assumes a plan shape to get its wall area, and one average air change rate. The room sum uses the wall lengths you entered and a ventilation rate per room use. Compare them as W/m², not kW — the two have separate floor areas, so the kW totals are for different houses until those agree. In a well-sealed home the room sum comes out higher, because an average hides the kitchen and the bathroom.
Ventilation. A bathroom is designed for around 3 air changes an hour against 1 for a bedroom, and it is held 4 K warmer. On a modern fabric that makes ventilation three-quarters of its heat loss, so it needs far more per square metre than its size suggests. It is why bathrooms get towel rails that look oversized and still struggle.
No. A real MCS survey measures every room, every surface and the actual construction. This is for understanding the shape of the problem before anyone quotes — and for sanity-checking a quote that looks wrong.
Releases in which this page changed, newest last. Derived from the archived copy of every release, not from notes written afterwards — so it reflects what actually shipped. Site-wide passes are left out; they are in the full changelog.