How we calculate: heating
Every formula and default figure our heating calculators use. Each figure can be changed in its calculator to match your product. See how we calculate for the method behind all of them.
Boiler size calculator
Open the boiler size calculator
- Combi hot water (kW) = flow (L/min) × 4.186 kJ/kg·K × temperature rise (K) ÷ 60 (1 L of water ≈ 1 kg)
- Combi size = the larger of the hot-water figure and the heating figure (a combi does one at a time)
- Heating (kW) = house heat loss (W) ÷ 1000
- System or regular: cylinder reheat (kW) = cylinder energy (kWh) ÷ reheat time (hours); boiler = heating + reheat
Don't oversize: a boiler much bigger than the heat loss cycles on and off and runs less efficiently. Combis are sized for hot water, so check the incoming mains flow can supply it.
A Gas Safe registered engineer must fit and commission the boiler and will confirm the size.
This calculator has no built-in defaults: every figure comes from you or your product's data.
Heat loss calculator
- Temperature difference ΔT (K) = room temperature − design outdoor temperature
- Fabric loss (W) = Σ (area A × U-value U × ΔT) for each wall, window, door, floor and roof, plus any thermal bridging allowance
- Ventilation loss (W) = 0.33 × air changes per hour n × room volume V × ΔT (V = floor area × ceiling height)
- Room heat loss = fabric loss + ventilation loss; house heat loss = the sum of the rooms
Not an MCS heat loss survey. An installer sizing a heat pump for the Boiler Upgrade Scheme must do their own room-by-room calculation to MCS MIS 3005-D.
Simplified BS EN 12831-1: no allowance for heat-up, intermittent heating or heat flow to neighbours. Floors on the ground use the floor U-value you enter.
Our regional design temperatures and air change rates are still being checked: enter your own until they are filled in.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Air changes per hour | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job Brief v2 H1: air change rate. No figure stated in the brief; Researcher to fill from CIBSE Domestic Heating Design Guide / BS EN 12831-1 by room type and age. Until then the user enters it. |
| Thermal bridging allowance on fabric loss | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job Brief v2 H1: thermal bridging allowance is an assumption. No figure stated; Researcher to fill. Blank means no allowance. |
| Table | Clause | Nations | Status |
|---|---|---|---|
| Design outdoor temperature by region | CIBSE Guide A / MCS MIS 3005-D design outdoor temperatures (Researcher to cite the exact table) | UK-wide | 0/11 values entered Not yet checked against the source |
Radiator size calculator
Open the radiator size calculator
- Mean water temperature = (flow + return) ÷ 2
- ΔT = mean water temperature − room temperature
- Correction factor = (ΔT ÷ 50)^n, where n is the radiator exponent from the maker (often about 1.3)
- Output to buy at ΔT50 = room heat loss ÷ correction factor
Radiator catalogues quote output at ΔT50 (BS EN 442). At the lower flow temperatures of a condensing boiler or heat pump, a radiator gives much less, so you need a bigger one.
Our exponent n is a typical figure; use the maker's own n or their correction table where they give one.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Radiator exponent n | 1.3 | Our assumption Not yet checked against the source | Our choice: change it to suit your job Radiator exponent n, default 1.3 as stated in brief v2 H2 ("typical from makers"). Stored as an assumption until the Researcher cites a maker's data sheet (e.g. Stelrad, Zehnder) and switches it to typical. |
Air conditioning size calculator
Open the air conditioning size calculator
- Room volume (m³) = length × width × height
- Cooling (W) = volume × base load (W/m³) + people × heat per person + equipment W + sunny glazing m² × solar gain W/m²
- BTU/h = W ÷ 1000 × 3,412.14
- Unit size = the smallest size on sale that is at least the cooling needed
A rule-of-thumb estimate, not a cooling load calculation (CIBSE Guide A). South and west-facing glass, a loft room or poor insulation need more.
Split systems contain fluorinated gas: they must be installed by an engineer holding an F-gas qualification. Portable units don't need one.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Base cooling load | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job Brief v2 H7: W/m³ base load is an assumption the user enters (test AC1 uses 35 W/m³ entered). Researcher to source (e.g. CIBSE Guide A summary). |
| Heat from each person | 100 W per person | Our assumption Not yet checked against the source | Our choice: change it to suit your job Brief v2 H7: 100 W/person. Researcher to source (e.g. CIBSE Guide A). |
| Solar gain through glazing | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job Brief v2 H7: solar W/m² of glazing, Researcher sources (e.g. CIBSE Guide A summary). Depends on orientation. |
| Unit sizes on sale | 5000, 7000, 9000, 12000 BTU/h | Common UK size Not yet checked against the source | From your product Common unit sizes named in brief v2 H7 (5k/7k/9k/12k BTU). Check the maker's range. |
Heat pump size calculator
Open the heat pump size calculator
- Required output (kW) = (heat loss at the design outdoor temperature + hot water allowance) ÷ 1000, with no extra margin added
- Compare it with the maker's output at your design outdoor and flow temperatures, not the headline rating
- Each radiator gives its ΔT50 output × (ΔT ÷ 50)^n, where ΔT = (flow + return) ÷ 2 − room temperature
An MCS installer sizes the heat pump from their own room-by-room heat loss (MCS MIS 3005-D). This is a planning guide.
Heat pump output falls as the outdoor temperature drops: use the maker's figure at your design outdoor and flow temperatures.
A heat pump needs its own circuit: check your supply's maximum demand with an electrician.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Radiator exponent n | 1.3 | Our assumption Not yet checked against the source | Our choice: change it to suit your job Radiator exponent n, default 1.3 as stated in brief v2 H2 ("typical from makers"). Stored as an assumption until the Researcher cites a maker's data sheet (e.g. Stelrad, Zehnder) and switches it to typical. |
Radiator ΔT converter
Open the radiator δt converter
- Correction factor = (ΔT ÷ 50)^n, where ΔT = mean water temperature − room temperature and n is the radiator exponent
- Output at your ΔT = ΔT50 output × correction factor
- ΔT50 output = output at your ΔT ÷ correction factor
Our exponent n is a typical figure; use the maker's own n or their correction table where they give one.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Radiator exponent n | 1.3 | Our assumption Not yet checked against the source | Our choice: change it to suit your job Radiator exponent n, default 1.3 as stated in brief v2 H2 ("typical from makers"). Stored as an assumption until the Researcher cites a maker's data sheet (e.g. Stelrad, Zehnder) and switches it to typical. |
Underfloor heating calculator
Open the underfloor heating calculator
- Wet output q (W/m²) = 8.92 × (floor surface temperature − room temperature)^1.1 (BS EN 1264 basic characteristic)
- Room output (W) = q × heated floor area
- Pipe (m) = heated floor area ÷ pipe spacing + tails to the manifold; loops = pipe ÷ the maker's longest loop, rounded up
- Electric: output (W) = area × mat W/m²; current (A) = W ÷ supply voltage
The floor finish (its tog or R-value) doesn't change the output at a given surface temperature, but a thicker or more insulating finish needs warmer water to reach it.
Electric mats need their own circuit with RCD protection; bathrooms have extra rules (BS 7671 Section 701).
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Output coefficient | 8.92 W/m²K^n | Our assumption Not yet checked against the source | Our choice: change it to suit your job Coefficient in q = 8.92 × (θfloor − θroom)^1.1, brief v2 H6 (BS EN 1264 characteristic). Brief says "Researcher verifies". |
| Output exponent | 1.1 | Our assumption Not yet checked against the source | Our choice: change it to suit your job Exponent in q = 8.92 × (θfloor − θroom)^1.1, brief v2 H6 (BS EN 1264 characteristic). Brief says "Researcher verifies". |
| Maximum floor surface temperature | To be confirmed from the source | Regulation or HSE figure BS EN 1264 (maximum floor surface temperature, occupied zone) Not yet checked against the source | Source to be added by the Researcher Brief v2 H6: max floor surface temperature is a limit from the BS EN 1264 explainer. Researcher to fill the value and cite the clause. |
| Supply voltage | 230 V | Our assumption Not yet checked against the source | Our choice: change it to suit your job UK nominal single-phase supply voltage (brief v2 E2). Researcher to cite the statutory source. |
kW to BTU converter
- BTU/h = kW × 3,412.14
- kW = BTU/h ÷ 3,412.14
- W = kW × 1000
BTU here means BTU per hour, the unit radiator and air conditioner makers quote.
This calculator has no built-in defaults: every figure comes from you or your product's data.