Heat loss figures
The figures our radiator size and BTU calculators use, from the MCS Heat Pump Calculator. MCS takes them from CIBSE Guide A and the Domestic Heating Design Guide. Checked 13 November 2026.
Outside design temperature by postcode area
The temperature a heating system is designed to cope with: a cold day for the area. Take off 0.6°C for every 100 m or so above 90 m, as the calculator does. The mean air temperature is used for solid ground floors.
| Postcode area | Outside design temperature | Mean air temperature | Weather station |
|---|---|---|---|
| AB | -4.2°C | 8.5°C | NE Scotland (Dyce) |
| AL | -2°C | 11.3°C | Thames Valley (Heathrow) |
| B | -3.4°C | 9.8°C | Midland (Elmdon) |
| BA | -1.7°C | 10.6°C | Severn Valley (Filton) |
| BB | -2.2°C | 10°C | W Pennines (Ringway) |
| BD | -2.5°C | 10°C | E Pennines (Finningley) |
| BH | -1.5°C | 10.4°C | Southern (Hurn) |
| BL | -2.2°C | 10°C | W Pennines (Ringway) |
| BN | -2.1°C | 10.4°C | Southern (Hurn) |
| BR | -3.2°C | 10.2°C | South-eastern (Gatwick) |
| BS | -1.7°C | 10.6°C | Severn Valley (Filton) |
| BT | -1.2°C | 9.4°C | Northern Ireland (Belfast) |
| CA | -3.7°C | 9.4°C | North-western (Carlisle) |
| CB | -2.5°C | 11.3°C | Thames Valley (Heathrow) |
| CF | -1.6°C | 10.6°C | Severn Valley (Filton) |
| CH | -2.2°C | 10°C | W Pennines (Ringway) |
| CM | -2.3°C | 11.3°C | Thames Valley (Heathrow) |
| CO | -2.3°C | 10°C | E Anglia (Honington) |
| CR | -2°C | 10.4°C | Southern (Hurn) |
| CT | -3.2°C | 10.2°C | South-eastern (Gatwick) |
| CV | -3.4°C | 9.8°C | Midland (Elmdon) |
| CW | -2.7°C | 10°C | W Pennines (Ringway) |
| DA | -3.2°C | 10.2°C | South-eastern (Gatwick) |
| DD | -3.8°C | 8.8°C | E Scotland (Leuchars) |
| DE | -2.2°C | 10°C | W Pennines (Ringway) |
| DG | -3.8°C | 9°C | Borders (Boulmer) |
| DH | -3.7°C | 9.4°C | North-eastern (Leeming) |
| DL | -3.7°C | 9.4°C | North-western (Carlisle) |
| DN | -3.4°C | 10°C | E Pennines (Finningley) |
| DT | -1.7°C | 10.4°C | Southern (Hurn) |
| DY | -3.4°C | 9.8°C | Midland (Elmdon) |
| E | -1.8°C | 11.3°C | Thames Valley (Heathrow) |
| EC | -1.8°C | 11.3°C | Thames Valley (Heathrow) |
| EH | -3.4°C | 8.8°C | E Scotland (Leuchars) |
| EN | -2.1°C | 10.2°C | South-eastern (Gatwick) |
| EX | -1.5°C | 11°C | South-western (Plymouth) |
| FK | -3.7°C | 8.8°C | E Scotland (Leuchars) |
| FY | -2.2°C | 9.4°C | North-western (Carlisle) |
| G | -3.9°C | 9.1°C | W Scotland (Abbotsinch) |
| GL | -3.3°C | 9.8°C | Midland (Elmdon) |
| GU | -2.2°C | 11.3°C | Thames Valley (Heathrow) |
| GY | -1°C | 10.2°C | South-eastern (Gatwick) |
| HA | -1.8°C | 11.3°C | Thames Valley (Heathrow) |
| HD | -3.5°C | 10°C | E Pennines (Finningley) |
| HG | -3.5°C | 10°C | E Pennines (Finningley) |
| HP | -2°C | 11.3°C | Thames Valley (Heathrow) |
| HR | -2°C | 9.8°C | Midland (Elmdon) |
| HS | -1°C | 8.6°C | NW Scotland (Stornoway) |
| HU | -3°C | 10°C | E Pennines (Finningley) |
| HX | -2.5°C | 10°C | W Pennines (Ringway) |
| IG | -1.8°C | 11.3°C | Thames Valley (Heathrow) |
| IM | -2.5°C | 10°C | W Pennines (Ringway) |
| IP | -2.3°C | 10°C | E Anglia (Honington) |
| IV | -4°C | 8.5°C | NE Scotland (Dyce) |
| JE | -1°C | 10.2°C | South-eastern (Gatwick) |
| KA | -3.7°C | 9.1°C | W Scotland (Abbotsinch) |
| KT | -1.8°C | 11.3°C | Thames Valley (Heathrow) |
| KW | -4.2°C | 8.5°C | NE Scotland (Dyce) |
| KY | -3.4°C | 8.8°C | E Scotland (Leuchars) |
| L | -2.4°C | 10°C | W Pennines (Ringway) |
| LA | -2.2°C | 9.4°C | North-western (Carlisle) |
| LD | -2.9°C | 9.9°C | Wales (Aberporth) |
| LE | -3°C | 9.8°C | Midland (Elmdon) |
| LL | -3°C | 10°C | W Pennines (Ringway) |
| LN | -3°C | 10°C | E Pennines (Finningley) |
| LS | -2.5°C | 10°C | E Pennines (Finningley) |
| LU | -2.4°C | 11.3°C | Thames Valley (Heathrow) |
| M | -2.2°C | 10°C | W Pennines (Ringway) |
| ME | -1.8°C | 11.3°C | Thames Valley (Heathrow) |
| MK | -3.4°C | 9.8°C | Midland (Elmdon) |
| ML | -3.9°C | 9.1°C | W Scotland (Abbotsinch) |
| N | -1.8°C | 11.3°C | Thames Valley (Heathrow) |
| NE | -3.4°C | 9.4°C | North-eastern (Leeming) |
| NG | -3.2°C | 10°C | E Anglia (Honington) |
| NN | -3°C | 9.8°C | Midland (Elmdon) |
| NP | -1.6°C | 9.8°C | Midland (Elmdon) |
| NR | -2.4°C | 10°C | E Anglia (Honington) |
| NW | -1.8°C | 11.3°C | Thames Valley (Heathrow) |
| OL | -3.2°C | 10°C | W Pennines (Ringway) |
| OX | -2.4°C | 9.8°C | Midland (Elmdon) |
| PA | -4°C | 9.1°C | W Scotland (Abbotsinch) |
| PE | -3°C | 10°C | E Anglia (Honington) |
| PH | -3.8°C | 8.5°C | NE Scotland (Dyce) |
| PL | -0.2°C | 11°C | South-western (Plymouth) |
| PO | -1.8°C | 10.4°C | Southern (Hurn) |
| PR | -3.2°C | 9.4°C | North-western (Carlisle) |
| RG | -2.2°C | 11.3°C | Thames Valley (Heathrow) |
| RH | -1.9°C | 11.3°C | Thames Valley (Heathrow) |
| RM | -1.8°C | 11.3°C | Thames Valley (Heathrow) |
| S | -2.8°C | 10°C | W Pennines (Ringway) |
| SA | -1.6°C | 9.9°C | Wales (Aberporth) |
| SE | -1.8°C | 11.3°C | Thames Valley (Heathrow) |
| SG | -2°C | 11.3°C | Thames Valley (Heathrow) |
| SK | -2.9°C | 10°C | W Pennines (Ringway) |
| SL | -2°C | 11.3°C | Thames Valley (Heathrow) |
| SM | -1.8°C | 11.3°C | Thames Valley (Heathrow) |
| SN | -2.2°C | 9.8°C | Midland (Elmdon) |
| SO | -1.8°C | 10.4°C | Southern (Hurn) |
| SP | -1.8°C | 10.4°C | Southern (Hurn) |
| SR | -3.7°C | 9.4°C | North-eastern (Leeming) |
| SS | -2.3°C | 11.3°C | Thames Valley (Heathrow) |
| ST | -3°C | 10°C | W Pennines (Ringway) |
| SW | -1.8°C | 11.3°C | Thames Valley (Heathrow) |
| SY | -3.3°C | 9.9°C | Wales (Aberporth) |
| TA | -2.1°C | 10.6°C | Severn Valley (Filton) |
| TD | -3.8°C | 9°C | Borders (Boulmer) |
| TF | -3.4°C | 9.8°C | Midland (Elmdon) |
| TN | -3.2°C | 10.2°C | South-eastern (Gatwick) |
| TQ | -1.3°C | 11°C | South-western (Plymouth) |
| TR | -1.4°C | 11°C | South-western (Plymouth) |
| TS | -2.9°C | 9.4°C | North-eastern (Leeming) |
| TW | -1.8°C | 11.3°C | Thames Valley (Heathrow) |
| UB | -1.8°C | 11.3°C | Thames Valley (Heathrow) |
| W | -1.8°C | 11.3°C | Thames Valley (Heathrow) |
| WA | -2.1°C | 10°C | W Pennines (Ringway) |
| WC | -1.8°C | 11.3°C | Thames Valley (Heathrow) |
| WD | -2°C | 11.3°C | Thames Valley (Heathrow) |
| WF | -2.5°C | 10°C | E Pennines (Finningley) |
| WN | -2.1°C | 10°C | W Pennines (Ringway) |
| WR | -3.3°C | 9.8°C | Midland (Elmdon) |
| WS | -3.4°C | 9.8°C | Midland (Elmdon) |
| WV | -3.4°C | 9.8°C | Midland (Elmdon) |
| YO | -3.7°C | 10°C | E Pennines (Finningley) |
| ZE | -3°C | 8.5°C | NE Scotland (Dyce) |
Room temperatures and air changes
Air changes an hour depend on when the home was built: before 2000, 2000 to 2006, and after 2006 to current building regulations. Homes built after 2006 use 21°C throughout, except bathrooms at 22°C and stores at 16°C.
| Room | Temperature | Before 2000 | 2000 to 2006 | After 2006 |
|---|---|---|---|---|
| Lounge | 21°C | 1.5 | 1 | 0.5 |
| Games | 21°C | 1.5 | 1 | 0.5 |
| Living | 21°C | 1.5 | 1 | 0.5 |
| Bedroom | 18°C | 1 | 1 | 0.5 |
| Breakfast | 21°C | 1.5 | 1 | 0.5 |
| Bedsitting | 21°C | 1.5 | 1 | 0.5 |
| Dining | 21°C | 1.5 | 1 | 0.5 |
| Bed & Ensuite | 21°C | 2 | 1.5 | 1 |
| Kitchen | 18°C | 2 | 1.5 | 1.5 |
| Family | 21°C | 2 | 1.5 | 1.5 |
| Bed/Study | 21°C | 1.5 | 1.5 | 0.5 |
| Hall | 18°C | 2 | 1 | 0.5 |
| Landing | 18°C | 2 | 1 | 0.5 |
| Cloaks/WC | 18°C | 2 | 1.5 | 1.5 |
| Bath | 22°C | 3 | 1.5 | 1.5 |
| Toilet | 18°C | 3 | 1.5 | 1.5 |
| Shower | 22°C | 3 | 1.5 | 1.5 |
| Utility | 18°C | 3 | 2 | 0.5 |
| Dressing | 18°C | 1.5 | 1 | 0.5 |
| Study | 21°C | 1.5 | 1.5 | 0.5 |
| Store | 16°C | 1 | 0.5 | 0.5 |
U-values
How many watts pass through a square metre for each degree of difference. Lower is better insulated.
| Construction | U-value, W/m²K |
|---|---|
| Solid brick, 102 mm (half a brick), plastered | 2.97 |
| Solid brick, 228 mm (9 inches), plastered | 2.11 |
| Solid brick, 343 mm, plastered | 1.64 |
| Solid brick, 102 mm, with 75 mm insulation outside, rendered | 0.3 |
| Solid brick, 102 mm, with 75 mm insulation and plasterboard inside | 0.3 |
| Solid stone, 305 mm (12 inches), unplastered | 2.78 |
| Solid stone, 457 mm (18 inches), unplastered | 2.23 |
| Solid stone, 610 mm (24 inches), unplastered | 1.86 |
| Cavity wall, brick both sides, empty cavity, plastered | 1.37 |
| Cavity wall, brick both sides, 50 mm mineral wool in the cavity, plastered | 0.56 |
| Cavity wall, brick outside and aerated block inside, empty cavity, plastered | 0.87 |
| Cavity wall, brick outside and aerated block inside, 50 mm mineral wool in the cavity, plastered | 0.45 |
| Brick outside, timber frame inside with 100 mm insulation | 0.32 |
| Solid concrete, 152 mm, plastered | 3.12 |
| Construction | U-value, W/m²K |
|---|---|
| Wood or PVC frame, single glazed | 4.8 |
| Single glazed with secondary glazing | 2.4 |
| Wood or PVC frame, double glazed | 2.8 |
| Wood or PVC frame, double glazed, low-E glass | 2.3 |
| Wood or PVC frame, double glazed, low-E glass, argon filled | 2.1 |
| Wood or PVC frame, triple glazed | 2.1 |
| Wood or PVC frame, triple glazed, low-E glass, argon filled | 1.6 |
| Metal frame, single glazed | 5.7 |
| Metal frame, double glazed | 3.4 |
| Construction | U-value, W/m²K |
|---|---|
| Solid wood door | 3 |
| Glazed wood or PVC door, double glazed | 2.8 |
| Glazed wood or PVC door, double glazed, low-E glass | 2.3 |
| Insulation | Ceiling under a pitched roof | Flat roof |
|---|---|---|
| None | 2.51 | 1.69 |
| 50 mm | 0.6 | 0.53 |
| 100 mm | 0.34 | 0.32 |
| 200 mm | 0.18 | 0.17 |
| 300 mm | 0.12 | 0.12 |
Ground floors
A ground floor loses most heat near its outside edges, so its U-value depends on the room's size and which of its walls face outside, as well as on its insulation. The calculator looks up the room's own floor U-value from the Domestic Heating Design Guide's tables, in the MCS calculator, for solid and suspended timber floors with up to 100 mm of insulation.
Radiator output at lower water temperatures
Radiators are rated with the water 50°C warmer than the room. At smaller differences they give out this share of that rating:
| Water to room difference | Share of the ΔT50 output |
|---|---|
| 5°C | 5% |
| 10°C | 12.3% |
| 15°C | 20.9% |
| 20°C | 30.4% |
| 25°C | 40.6% |
| 30°C | 51.5% |
| 35°C | 62.9% |
| 40°C | 74.8% |
| 45°C | 87.2% |