Room heat loss
room_heat_lossCalculate the design heat loss of a single room in watts, following BS EN 12831 as MIS 3005-D applies it. This is the figure a heat pump is sized from and the figure the radiator in that room must be able to deliver.
BS EN 12831 is a room-by-room method. Run this once for each heated room and add the results; sizing a heat pump from floor area alone is what produces the oversized installations that cycle all winter.
Internal design temperatures come from Table 1 of MIS 3005-D and follow from the room type: 22 for a bathroom, 21 for living and dining rooms, 18 for bedrooms, halls, kitchens and toilets. These are minimums, so a higher figure is accepted and a lower one is refused.
External design temperatures come from Table 2, for eight locations, in two columns: 'a' is the 99% figure and 'b' the colder 99.6% one, which needs no uplift for intermittent heating.
Each element needs an area, a U-value, and a kind. The kind matters because it fixes which temperature difference applies. Walls, windows, doors, roofs and suspended floors lose heat to the outside design temperature. A SOLID FLOOR ON THE GROUND loses heat to the local annual average air temperature instead, which is far warmer, and getting this wrong is the commonest error in a heat loss calculation. Window and door areas belong to the window and the door, not to the wall behind them.
Method from MIS 3005-D. These calculators follow the published MCS methods, for understanding a system and checking a quotation. Anything that gets installed still needs a design and a site survey from a certified installer.
Input Schema
| Name | Required | Description | Default |
|---|---|---|---|
| elements | Yes | Every surface through which the room loses heat. At least one is required. | |
| location | Yes | Which of the eight tabulated locations the property is nearest: 'belfast', 'birmingham', 'cardiff', 'edinburgh', 'glasgow', 'london', 'manchester' or 'plymouth'. Required. | |
| roomName | No | A name for the room. | |
| roomType | No | Which of the tabulated room types this is: 'living_room', 'dining_room', 'bedsitting_room', 'bathroom', 'bedroom', 'kitchen', 'hall' or 'toilet'. Sets the internal design temperature. Defaults to 'living_room'. | |
| designBasis | No | 'a' for the 99% outside design temperature or 'b' for the colder 99.6% figure. Column B needs no uplift for intermittent heating. Defaults to 'a'. | |
| floorAreaM2 | Yes | Floor area of the room in square metres. Required, and used with the ceiling height to work out the ventilation loss. | |
| internalTemp | No | Internal design temperature in Celsius, if it should differ from the tabulated value for the room type. MIS 3005-D sets the tabulated values as a floor, not a target, so a higher figure is allowed and a lower one is not. | |
| ceilingHeightM | No | Floor to ceiling height in metres. Defaults to 2.4. | |
| airChangesPerHour | No | How many times an hour the air in the room is replaced. A draughty old room may be 1.5 or more; a well sealed new one 0.5. Defaults to 1. |
Output Schema
| Name | Required | Description | Default |
|---|---|---|---|
| notes | Yes | ||
| method | Yes | ||
| elements | Yes | ||
| location | Yes | ||
| roomName | No | ||
| roomType | Yes | ||
| references | Yes | ||
| designBasis | Yes | ||
| fabricLossW | Yes | ||
| largestLoss | No | Which single element loses most, which is where insulating first pays best. | |
| internalTemp | Yes | ||
| roomVolumeM3 | Yes | ||
| totalHeatLossW | Yes | The room's design heat loss in watts. This is what the emitter in the room has to deliver on the coldest day. | |
| ventilationLossW | Yes | ||
| externalDesignTemp | Yes | ||
| fabricSharePercent | Yes | ||
| wattsPerSquareMetre | Yes | Heat loss divided by floor area. A useful sanity check: a well insulated room lands near 40, a poor one well over 100. | |
| temperatureDifference | Yes | Internal minus external design temperature, in kelvin. The figure most of the fabric loss is worked out against. | |
| ventilationSharePercent | Yes |