Skip to content
Ampworth — home Ampworth
Calculators Guides Articles Questions Developers

Room heat loss

Works out one room's design heat loss, the figure a heat pump and its radiators are sized from.

Follows MIS 3005-D

The room

This sets the internal design temperature, from Table 1 of MIS 3005-D.

This sets the outside design temperature, and the annual mean a solid ground floor is worked out against.

Column B needs no uplift for intermittent heating.

About 0.5 for a well sealed modern room, 1.0 for an average one, 1.5 or more for a draughty old one.

What it loses heat through

One row per surface. Take window and door areas out of the wall area so they are not counted twice.

Typical U-values, if you have no survey

These are indicative figures for common British construction, not measurements of your house. An assumed cavity fill that was never installed is the usual reason a heat loss comes out far too low.

Walls
Solid brick, 225 mm, uninsulated 2.1
Solid stone, uninsulated 1.7
Cavity, unfilled 1.5
Cavity, filled 0.5
Insulated to modern building regulations 0.25
Windows
Single glazing 4.8
Double glazing, pre-2002 3.1
Double glazing, modern A-rated 1.4
Triple glazing 0.8
Doors
Solid timber 3
Insulated modern door 1.4
Roofs
Uninsulated loft 2.3
Loft, 100 mm insulation 0.4
Loft, 270 mm insulation 0.16
Insulated flat roof 0.25
Floors
Solid, uninsulated 0.7
Solid, insulated 0.22
Suspended timber, uninsulated 0.7
Suspended timber, insulated 0.25

Nothing is stored. Nothing is sent anywhere.

Your answer appears here

Most of the form is filled in already with figures that give a sensible answer. Adjust what you know and press the button.

What this calculator does

Calculate 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.

Also available to software

The same calculation answers on POST /api/calculators/room-heat-loss and as the MCP tool room_heat_loss. How to call them.

What this is doing, explained

Questions Heat pumps

Heat pumps: common questions

Short answers on whether a heat pump suits your house, what it costs to run, how loud it is and what has to change before one goes in.

14 questions
Guide Heat pumps

How a heat pump heats a house

A heat pump moves heat instead of making it, which is why it can deliver more heat than the electricity it consumes. What that means for your radiators and for the way the house is heated.

7 min read
Article Heat pumps

Reading a heat loss calculation

A heat pump quotation should come with a room-by-room heat loss calculation. Here is what its columns mean and the handful of checks that catch most errors.

5 min read