Room Heating Load and Heat Loss Calculator (Preliminary)

How is a room heating load calculated?

Keeping a room at the desired temperature takes as much heat as the room loses to outdoors. Heat leaves in two ways: by transmission through the building elements and with the air that is exchanged.

Transmission loss for each external surface is Q = U × A × (t_i − t_e). U is the thermal transmittance of the element: the lower it is, the better the insulation. External wall, windows, roof and floor are worked out separately and added.

Ventilation loss is the heat needed to bring incoming cold air up to room temperature: Q_v = 0.335 × n × V × (t_i − t_e), where n is the air change rate per hour and V the room volume.

Example

A room of 20 m² with a 2.7 m ceiling. Indoor temperature 20 °C, outdoor design temperature minus 3 °C, a difference of 23 K. The external wall is 12 m² at a U-value of 0.5 and the window 3 m² at 1.8. The wall loses 138 W and the window 124 W. With half an air change per hour the ventilation loss is 208 W. With a 10 percent allowance the heating load is 517 W.

Here 3 m² of window loses as much as 12 m² of wall, and air exchange is larger than either. In well insulated buildings ventilation is often the largest item.

Where do I get U-values?

A U-value is calculated from the thickness and thermal conductivity of the layers in an element, or given by the manufacturer for windows and doors. This tool suggests no values: the defaults are only there so that it shows a result when it opens.

Limits

This is a preliminary estimate. Thermal bridges, proper ground loss, adjacent spaces, orientation and wind allowances and reheat after setback are handled by the EN 12831 method. Have a full calculation done before basing radiator, boiler or heat pump selection on this result.