The object of this project was to develop a graphic method for selecting the minimum capacity of a heating system which is to operate in a building subjected to periodic temperature setbacks.

The graphic method developed utilises a selection factor which was defined as the ratio of the maximum system design output to the design heat loss of the building.

Graphs relating the selection factor, the desired recovery time, the^amount of temperature setback, and the building and heating system parameters are presented. These graphs wore formed from the computer solutions to the equations describing the building and heating system response to a step change in boiler input.