Taken from the SAP 2005 (revision 1) section A2.1: "Identifying the secondary system (1) Count the number of habitable rooms and the number heated habitable rooms (for the definition of habitable rooms see S9.1 in Appendix S). (2) If a fixed secondary heater is found in a habitable room, that heater is the secondary, whether or not there are any unheated habitable rooms. (3) If there is an unheated habitable room and no fixed secondary heater in any habitable room, but there is a fixed heater in a non-habitable room, that heater is the secondary. (4) If no heater is identified in (2) or (3) there is no fixed secondary heater. In some cases it may not be immediately clear which of two systems present should be classified as the main system and which as the secondary. In these cases the system which is cheapest to use should be taken as the main system, and if there is still doubt, i.e. if they are both equally cheap to use, select the system that heats the living room. The other system can still be input as a secondary system but it needs to be input as a room heater. A room heater system should be chosen so that its efficiency closely reflects (but does not exceed) that of the actual system (as defined by Table 4a). The chosen room heater should also use the same fuel/tariff as the actual system. If two types of secondary heater are present, that which heats the greater number of rooms should be specified as the secondary system. If that condition does not resolve the choice, the system which is the cheapest to run should be specified." The Lifespan SAP Team