How are ratings calculated when there is more than one heating system in a house?
The new HERS council rating method uses the Modified End-Use Load method. The loads are modified by applying a ratio of the efficiency in the building to the minimum efficiency specified by the HERS Guidelines as a function of system type and fuel type. If two or more systems are present in a building, each portion of the load is applied to its own specific ratio. You can think of it as the same method, but having one end-use per fuel type. For example: If a user specifies a gas fired air distribution system servicing 60% of the heating load, and electric resistance for the remaining 40% of the heating load. Then 60% of the heating load is applied to the ratio of the specified gas AFUE to the HERS Reference AFUE (78 AFUE), and the remaining 40% of the load is applied to the ratio of the efficiency of the specified electric resistance (100%) to the HERS Reference efficiency (which is a HSPF of 6.8, as the HERS reference mandates a conversion to an air-source heat pump).
