Two-stage/modulating furnaces

[shorowit]

Presumably would not have different efficiencies at different stages, but could account for reduced fan energy and different distribution losses (e.g., due to different airflow rates and runtime fractions).

There is a Coil:Heating:Gas:MultiStage object, but it says it only works with AirLoopHVAC:UnitaryHeatPump:AirToAir:MultiSpeed and not AirLoopHVAC:UnitarySystem? Though the IDD shows that it can be used with AirLoopHVAC:UnitarySystem. Would need to check that OS/E+ support this.

Could also consider defaulting the number of stages based on AFUE (similar to what we do for ACs based on SEER).