I am digging into the heat pump modeling and ResStock and was surprised at the amount of supplemental heating energy used by ASHPs in the simulations. For instance, in climate zone 5A, the supplemental heating coil consumes almost as much electricity as the heat pump over the course of the year for one particular simulation I looked into. One thing that stuck out to me is that the heat pump heating coils are sized smaller than I would think they are in the field. For instance, a 1700 sqft single family home gets a 2 ton heat pump. Would think that most contractors would put a 3 ton system since unfortunately rules of thumb seem to be widely used for existing home installations. This smaller sizing would result in the heat pump satisfying less load than it could if it were sized larger and greater use of the backup electric resistance heat.
Since these values are hard coded into the IDF files, can anyone from NREL (tagging you guys @Eric Wilson and @shorowit) explain the sizing or point me towards some documentation on it?
In addition, how does the simulation call the DX heat pump and supplemental coils? If the DX coil capacity is able to satisfy the load, then does the supplemental coil stay off? Once both are required, does it call them simultaneously and each coil is activated at their rated capacity at given temp/conditions? So for instance, if 100 BTUs are required in an hour and the heat pump can only supply 78 BTU/h, does the backup coil get activated and let's assume it's capacity is 140 BTU/h so that the heat pump only ends up supplying 100*(78/(78+140)) = 35 BTUs for that hour.
Or is it a topping off method, where the heat pump supplies 78 BTU for the hour and the supplemental heat supplies 22 BTU. This would obviously be preferable energy consumption wise but would require a lot more complicated control algorithm to modulate the supplemental heat pump heating coil contribution which probably isn't industry standard for heat pumps in the field right now.
