heat pump sizing with fossil fuel back up
I know it's ok to upsize a heat pump 25% when used for heating and cooling when using elec. em/aux heat. But when you are sizing for a dual fuel set up do the same rules apply. The reason I am asking is because I did a manual j load calc. on a new 1250 sq. ft. residential home and came up with a loss of 50k btuh and a gain of about 26K btuh. The general contractor told me he wants a 4 ton 13 seer heat pump! When I asked how he came up with this size he told me he received two other bids and both were quoting for a 4 ton. Because you cannot run the heat pump and furnace at the same time, is this the reason for oversizing in an attempt to reach the heat loss with just the heat pump or did I just miss the memo altogether? Also, I have seen evap coils installed on the RA side usually for less than ideal situations. Does this make a significant difference on blower and overall system performance of equipment? Thanks in advance for your professional input!