Hi. I live in Washington DC and am looking to install a dual fuel (heat pump and gas) system in our new home (a 1939 brick colonial with ~2300 sq ft). All three of the contractors I've had out have basically refused to do a Manual J so I bought a program from hvaccomputer.com and have done it myself. I'm pretty confidant I did it correctly since the program was pretty straightforward, but now I'm having some trouble deciding what to do with the data since it seems to be on the line between different sizes.
The program recommends design criteria of 91 degrees for summer and 17 for winter based on my zip code. The instructions stress that the program adds no safety margin to any calculations but suggests that in many situations people add 20% margin on furnances but little or none on A/C to maximize efficiency. However, since I'd like to do dual fuel I imagine I need to look at both the heating and colling capability of the pump.
Here's what I get:
91 34,679 (sensible gain = 31K; Latent gain = 4K)
It seems like a for the furnace, a 80% dual stage with 100K Btu's in and 80K out is probably the right size. Am I reading this right?
As for the heat pump, do you think a 3-ton or 4-ton is more appropriate? It seems 3 tons might be enough for the A/C but the switch over point would then need to be above 40 degrees which seemed high to me but maybe it isn't. Does your recommendation depend on whether I go single or dual stage (on the logic that in a dual stage, oversizing may be less problematic)? I had been thinking about either a Trane or a Bryant Evolution but neither seems to come in a 3.5 ton option. Lastly, since all the quotes I have so far are for 3 ton units, how much more could I expect to pay to go to 4 tons so I'm prepared for sticker shock if I go that way?