Posted By SkyHeating on 18 Jan 2012 02:08 PM
I don't know your area well but why are you thinking a 3 ton and not a 4 ton doc? If they are beyond the 3 ton limit of the unit for heating using 25% of their system on backup heat it will negate the savings. What indoor and outdoor temperatures did they design for. What are your entering water temperatures from your well? You should have a professional in your area tell you what heat pump and what size. I would go with a two stage unit that way your unit will have close to full heating capacity and in its first stage(most run at 60%) you will still have 28,800 BTU's (assuming 48K cooling of your unit). So your unit will be almost locked into first stage cooling sizing it perfectly.
See the attached report. A 3 ton unit with DSH will only use $35 of supplement heat in Greensboro, NC climate, with the above design parameters. A 4 ton unit will actually run $17 more per year, give you less comfort since it is running in single stage in A/C, and will cost you usually $3K more upfront. Deep ground temps are 60F, Design temp is 18F for heating in Greensboro, NC.
Designing for 1st stage running only is not perfect, it is wrong.
Supplement the heating extremes with aux heat, not full heatpump capacity design. The art is to to build the most efficient system for the least amount of upfront money. You in contrast suggest a system which is designed like a conventional system.