Having trouble getting subs to pay attention to what my new house needs. Two different subs say they ran a Manual J on the same plans, but one recommends a 2.5 ton 2-stage unit and one a 4 ton 2-stage unit. I decided I better check, so I bought software and did Manual J calcs on the zone covering the lower two floors of this 3-1/2 floor, tight house with spray foam insulation and good windows. I get 18,500 Btus of sensible heat gain and 3,500 Btus of latent gain. I like Ruud/Rheem equipment, and when I look at the specs for their hardware a 2-ton unit does look like enough even for the very hot, humid days we get here in Virginia.
I get-it that I don't want to oversize. But what I'm worried about is dehumidification when the temperature is moderate. The latent load is still about 3,500 at 80 degrees, but the sensible load drops a lot. If I'm reading the equipment specs right, I'm gonna have to run the compressor and blower on low speed just about all the time to handle the humidity, but if I do that it looks like I'll deliver too much sensible cooling. However, if I use a 3-ton unit with a big inside coil, the latent capacity at low speed is 2.5 the latent capacity of the 2-ton but the sensible capacity only goes up by a factor of 1.17. Seems like it would have to run a lot less to handle the humidity and would be less likely to overcool.
But on a total capacity basis, 3 tons looks way oversized relative to the Manual S 1.15x limit. And just going back to rules of thumb, 3 tons would be a bit under 1200 square feet per ton, a pretty high number by traditional standards, but 2 tons would be 1700 square feet per ton, which is way out there.
Anyone have any experience with this sort of situation? All advice gratefully appreciated.