Conflicting HVAC Quotes
The original design for my house was 1900 sq ft 1st floor (front faces due North). with a 1900 sq foot finished basement. 450 sq ft attached garage. It is a walkout that faces due West. Only the walkout wall is fully exposed (and will be covered with slate). Remaining basement wall exposure is 12″ at the top (also slate covered). First floor and basement ceilings were to be 9′. Using Marvin Integrity fiberglass windows with LoE2 coating for North & South facing windows. LoE3 for East and West facing windows (only 1 5×5 facing East, and one 5×5 and one 2.5×5 facing West). The planned insulation was 2″ sprayed closed cell foam in basement and around band. 2×4 framing cavities filled with sprayed open cell upstairs, and attic floor was to be sprayed with open cell to R-38. I’m in KY (climate zone 4).
Upfront, I got one HVAC quote from an installer who lives across the street from my build (installs heat pumps). He initially quoted me prices on a 3-ton system, given those design parameters. His quote is about 6 months old.
As construction has progressed, only a few things have changed. Basement ceilings only turned out to be 8’10” (so only about 8′ where the duct chase will be). To save $2000, my insulator suggested using 4″ of spray foam on attic floor, then blown in cellulose on top of that to R-38. Otherwise insulation is unchanged.
Last week I got two quotes from Geotherm installers (Water Furnace & Climate Master), both recommend 3 ton systems (one 36,000 BTU/hr manual J and the other 39,000 BTU/hr). Just to make sure I had the most current prices, I went back to the original installer to refresh the quote. I told him about the change to the attic insulation. He came back with a quote for a 4 ton system (raising the cost). He said his original calculation was 36,000 BTU/hr with an all foam attic, but now that I’m doing foam/cellulose, it goes up to 46,000 BTU/hr.
So I guess my question is, does this sound reasonable? Is there a reason why a heat pump tonnage might differ from a Geotherm tonnage? Ground to inside air differential vs outside air to inside air? Is going from all foam to a foam/cellulose hybrid in the attic really going to increase my heat load by that much? I though open cell at 4″ was an effective air barrier, so why would it matter what is providing the additional R value?
I don’t want to buy an over or under-sized system. I also don’t want some dealer giving me an undersized price to win the job. The heat pump dealer is sort of old school though, using software that is a decade old (even though he does go to regular training updates). I don’t know if that could effect the calc too. Do the calculations change with time? The Water Furnace dealer has 14,000 units installed in his service area, so I wouldn’t think he is in the habit of under-sizing people or he wouldn’t do that much business.
Not sure how to proceed with conflicting info. Any suggestions?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part