I am a homeowner located in the Pacific NW. I need a new gas furnace for my home. One HVAC guy I spoke with is recommending a 75K BTU unit, and another is recommending 90K. In both cases, it's a 90+ multistage/multispeed furnace, so the effective BTU recommendation differs by 15K.
From the best that I can tell by doing my own calculation, my heat loss is just about 70K BTU at the coldest outside temperature. The guy who recommended a 90K unit said that I'd be "safe" with the bigger unit running on its low speed, and the guy who recommended the smaller one said that I'd be better off and more efficient to run a 75K closer to the max design capacity, since it would be cycling less often. It's rare in this part of the country to be truly at the ends of the temperature range, which are 24 and 84F. We're usually 15 degrees or so inside of that year-round.
The 90K guy also said that he can tweak the unit to use less gas if I was concerned about cost, but I don't understand why I'd buy a V8 to run on 7 cylinders. I also got an earful about CFM, and how the blower on the 90K unit would work out better for me.
Every time I read about buying a furnace, homeowners are told to be careful to right-size their equipment, but every installer I spoke with seemed very casual about it, and some of these places are big, well-regarded outfits. None of them did an on-site calculation, or even took a look at the rest of the house.
What should I know as a homeowner about sizing, or is it just not that big a deal?