I have an '88 or '89 Lennox Gas 165,000 Btu furnace that died. I know this one is too large* by today's standards. Everything I've read says you don't want a furnace too large or too small, but they don't say anything about what is the acceptable range of size. Practically speaking, there must be a target range of what is the right size.
I have estimates from qualified and long-experienced contractors who are quoting a new furnace from 75,000 btus on the low end to 115,000 btus.
There does appear to be something of a judgement call in load calculations, at least the ones that you get when receiving an estimate. If time and money were infinite (for contractors or consumers), I suppose the exact number for a house could be achieved, but that is not reality.
*So here are my house stats: 1600 sq. ft total, approx 900 in basement. House was built in the 1940s, 2 story, frame and brick. Single pane windows with storm windows. House is insulated, average+ (but not superior--there's only so much you can do on an existing vintage house). Located in Denver, CO.
So what are your thoughts on what size is "too small" of a furnace, and we'll be cold, and what is "too big"? Are all of these furnaces "right sized" because a range that wide is still fine?
And if that wide of range is too wide, what should I ask these contractors, or how should I handle the discussion (particularly with one I'd like to work with), if I'd feel better if the btus should be "zeroed in on" more before install, without getting their fur up like I'm questioning whether they did the calc or rule of thumb right or it should be rechecked or telling them how to do their job? (I don't know how to do their job, just trying to do mine as a homeowner doing research.)
Appreciate your expertise/advice,