Given that sizing of the equipment is the most important issue to get right to maximize efficiency, what are the factors an installer should take into account to determine the correct size? Is it possible for me to calculate it?
For example, this homeguide describes two formula for sizing a furnace — one based on square footage (understood to be too crude) and one based on heating degree-days:
50 x heating therms x furnace efficiency = load in BTUs/hour
Oddly, this formula suggests the required BTUs/hour increases as furnace
efficiency (AFUE) increases. Surely that can't be right, since the furnace size
should not need to increase as the furnace gets more efficient.
So if these formula are not correct, what is a good method of calculation?
Correction: In the formula, as furnace efficiency increases, actual heating used would decrease. So I'm wrong; the formula does, at least, pass the
therms
smell test.
Best Answer
With a Manual J heat loss calculation. The number depends on your house's size, insulation, and air sealing, among other things. So you (or the installer) need to know those things first. You can do a rough, ballpark calculation yourself at http://loadcalc.net/ (no affiliation). It won't be 100% accurate, but it's a heck of a lot better than using antiquated rules of thumb. Dismiss anyone who tries to use rules of thumb or blindly recommends replacing equipment with something of identical size. They don't know that they don't know what they're doing.