Question 1

What size furnace do I need for a 2,000 sq ft house?

Accepted Answer

As a starting estimate, 60,000 to 120,000 BTU depending on your climate zone — roughly 60,000–80,000 BTU in the mild South, 80,000–100,000 BTU in the middle of the country, and 100,000–120,000 BTU in the cold North. The range is deliberately wide because square footage is the weakest of the inputs that set heating load. A Manual J calculation on a well-insulated 2,000 sq ft home often comes back far lower than the rule of thumb suggests.

Question 2

How many BTU do I need per square foot?

Accepted Answer

Between 30 and 60 BTU per square foot, set mostly by climate: about 30–35 in the hot South, 35–45 in the lower-middle states, 45–50 in the upper-middle, and 50–60 in the cold North. A tight, well-insulated house sits at the bottom of its zone's range and a leaky one at the top. Multiply by heated square footage for a ballpark, then treat it as the number you sanity-check a contractor against — not the number you buy from.

Question 3

Does furnace size depend on AFUE?

Accepted Answer

Yes. Furnace capacity is advertised as input BTU, but what heats your house is output BTU — input multiplied by AFUE. An 80,000 BTU furnace at 80% AFUE delivers 64,000 BTU of heat; at 96% AFUE it delivers 76,800 BTU. That is why replacing an old 80% unit with a condensing model often means a smaller input rating for the same heat — this calculator does that conversion for you.

Question 4

Is it better to oversize or undersize a furnace?

Accepted Answer

Neither, but oversizing is the more common and more expensive mistake. An oversized furnace short-cycles: it heats the thermostat's location quickly, shuts off, and repeats — uneven temperatures, more wear on the ignitor and blower, worse humidity control, and lower real efficiency than the AFUE label implies. Undersizing leaves you cold on the coldest days. Correct sizing beats both, and an ACCA Manual J load calculation is what produces it.

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