What size furnace do you need?

What size furnace do I need?

One that covers your home's heat loss on a cold design day. A 1,500 square foot one-story house with R-13 walls, an R-38 attic, 200 square feet of windows at U-0.30 and average air leakage loses about 18,700 BTU per hour at 5°F outside. A 95% furnace needs 18,700 ÷ 0.95 = 19,700 BTU/h of input, so even the smallest common 40,000 BTU furnace is enough.

How do I calculate heat loss?

Add up the heat that leaves through each surface and with the leaking air. Walls and ceilings: area × temperature difference ÷ R-value — 1,039 square feet of R-13 wall at a 65°F difference loses 5,197 BTU/h. Windows: area × U-factor × difference. Air: 0.018 × the house volume × air changes per hour × difference — 12,000 cubic feet at 0.5 air changes and 65°F is 7,020 BTU/h.

Why not just use BTUs per square foot?

Because it ignores the climate and the insulation. The house above needs about 12.5 BTU/h per square foot; an older, leaky house with thin insulation in a colder place can need several times as much. A rule of thumb that fits one of them oversizes or undersizes the other.

What is AFUE?

Annual Fuel Utilization Efficiency: the share of the fuel's heat that reaches the house. A 100,000 BTU/h furnace at 80% AFUE delivers 80,000 BTU/h. Standard furnaces are 80%; condensing furnaces are 90–98%. It is on the yellow EnergyGuide label and the nameplate.

Is this the same as a Manual J?

No. A Manual J calculation from ACCA goes room by room, with your exact walls, windows, orientation and local design temperature, and is what an HVAC contractor should do before sizing equipment. This whole-house estimate is for checking a quote: a furnace far larger than your heat loss runs in short cycles and wastes fuel.

What does the outdoor design temperature mean?

It is a cold-day temperature for your location that the heating system is sized to handle, not the coldest night ever recorded; on rare colder nights the house may fall a little behind. Moving the design temperature from 10°F to −10°F in the example below raises the heat loss from 21,710 to 28,946 BTU/h, because the temperature difference grows from 60°F to 80°F.

Should I choose the next size up to be safe?

Not by default. A furnace much larger than the heat loss turns on and off in short cycles, which wastes fuel and gives uneven temperatures. Choose the smallest standard size at or above the calculated input: 22,614 BTU/h of input points to a 40,000 BTU/h furnace here, the smallest size the calculator lists. A result that is several sizes below the old furnace in the house means the old one was oversized, not that the calculation is wrong.

The parts of the heat loss

Heat leaves a house through five paths. For walls, ceiling and floor the loss in BTU per hour is area × temperature difference ÷ R-value. Windows lose area × U-factor × temperature difference, and U is the reciprocal of R. Air leakage and ventilation lose 0.018 BTU per cubic foot per degree Fahrenheit (the volumetric heat capacity of air) × the house volume × air changes per hour × temperature difference. The sum is the heat the furnace must supply at the design temperature; dividing by the AFUE gives the fuel input.

Worked example: a 2,200 square foot two-story house

With R-19 walls, an R-49 attic, 280 square feet of windows at U-0.30, 0.5 air changes per hour, 70°F inside and 10°F outside, the heat loss is 21,710 BTU/h, split into 5,819 through the walls, 5,040 through the windows, 1,347 through the ceiling and 9,504 by air leakage. At 96% AFUE the input is 22,614 BTU/h, so a 40,000 BTU/h furnace; at 80% AFUE it is 27,137 BTU/h and a 40,000 BTU/h size. A tight house at 0.3 air changes loses 17,908 BTU/h; a leaky one at 1.0 loses 31,214 BTU/h and calls for a 40,000 BTU/h furnace.

Common mistakes

Using the nameplate capacity of the old furnace as the load repeats whatever oversizing it had. Using the output and input mixed up is another: a 100,000 BTU/h input furnace at 80% AFUE delivers 80,000 BTU/h, and the load is compared with the output. Third, a whole-house figure says nothing about the ducts or about individual rooms, which is why a contractor's room-by-room calculation and a duct design matter. Finally, insulation levels and air leakage are the most uncertain inputs, so try a few values to see how much the answer moves.

Sources: heat loss by conduction (area × temperature difference ÷ R) and by air (0.018 × volume × air changes × temperature difference) as used in the ACCA Manual J residential load method and in the ASHRAE Handbook of Fundamentals; AFUE — U.S. Department of Energy, Furnaces and Boilers. Related: heat loss calculator, BTU calculator, heating cost calculator and insulation calculator.