Furnace sizing is one of the few HVAC calls where climate matters more than house size. A 2,000 ft² house in Atlanta needs a 60,000 BTU/hr furnace; the same house in Minneapolis needs 100,000+. The rule of thumb scales BTU per square foot by climate zone, then divides by AFUE to convert delivered output into the input BTU printed on the furnace nameplate. Here's the math and the gotchas that change it.

The climate-zone formula

Residential furnace heating load:

output_BTU = ft² × climate_factor input_BTU = output_BTU ÷ AFUE

Example: 2,000 ft² house in Chicago (climate factor 45 BTU/ft²), 95% AFUE condensing furnace. Output = 2,000 × 45 = 90,000 BTU/hr delivered. Input = 90,000 ÷ 0.95 = 94,737 BTU/hr nameplate. Round up to a 100,000 BTU/hr input furnace.

The furnace size calculator runs both numbers and snaps to the nearest standard size.

Climate factors by region

Pull the right factor from where the house actually is, not where the supplier's rule of thumb came from:

  • 30 BTU/ft² (mild): Atlanta, Dallas, Charleston, LA, Phoenix in mild winter zones
  • 35 BTU/ft²: Memphis, Nashville, Norfolk, Sacramento
  • 40 BTU/ft² (moderate): Washington DC, St Louis, Kansas City, San Francisco
  • 45 BTU/ft² (cool): Chicago, Boston, Denver, Cleveland, Pittsburgh
  • 50 BTU/ft² (cold): Minneapolis, Buffalo, Salt Lake City
  • 60 BTU/ft² (very cold): Burlington VT, Fargo, Anchorage south, Duluth

The factors assume average residential construction (2x6 walls R-19, R-30 to R-49 ceilings, double-pane low-E windows). For pre-1980 homes with R-11 walls and single-pane windows, add 10-20%. For Passive House or super-tight builds, subtract 30-40%.

Output BTU vs Input BTU — read the right number

Furnaces are sold by input BTU — the gas consumption rating on the nameplate. Output BTU is what the furnace actually delivers to the house, after combustion losses and flue venting:

  • 80% AFUE (standard non-condensing): 80,000 BTU input → 64,000 BTU output
  • 90% AFUE (entry condensing): 80,000 BTU input → 72,000 BTU output
  • 95% AFUE (high-efficiency condensing): 80,000 BTU input → 76,000 BTU output
  • 98% AFUE (premium condensing): 80,000 BTU input → 78,400 BTU output

When a contractor says “you need an 80,000 BTU furnace,” ask whether they mean input or output. Most quote input. The calculator returns both so you can verify.

Standard residential furnace sizes

Manufacturers ship in 20,000 BTU/hr increments at the input rating:

  • 40,000 BTU input — small house under 1,200 ft² in mild climate
  • 60,000 BTU input — 1,200-1,800 ft² mild, smaller in cold
  • 80,000 BTU input — most common residential size, 1,800-2,500 ft²
  • 100,000 BTU input — 2,500-3,500 ft² in cool/cold climate
  • 120,000 BTU input — large or very-cold-climate houses

Above 120,000 BTU, the right answer is usually two zones with smaller furnaces, not one larger furnace. Single-stage furnaces over 120,000 also need higher gas line pressure that some residential meters can't supply.

The oversizing trap (also applies to heating)

Same problem as AC oversizing — short cycles. An oversized furnace heats the thermostat zone in 5 minutes, shuts off, and cools off. The opposite end of the house never warms up because the air handler doesn't run long enough to push warm air everywhere. You feel cold rooms even though the thermostat says 72°F.

Modulating furnaces (variable input BTU) are the modern fix — they ramp down to 40-50% of nameplate during mild weather, run longer, and balance the house better. Premium price; worth it in cold climates.

What pros do differently

Verify the gas line size. An 80,000 BTU input furnace needs a 1/2 inch gas line on a short run, 3/4 inch on anything over 25 ft. 100,000+ BTU often needs 3/4 in or 1 in. Old houses with original 3/8 in or 1/2 in lines starve larger furnaces.

Match the AC tonnage to the same air handler. A 100,000 BTU furnace with a 4-ton AC needs an air handler rated for both. Mismatched air handlers run efficiency lower on one side and starve the other.

Run a Manual J for new construction or major renovation. The rule of thumb breaks on tight modern envelopes (over-sizes by 30-50%) and on poorly insulated older homes (under-sizes). Manual J is required by most jurisdictions for permitted work over 100,000 BTU input.

Quick FAQ

What size furnace for 2,000 ft²? Depends on climate. Atlanta: 60,000 BTU output (~75,000 input at 80% AFUE). Chicago: 90,000 BTU output (~95,000 input at 95% AFUE). The calculator handles the climate scaling.

Is 95% AFUE worth the upgrade from 80%? Payback in 5-8 years in cold climates (saves 15-20% on gas). Less compelling in mild climates where heating is a small bill item.

Do I need a permit to replace a furnace? Yes in most jurisdictions. Inspectors check input BTU against gas line capacity and venting. DIY furnace swaps without permit can void homeowner's insurance after a fire.

Hydronic system instead of forced air? Run the boiler size calculator — same climate-zone math but with boiler-specific AFUE and a discount for radiant floor systems.