Furnace Size Calculator

How Furnace Sizing Works

Furnace sizing is two steps. First, find the design heating load: how many BTU per hour the house loses on a cold day at the local winter design temperature. Second, pick a furnace whose heat output covers that load without going too far over it.

The industry method for the first step is an ACCA Manual J load calculation, done room by room from the actual walls, windows, insulation and air leakage. The second step is ACCA Manual S, which says a furnace’s output should be between 100% and 140% of the Manual J heating load.

Most online furnace calculators skip both and multiply square footage by a BTU-per-square-foot number for your climate zone. Those numbers disagree from site to site, from 25 to 100 BTU per square foot, and none of them know how cold your winters get or how well your house holds heat. This calculator does a simplified heat loss instead, using the temperature difference between inside and your local design day, and shows where the heat goes.

What Drives the Heating Load

Heat leaves a house two ways: through the walls, windows, ceiling and floor (conduction), and with the cold air that leaks in and has to be warmed (infiltration). Both scale with the temperature difference between inside and outside.

• Design temperature: the outdoor temperature that is colder only 1% of the hours in a typical year (the 99% value). Manual J uses this, not the record low. Minneapolis is -8°F, Atlanta is 26°F.
• Insulation: an uninsulated 2x4 wall loses about three times as much heat per square foot as an R-13 wall, and more than five times as much as a current code wall.
• Windows: even good double-pane windows lose several times more heat per square foot than an insulated wall. Single-pane glass is worse still.
• Air leakage: a drafty older house can lose nearly as much heat to air leakage as it does through all its walls.
• Ceiling height and stories: taller ceilings add wall area and air volume. A two-story house has less roof and slab per square foot of floor than a ranch of the same size.

Heat Loss and Furnace Sizing Formulas

Each surface loses heat in proportion to its area, its U-value and the temperature difference. Air leakage is converted to CFM and multiplied by the sensible heat factor for air. The indoor temperature is 70°F, the Manual J default.

Q

Heat loss in BTU per hour

U

Heat flow through one square foot per °F (lower is better)

A

Area of the wall, window or ceiling in square feet

F, P

Slab edge F-factor and the slab perimeter in feet

ACH

Natural air changes per hour at design conditions

AFUE

Annual fuel utilization efficiency, as a decimal

The estimate assumes a square footprint, windows at 15% of floor area and a slab on grade. U-values come from the insulation level you pick: older code minimums (R-13 walls, R-30 attic) for average and current code maximums for new homes.

Worked Example: 2,400 Sq Ft Two-Story in Minneapolis

A two-story house with 2,400 sq ft of heated floor, 8 ft ceilings, average insulation (R-13 walls, R-30 attic, double-pane windows) on a slab, in Minneapolis (99% design temperature -8°F), with a 96% AFUE furnace.

1. Temperature difference: 70 − (−8) = 78°F.
2. Geometry: 1,200 sq ft footprint, 138.6 ft perimeter, 360 sq ft of windows, 1,857 sq ft of net wall, 19,200 cubic feet of air.
3. Walls: 0.082 × 1,857 × 78 = 11,878 BTU/h. Windows: 0.50 × 360 × 78 = 14,040 BTU/h. Ceiling: 0.035 × 1,200 × 78 = 3,276 BTU/h.
4. Slab edge: 0.73 × 138.6 × 78 = 7,890 BTU/h. Air leakage: 0.45 ACH × 19,200 ÷ 60 = 144 CFM, and 1.08 × 144 × 78 = 12,131 BTU/h.
5. Total: 49,214 BTU/h, about 20.5 BTU/h per sq ft. Manual S output window: 49,214 to 68,900 BTU/h.

Result: a 60,000 BTU/h input furnace at 96% puts out about 57,600 BTU/h, 117% of the load, which fits. An 80,000 input unit (76,800 output, 156%) is oversized.

Worked Example: Picking a Furnace From a Manual J Load

A Manual J shows a design heating load of 56,000 BTU/h. The homeowner wants an 80% AFUE furnace. This is the sample problem from Contracting Business’s introduction to Manual S.

1. Output window: 56,000 to 1.40 × 56,000 = 78,400 BTU/h.
2. Input window at 80%: 56,000 ÷ 0.80 = 70,000 to 78,400 ÷ 0.80 = 98,000 BTU/h.
3. A 60,000 input furnace puts out 48,000 BTU/h, only 86% of the load. Too small.
4. An 80,000 input furnace puts out 64,000 BTU/h, 114% of the load. Fits.
5. A 100,000 input furnace puts out 80,000 BTU/h, 143% of the load. Just over the limit.

Result: the 80,000 BTU/h input furnace is the right pick.

Worked Example: When the Smallest Furnace Is Too Big

A new one-story home, 2,000 sq ft, built to current energy code, with a 0°F design temperature and a 96% furnace.

1. The estimated load is 26,396 BTU/h, about 13.2 BTU/h per sq ft.
2. A 40,000 input furnace at 96% puts out 38,400 BTU/h, 145% of the load.

Result: even the smallest common furnace is a little over the 140% limit. This is common in new, tight homes. Look for a modulating furnace whose low fire is well below its rating, or consider a heat pump.

99% Winter Design Temperatures for Major Cities

The outdoor temperature used for heating load calculations. Pick the station closest to the job, and use your local building department’s value if it publishes one.

Furnace Size Chart: Heating Loads Each Size Fits

The range of design heating loads each common furnace size can serve within the Manual S 100-140% window. Output is estimated as input × AFUE.

Field Tips

Practical advice from heating contractors on the HeatingHelp forum and from Green Building Advisor, ACHR News and HVAC School.

1. 1

   Don’t copy the old nameplate

   Most installed heating equipment in the U.S. puts out two to four times the actual heating load. Matching the old furnace’s size carries that mistake forward, and insulation or window upgrades since then make it worse.

2. 2

   Check the estimate against the gas bill

   On a replacement, the old furnace already measured the load. Divide a winter bill’s gas by its heating degree days: 182 therms at 80% efficiency over 938 base-65 degree days is about 647 BTU/h per degree. Multiply by 65 minus the design temperature (45 for a 20°F design) and the load is about 29,100 BTU/h.

3. 3

   Derate for altitude

   Above 2,000 feet, most gas furnaces must be derated, commonly 4% per 1,000 feet of elevation. A 100,000 BTU/h furnace at 5,000 feet ends up near 80,000 BTU/h input. Some models derate less, so use the manufacturer’s high-altitude table.

4. 4

   Clock the meter and check the rise

   After install, time the gas meter with everything else off. One cubic foot per minute is 60 cubic feet per hour, about 61,800 BTU/h at 1,030 BTU per cubic foot (ask the utility for the actual heat content). Then confirm the temperature rise sits near the middle of the nameplate range.

5. 5

   Size the blower for the AC too

   If the furnace also moves air for central AC, the blower has to deliver about 400 CFM per ton, around 1,200 CFM for 3 tons. A small furnace can carry a big blower, so check the blower table at the duct system’s static pressure instead of upsizing the burner.

Sources: Green Building Advisor: Replacing a furnace or boiler, HeatingHelp: Calculate needed boiler size from actual usage?, ACHR News: Furnaces must be modified for high altitude applications, HVAC School: Furnace air temperature rise