Furnace Size Calculator

How to use this tool

Enter the heated floor area and the number of stories, pick your climate zone or type the design temperature for your town, and choose the level of insulation and windows that best describes the house. Set the efficiency of the furnace you are looking at. The heat loss on the design day, the minimum furnace input and the common size that covers it appear at the top.

The first chart shows where the heat goes, the second how the heat loss grows as it gets colder against the output of the chosen furnace, and the table checks every common furnace size against the Manual S range, so you can see how far a bigger unit would oversize the house.

How the size is worked out

A house loses heat through its walls, windows, ceiling and floor, and through air leaking in and out. Each surface loses heat at a rate set by its U value, the inverse of its R value, and the page builds a simple model of the house from the floor area: a square plan on each story, walls as high as the ceilings, windows making up 15 percent of the walls. Air leakage is 0.018 BTU per cubic foot per degree for every air change an hour.

Added up, those give how many BTU an hour the house loses for every degree Fahrenheit colder outside. Multiplied by the difference between indoors and the design temperature, that is the heat loss on the design day, the output the furnace must deliver. Furnaces are sold by input rating, so the input needed is the output divided by the AFUE, and the page picks the smallest common size that covers it and checks that it stays within the 100 to 140 percent of load that ACCA Manual S allows.

Furnace size by floor area and climate zone

Heat loss in thousands of BTU per hour, then the furnace input size at 80 percent AFUE. One story, 8 ft ceilings, average insulation, typical design temperatures for each zone.

The insulation levels

U values in BTU per hour per square foot per °F, the inverse of the effective R value, including framing. Lower is better.

The levels are broad: poor is an older house with little wall insulation, a thin attic layer, single glazing and noticeable drafts; average has insulated walls, about R20 in the attic and older double glazing; good has full cavity insulation, about R38 overhead, low e windows and sealed air leaks; very good is a new house built to current energy code.

Climate zones and design temperatures

Rounded typical 99 percent design temperatures. Your own town’s figure, from ACCA Manual J tables or the local building department, is better.

A worked example you can check on paper

A one story 2,000 sq ft house in zone 5 with average insulation has a square plan 44.7 ft on a side, 179 ft of wall around it and 1,431 sq ft of wall at 8 ft, of which 215 sq ft is window. Walls lose 1,216 times 0.09, 109 BTU an hour per degree; windows 215 times 0.55, 118; the ceiling 2,000 times 0.05, 100; the floor 2,000 times 0.06, 120; and air leakage at 0.6 air changes through 16,000 cu ft, 0.018 times 0.6 times 16,000, 173. The total is 620.

At a 2 °F design temperature and 70 °F indoors, the difference is 68 °F, so the heat loss is 620 times 68, about 42,200 BTU an hour. An 80 percent furnace needs at least 52,700 BTU of input; the 60,000 BTU size delivers 48,000, which is 114 percent of the load and inside the Manual S range. An 80,000 BTU unit would be 152 percent, oversized.

Why rules of thumb oversize furnaces

Contractors have long used rules such as 30 to 60 BTU per square foot, higher in colder places. They were built for leaky older houses and include generous safety margins, so on a reasonably insulated house they often give a furnace twice the size it needs. Since furnaces are often replaced like for like, an oversized unit can be passed down for decades. Working from heat loss, as a Manual J does in far more detail, is the way out.

Mistakes that catch people out

Sizing by square feet alone. Insulation and air leakage change the answer by a factor of four.

Replacing like for like. The old furnace may have been oversized from the start, and insulation or new windows since then shrink the load further.

Confusing input and output. Labels show input; the heat delivered is input times AFUE.

Using the record low. Systems are sized for the 99 percent design temperature, not the coldest night ever recorded.

Forgetting the ducts. Ducts running through an unheated attic or crawl space lose heat and can add noticeably to the load; a Manual J accounts for them.

Popular calculations

Each opens the calculator with its own address, and every value can be changed once it is open.

Zone 5 houses

Other climate zones

Insulation, stories and efficiency

Metric

How this was checked

The engine was tested in Node against the hand calculation above: a one story 2,000 sq ft house with average insulation gives a heat loss coefficient of 620.4 BTU an hour per °F and 42,183 BTU an hour at a 68 °F difference, 60,000 BTU of input at 80 percent AFUE and 114 percent of load. The four insulation levels give 77,900, 42,200, 28,300 and 19,800 BTU an hour for the same house, and metric inputs were checked by converting square meters, meters and °C by hand. The results sit within the range expected for real houses of these types, but the model is simple, and a Manual J remains the proper basis for buying equipment.

Every address in the lists above was then opened and checked for a distinct title, the right answer, charts that render, and clean CSV, image and print output containing only that calculation, at desktop, tablet and phone widths.

Common questions

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

It depends on the climate and the insulation more than the floor area. With average insulation in zone 5, a one story 2,000 sq ft house loses about 42,000 BTU an hour on the design day, so an 80 percent furnace of 60,000 BTU input fits. The same house poorly insulated needs about 100,000 BTU; well insulated, 40,000.

How many BTU per square foot do I need for heating?

Worked out properly, most houses need 10 to 40 BTU an hour per square foot on the coldest design day, depending on climate, insulation and air leakage. Rules of thumb of 30 to 60 BTU per square foot are common, and they usually oversize the furnace.

What is AFUE?

Annual fuel utilization efficiency, the share of the fuel’s heat that ends up in the house over a season. A furnace rated 80,000 BTU input at 80 percent AFUE delivers about 64,000 BTU an hour; a 95 percent condensing furnace of the same input delivers about 76,000.

Is it bad to oversize a furnace?

Yes. An oversized furnace heats the house in short bursts, which wastes fuel, wears the equipment, makes rooms swing between warm and cool and can be noisy. ACCA Manual S limits furnace output to 100 to 140 percent of the design heat loss.

What is a design temperature?

The outdoor temperature a heating system is sized for: in the usual 99 percent figure, only 1 percent of winter hours are colder. It is around 2 °F for Chicago and about minus 11 °F for Minneapolis, and much milder in the South.

Do I need a Manual J load calculation?

For new equipment it is the right way to size it, and many building codes based on the International Residential Code require heating and cooling equipment to be sized from a Manual J load and Manual S selection. This page gives a planning estimate to check a quote against.

Related Calculators and Reference

• BTU Calculator: the cooling side, room air conditioner size by floor area.
• Insulation Calculator: how much to add to the attic to cut the heat loss.
• R Value Converter: R values and RSI for insulation and windows.
• Energy Converter: BTU, kWh, therms and joules.
• Power Unit Converter: BTU an hour and kilowatts.
• Temperature Converter: design temperatures in °F and °C.
• Water Heater Size Calculator: tank or tankless for your peak demand.
• Air Changes per Hour Calculator: the airflow side of a room, because extra air changes bring outdoor air that must be heated.
• ACCA Manual J: the national standard for residential heating and cooling load calculations.