BTU heating calculator for furnaces and boilers

From the house. Add up the heat that leaves through each part of the house at the design temperature difference: walls, attic, windows, the floor over a crawlspace, air leaking out, and ducts outside the living space. Each part is U-factor × area × temperature difference; air leakage is 1.08 × cfm × temperature difference. This is what a Manual J heating calculation does, room by room.

From last winter's fuel use. A house that already has a heating system has measured its own heat loss every day it ran. Take the fuel it burned for heat, multiply by the fuel's heat content and the old system's efficiency, and divide by the heating degree days for the same period. That gives the heat loss per degree. Multiply by the design temperature difference and you have the load. This is often called the fuel-use method, and for replacements it is hard to beat, because it measures the house as it really is, leaks and all.

Use both when you can. If they agree, you can quote with confidence. If the house method comes in much higher, the inputs probably assume a leakier or less insulated house than the bills show.

The calculator opens on a two-story, 2,000 sq ft house in Chicago: 8 ft ceilings, R-13 walls, an R-30 attic, 250 sq ft of double-pane clear windows, average air tightness (7 ACH50) and a slab or heated basement. Chicago's ASHRAE 99% winter design temperature is 3.7°F, so with 70°F inside the difference is 66.3°F.

That is 13.4 Btu/h per sq ft. Windows are 30% of the loss on 12% of the wall area, and air leakage is another quarter. Those are where upgrades pay.

No credit is taken for sun, people or appliances, the same as Manual J. The design night is cold and dark, and the house might be empty.

Winter design temperatures

The design temperature is a cold that the area reaches or drops below only about 1% of the hours in a year (the ASHRAE 99% value). It is not the record low. Sizing to the record would leave the system oversized every other night of the winter; on the rare night colder than design, the house drifts a degree or two and recovers. The calculator includes these sample cities:

For anywhere else, choose "Somewhere else" and enter the 99% value for the nearest weather station from ACCA Manual J or the ASHRAE climate tables.

Say the same Chicago house burned 800 therms of gas for heat last year in an 80% AFUE furnace, in a year with 6,000 heating degree days.

The two methods land within 2% of each other. The 65°F in the last step is the base the degree days are counted from: the outdoor temperature at which a typical house starts needing heat, because people, lights and appliances cover the rest.

Getting the inputs. Add up 12 months of bills. If the same fuel heats water or runs a stove or dryer, take the summer months' average use, multiply by 12, and subtract it. Get the heating degree days for the same 12 months for your town from NOAA's degree day data or the utility; many utilities print them on the bill. The calculator's 6,000 is only a placeholder. Heat contents are EIA's:

AFUE (annual fuel utilization efficiency) is the share of the fuel's heat that ends up in the house over a season. The rest goes up the flue. Furnace and boiler nameplates list input, so:

Output = input × AFUE and input needed = load ÷ AFUE

The federal minimum for most non-weatherized gas furnaces is 80% AFUE today and rises to 95% for furnaces made from December 18, 2028. Oil furnaces must be at least 83%.

When you size from fuel bills, use the old system's AFUE in the fuel step, because that is how much of the burned fuel reached the house. Use the new system's AFUE to turn the load into an input rating.

Charts that size a furnace at a set number of Btu per square foot for each climate zone are quick, and they are the reason so many furnaces are two or three times too big. They cannot see the things that actually drive heat loss:

• Shape. A two-story house has half the roof and floor of a ranch with the same square feet, and a different amount of wall.
• Glass. In the Chicago example, windows lost 30% of the heat. A house with twice the glass loses far more.
• Air leakage. Going from a tight house to a leaky one changes the load more than going from one climate zone to the next.
• Ducts. Ducts in a cold attic or crawlspace add a fifth or more.

The sample house needed anywhere from about 7 to 34 Btu/h per sq ft in the same city depending on those four things. No single number per square foot can cover that range, so the calculator works from the parts instead, and shows each one so you can see which upgrade matters most.

Boilers are sized from the same heat loss. Boiler ratings list input and output (often called heating capacity or net rating, which allows for piping losses). Match the output to the load, and check that the emitters, whether baseboard, radiators or radiant floor, can deliver the load at the water temperature the boiler will run. A condensing boiler only condenses, and reaches its rated efficiency, when the return water is cool enough, so emitters sized for lower water temperatures pay off.

Heat pumps are sized differently. Their heating capacity falls as it gets colder outside, so look at the maker's capacity at your winter design temperature, not the nominal rating. Where the heat pump cannot meet the full load at design, plan backup heat for the difference and size the backup from this load. Cold-climate heat pumps hold much more of their capacity at low temperatures.

For either, the fuel-use method still works on the existing house: it tells you the heat the new system has to deliver, whatever burns or pumps it.

ACCA Manual S allows a furnace's output to be from 100% to 140% of the heating load. The calculator shows that band and picks the smallest common input size (40,000 to 140,000 Btu/h) whose output lands in it.

For the 27,000 Btu/h Chicago house, even a 40,000 Btu/h input furnace at 96% is 141% of the load, just over the band. That is common with tight, well insulated homes: the smallest standard furnaces are bigger than the house needs. A modulating furnace that turns down well below its rating, or a boiler or heat pump matched to the load, fits better.

Oversizing a furnace makes it short cycle: it heats the air fast, shuts off, and starts again. Short cycles waste fuel, wear the igniter and heat exchanger, and leave hot and cold spots. Many existing furnaces are two or three times the load, because they were sized by square feet or by matching the last one.

Airflow matters too. A bigger furnace needs more airflow to stay inside its temperature rise, and existing ducts may not carry it. Check with the duct size calculator, and for cooling, the load calculator.

Once the size is set, send the replacement estimate from Koira with an 80% and a high-efficiency option side by side, so the homeowner sees the price of each and can sign from their phone.