How to use the gas furnace cost calculator
- Enter the furnace rating in BTU per hour, and say whether it is the gas input or the heat output. Enter the efficiency (AFUE) from the nameplate.
- Enter the hours running per day on heating days, the number of heating days, and your gas price per therm.
- In More options, add a monthly fixed charge and the heating months, or a comparison efficiency for a replacement furnace.
- Read the headline for the gas cost of the season, the therms used, and the cost per heating day.
Formula
gas input per hour = heat output / AFUE, or the input rating as given
therms = BTU of gas per hour x hours per day x heating days / 100,000
gas cost = therms x price per therm
A therm is 100,000 BTU by definition. For a comparison, the same heat output is divided by the other efficiency to find the gas the other furnace would burn.
An 80,000 BTU input furnace
80,000 BTU per hour x 8 hours x 150 days is 96,000,000 BTU, or 960 therms. At $1.50 a therm that is $1,440 for the season, or $9.60 per heating day.
A 76,000 BTU output rating at 95% efficiency burns 76,000 / 0.95 = 80,000 BTU of gas an hour, so it costs the same $1,440. A fixed charge of $12 over 5 months adds $60, for $1,500. A furnace delivering the same heat at 80% efficiency would burn 95,000 BTU an hour, costing $1,710, which is $270 more, shown as ($270.00).
Input rating, output rating and efficiency
A furnace nameplate lists the gas it can burn per hour (the input) and the heat it delivers (the output). The ratio is the efficiency, often called AFUE. If you know the input, the gas burned is the input itself. If you only know the output, divide it by the efficiency to find the gas. The page handles both so you can use whichever number you have.
The hours per day are an average run time on heating days, not the hours the heat is set to be on. A furnace cycles on and off. If you have a past gas bill, working backward from the therms used gives a more reliable run time than a guess.
Comparing furnaces
The comparison shows the saving or extra cost of a different efficiency for the same heat. It does not include the cost of the new furnace or installation. For that side of the decision, try the Appliance Replacement Payback Calculator, and for sizing, the BTU Calculator. Electric and propane versions of the same question: the Electricity Cost Calculator and the Propane Tank Runtime Calculator.
Assumptions and limits
- The furnace runs at the average hours you enter on every heating day. Weather and duct losses are not modeled.
- The price per therm is the one on your bill. If your bill uses CCF, check the therm conversion on it.
- Efficiency applies to the whole season. Real efficiency changes with how the furnace cycles.
- Money is rounded to the cent, half up.
Frequently asked questions
How much does it cost to run a gas furnace?
Multiply the gas input in BTU per hour by the hours run, divide by 100,000 to get therms, and multiply by the price per therm. An 80,000 BTU furnace running 8 hours for 150 days uses 960 therms, which costs $1,440 at $1.50.
What is a therm?
A therm is a unit of heat energy equal to 100,000 BTU. Gas bills often list usage in therms, or in a volume such as CCF that the utility converts to therms. Check your bill for the conversion before you enter a price.
Should I enter the input or the output rating?
Use whichever the nameplate gives. Input is the gas burned. Output is the heat delivered, so the page divides it by the efficiency to find the gas. A 76,000 BTU output at 95% burns 80,000 BTU an hour.
How do I compare a new furnace with my old one?
Enter the old furnace first, then type the new efficiency in the comparison box. The page finds the gas the new one needs for the same heat and shows the difference per season, with a higher cost in parentheses.