Chimney & fireplace
Fireplace Operating Cost Calculator
What an hour of fire actually costs, across the five fuels, on a common basis of delivered heat rather than fuel units.
Result
Comparing fuels by their own units is meaningless, because a cord, a therm and a kilowatt hour carry wildly different amounts of heat. The cost per million BTU delivered is the figure that makes them comparable, and it is the output worth looking at. Efficiency is doing most of the work in that number: an open masonry fireplace delivers very little of the fuel's heat into the room and draws heated household air up the flue while it burns, which is why it can raise a heating bill rather than lower it. Heat contents are the standard published values but stay editable, since cordwood varies by more than two to one between species and moisture levels.
Get a Free Business AuditHow the calculation works
Heat delivered divided by appliance efficiency gives the fuel input the appliance has to burn, which is the step that makes fuels comparable. Each fuel then has a published heat content per unit, so dividing the price per unit by that content gives a cost per BTU, and multiplying by the input gives the cost per hour. Cordwood, therms, propane gallons, pellet tons and kilowatt hours all carry wildly different energy, which is why the cost per million BTU delivered is the only honest comparison between them.
What moves the number
- Appliance efficiency, which on an open fireplace is low enough to dominate the result.
- Fuel heat content, since a cord and a therm are not comparable units.
- Local fuel prices, which change the ranking between fuels by region.
- Hours of actual use, which people routinely overestimate.
Where this one goes wrong
Efficiency does most of the work in this calculation. An open masonry fireplace delivers very little of the fuel's heat into the room while drawing heated household air up the flue, so the fuel input needed for a given amount of warmth is very large and the net seasonal contribution can be negative. A closed stove or insert changes the result by a wide margin on the same fuel. Heat contents here are the standard published figures, and cordwood in particular varies by more than two to one with species and moisture, which is why it stays editable.
Before you quote
- Use the appliance's actual efficiency rather than an optimistic figure.
- Compare fuels on cost per million BTU delivered, not per unit.
- Adjust cordwood heat content for species and seasoning.
Where the rules come from
Questions
About this calculator
Which fuel is cheapest for a fireplace?
Compare them on cost per million BTU delivered rather than on the price of a cord or a therm, and the ranking depends entirely on local fuel prices and on the appliance efficiency. Run the calculation for each fuel at your own prices; the answer differs a lot by region and changes with the market.
Why is an open fireplace so expensive to run?
Because most of the fuel's heat goes up the flue rather than into the room, and the fire pulls already heated household air out with it. Enter a low efficiency and the fuel input needed for a given amount of delivered warmth becomes very large, which is the honest picture. An insert or a stove changes that substantially.
How much wood is a cord?
A cord is a stacked volume of 128 cubic feet, conventionally four feet by four feet by eight feet. What matters here is the heat in it, which depends on species and dryness: dense hardwoods carry considerably more than softwoods, and unseasoned wood wastes energy boiling off its own moisture.
Does this include the chimney sweeping and maintenance?
No, this is fuel only. Solid fuel appliances need regular inspection and sweeping, and that is a real annual cost that gas and electric do not carry to the same degree. Add it separately when comparing fuels on total cost of ownership.
The formula
fuel input = heat delivered / efficiency; cost per hour = fuel input x price per unit / heat content per unit
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How to use this calculator
- Fuel. Enter your own figure; the page starts at Cordwood, priced per cord.
- Price per unit of fuel ($). Per cord, per therm, per gallon, per ton or per kWh depending on the fuel chosen.
- Appliance efficiency (%). An open masonry fireplace is very low and can lose more heat up the flue than it delivers. Inserts and stoves are far higher.
- Heat you want delivered (BTU/h). Enter your own figure; the page starts at 25,000.
- Hours of use per day. Enter your own figure; the page starts at 4.
- Days in the season. Enter your own figure; the page starts at 120.
The result updates as you type, so you can try a figure, see what it does to cost for the season, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Fuel | Cordwood, priced per cord |
| Price per unit of fuel | $250 |
| Appliance efficiency | 15% |
| Heat you want delivered (BTU/h) | 25,000 |
| Hours of use per day | 4 |
| Days in the season | 120 |
What comes out
| Result | Value |
|---|---|
| Cost for the season main result | $1,000 |
| Cost per hour of use | $2.08 |
| Fuel input needed (BTU/h) | 166,667 |
| Cost per million BTU delivered | $83.33 |
| Fuel units per season | 4.0 |
| Cost per day of use | $8.33 |
Delivering 25,000 BTU an hour at 15% efficiency needs 166,667 BTU of fuel input, costing $2.08 an hour. Over the season that is $1,000 and about 4.0 units of fuel. On delivered heat it works out at $83.33 per million BTU.
How sensitive the answer is
Holding everything else steady and changing only price per unit of fuel, this is what happens to cost for the season. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Price per unit of fuel ($) | Cost for the season |
|---|---|
| $125 | $500.00 |
| $188 | $752.00 |
| $250 example above | $1,000 |
| $375 | $1,500 |
| $500 | $2,000 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Fuel
- Choose from Cordwood, priced per cord, Natural gas, priced per therm, Propane, priced per gallon, Wood pellets, priced per ton, Electricity, priced per kWh, Something else, I will enter the heat content.
- Price per unit of fuel ($)
- Per cord, per therm, per gallon, per ton or per kWh depending on the fuel chosen.
- Appliance efficiency (%)
- An open masonry fireplace is very low and can lose more heat up the flue than it delivers. Inserts and stoves are far higher.
- Heat you want delivered (BTU/h)
- A number, between 0 and 500000, starting at 25,000.
- Hours of use per day
- A number, between 0 and 24, starting at 4.
- Days in the season
- A number, between 0 and 366, starting at 120.
What the results tell you
| Result | What it is |
|---|---|
| Cost for the season | The headline result. A money figure. |
| Cost per hour of use | A money figure. |
| Fuel input needed (BTU/h) | A number. |
| Cost per million BTU delivered | A money figure. |
| Fuel units per season | A decimal figure. |
| Cost per day of use | A money figure. |
Keep going
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