All of it is heat
A compressor takes electricity in and puts compressed air out, and the compressed air arrives at the tool at roughly room temperature. Nearly all of the energy that went in came back out as heat, because that is what happens to work done on a gas that is then cooled back down. At 22 kW input that is 75,067 BTU an hour, which is a small furnace running continuously in the compressor room.
How much of it you can catch is the question the page cannot answer for you, and it is a real fraction rather than a formality. Some leaves through the enclosure skin, some through the motor frame, some through the discharge piping before it reaches anything you have ducted, and some rides out with the air and the condensate. The 80 percent default is a placeholder for whatever the heat recovery data for your machine says or your own measurement across the duct shows.
What comes out of the duct
60,054 BTU an hour into 3,000 cfm of cooling air raises it 18.5 degrees. In a 70 degree room that is an 88 degree discharge — warm, not hot, and exactly the temperature range that is useful for space heating and useless for anything else. Air-cooled packages produce a lot of low grade heat, which is the reason heat recovery from them almost always means blowing it into a shop rather than doing anything cleverer with it.
If your numbers give a rise far above that, check the airflow figure against the data sheet before believing it. A large calculated rise is usually a small cfm figure rather than a hot machine.
The money, framed honestly
At 1,200 overlapping hours, the recoverable heat is 72 million BTU a year. Displacing an 80 percent efficient gas appliance means 901 therms of gas at 1.30 dollars, which is 1,171 dollars. The electricity the compressor drew during those same 1,200 hours cost 3,696 dollars, so 31.7 percent of it came back as heat value.
That framing matters. Nobody saves money on the compressor by doing this — the electricity is spent whether the heat goes out of the louvre or into the shop. The saving lands on the heating bill, it only lands during the heating season, and it is worth several times more against resistance heat than against cheap gas.
The overlap hours are the whole decision
A machine that runs 4,000 hours a year in a building that wants heat for 1,500 of them does not get 4,000 hours of benefit, and it does not get 1,500 either. It gets the overlap, and the overlap is lower than either because plenty of the heating hours are nights and weekends when the compressor is off. Guessing this optimistically is how heat recovery projects end up with paybacks that were never real.
The reverse case is worth naming too. The same duct that helps in January is a nuisance in July, and a scheme with no damper and no thermostat on it heats the shop all summer as well.
Distance is the other constraint
Low grade heat is cheap to produce and expensive to move. A compressor room sharing a wall with the space being heated is a short duct and a damper; the same machine at the far end of a building is a long insulated run, fan power to push the air along it, and a temperature that has fallen on the way. The arithmetic on this page describes the heat available at the package. What it costs to get it somewhere useful is a separate estimate and often the one that decides the answer.
Questions people ask
How much heat does an air compressor produce?
Essentially all of the electricity it draws. A 22 kW input package rejects about 75,067 BTU an hour while loaded, which is 22 kilowatts of heat by another name. The compressed air itself carries almost none of it away, because it arrives at the tool at roughly the temperature it started at. The useful question is not how much heat there is but how much of it you can capture, which depends on the package and is a figure from the manufacturer or from a measurement.
Can I heat my shop with the compressor?
A duct off an air-cooled package delivers a lot of low grade heat — at the defaults here, 3,000 cfm at about 18 degrees above room temperature. Whether that is worth doing depends on how many hours the compressor is loaded while the building wants heat, how far the heat has to travel, and what fuel it displaces. It also depends on the machine tolerating the ducting, which is a manufacturer question. Nothing on this page says a particular arrangement is acceptable or safe.
Does heat recovery make my compressed air cheaper?
No. The electricity is spent either way, and the compressor bill does not move. What changes is that a heating appliance somewhere else in the building runs less, so the saving appears on the gas or heating oil bill instead. At the defaults that is 1,171 dollars a year against 3,696 dollars of compressor electricity during the same hours, which is 31.7 percent of it coming back in a different currency.
Why is the recoverable share not 100 percent?
Because the heat leaves the package by more routes than the one you can duct. Some radiates off the enclosure and the motor frame into the room, some leaves through the discharge piping upstream of anything you have captured, and some goes out with the compressed air and the condensate. The figure to use comes from the heat recovery data for your machine or from measuring the temperature rise and airflow across the duct yourself. The value on the form is a placeholder.
What should I use for the overlap hours?
The hours the compressor is loaded and the building wants heat at the same time, which is lower than either figure alone. Nights and weekends usually count as heating hours and not as compressor hours. If the machine has a load hour meter and you know your heating season, you can bracket it; if you cannot, run the page at an optimistic and a pessimistic figure and see whether the decision changes. If it does, the estimate needs to be better before anyone spends money.