# What causes more gas consumption - Idling with AC or Heat?

So I have begun doing some research on the effects of idling. So my question is, does idling with the AC or heater on affect how much gas is given off? If possible, please give the percent of extra gas given off (e.g. AC causes 10% of extra gas to be given off). Thanks for your help!

• @Chenmunka looks like the linked question was an original one which was migrated here after OP was directed here instead of earthscience Commented Jan 11, 2016 at 11:03
• I posted that question at Earth Science too, however, it was migrated here without me knowing. Sorry about the confusion! xD Commented Jan 11, 2016 at 12:15
• Don't they teach people where the heat comes from? I recall explaining to people many many times that, no, you can't have heat (in bitter cold) until the engine warms up. Why don't people get this?
– user15009
Commented Apr 13, 2016 at 2:04

In most cars (except for some of those sold in very cold countries, I believe) the heating works by using waste heat from the engine, so the only additional energy used for heating is that to run the fans. Conversely, air conditioning requires energy to operate. Therefore, using AC will usually use more energy than using heating.

Translating this to answer your question is more complicated. I assume by "how much gas is given off" you mean exhaust gas. Since an engine that is idling is running inefficiently at very low load, I'm not sure whether adding the load of an air conditioner would necessarily increase its fuel consumption, or exhaust output, significantly. Perhaps somebody with a better understanding or car engines could elaborate!

However, if there is a noticeable effect, then that effect will be greater from A/C than from heating.

• My engine has +10% idle speed when the compressor is running; so I'd expect it to have +10% gasoline consumption.
– Alexander
Commented Jan 10, 2016 at 19:52
• That's strange, Alexander, shouldn't the IACV maintain idle speed? I believe A/C in idle will increase the fuel consumption, as even the slightest electrical load will make the generator work harder, which will make the engine work harder, which will mean the idle will drop and will have to be restored back by increasing mixture volume. The question is if this change is noticeable. Commented Jan 11, 2016 at 17:05

Definitely air conditioning requires more energy, since heat is, as correctly stated in the other answer "basically free" because it's the waste heat from your engine.

According to this study by R. Farrington and J. Rugh of the National Renewable Energy Laboratory (NREL),

Current air-conditioning systems can reduce the fuel economy of high fuel-economy vehicles by about 50% and reduce the fuel economy of today ís mid-sized vehicles by more than 20% while increasing NOx by nearly 80% and CO by 70%.

This study was done in 2000, but is probably not that far off more recent figures, and it also shows that the cost is not merely in the energy used but in the increased pollution that results.

Heat is basically free: your car's heating system works by diverting some of the engine coolant through the heater block rather than the radiator, with the result that waste heat gets dumped into the cabin rather than the outside world.

In contrast, air-conditioning works by connecting the AC's compressor to the engine output, drawing off energy that would otherwise go to the wheels (and potentially requiring a higher idle fuel flow to prevent stalling).

• Thanks for the answer! By any chance, do you know how much gas is wasted as a result of this? That would be helpful as I am doing a project on this. Commented Jan 10, 2016 at 20:53
• @penatheboss Depends on the car/engine type. Commented Jan 10, 2016 at 21:37
• What do you think is the average? Commented Jan 10, 2016 at 21:38
• to the point that on some older cars that tend to overheat, it is suggested to put the heat on and open the windows to help with dissipation (it actually works) Commented Jan 10, 2016 at 23:46
• @penatheboss "do you know how much gas is wasted as a result of [running the AC]" Running the AC isn't wasting gas: it's using gas, assuming that you actually want the interior of the car to be cooler. Waste is when you consume something for no benefit Commented Jan 11, 2016 at 4:46