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Its well known that using heat in a car is almost free (see for example here). It is also known that using the heat can reduce the engine temperature which otherwise needs to rely on water cooling. Since the water cooling has a price it is presumable that this would consume gas, possibly more gas then the cost of using the heat itself.

So my question is whether its actually more gas efficient to use the heat in your car then have it off and if so how big an impact can this have on gas efficiency? How big an impact this is, is also likely a function of the outside temperature, though I understand its likely hard to get that data...

Note: After forming this question I saw the same possibility posed in an answer to the linked question by JuannStrauss.

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    FWIW, there already is a device that utilizes the heat generated by combustion to improve engine efficiency. It's called a turbocharger.
    – Zaid
    Commented Oct 2, 2015 at 14:04
  • Turboes don't improve efficiency by scavenging heat. They improve efficiency by artificially increasing the Volumetric Efficiency.
    – Captain Kenpachi
    Commented Oct 2, 2015 at 14:08
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    @JuannStrauss : Increased volumetric efficiency is the impact of the turbo. The energy source that allows the turbo to do this is the hot exhaust gases that result from combustion.
    – Zaid
    Commented Oct 2, 2015 at 15:57
  • In an emergency situation I have seen a difference like when the engine fan goes out where using the heater with the windows down can help lower the temperature as a redundant radiator. An overheating engine is less efficient and if using the heater core on high makes it less hot then yes. If you must drive an overheating vehicle take the hood off and it will help air cool the engine. Turning off the A/C is a big one. Adding after market products like a cold air intake and duel exhaust my also help save gas.
    – user22295
    Commented Sep 13, 2018 at 20:19

2 Answers 2

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Using the heater has no effect on fuel consumption. Using the heater won't lower the engine temperature because the thermostat will keep the engine temperature constant. It may actually use a tiny bit more gas from the electrical draw because of the blower motor.

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  • The thermostat might keep the engine temperature constant but if the engine loses heat by blowing out hot air it will take less energy to cool? I assume moderating the engine temperature requires energy?
    – JeffDror
    Commented Oct 2, 2015 at 2:40
  • @JeffDror What your talking about is a game of robing Paul to pay Peter. If you run the blower motor you will have to run the radiator fan less, but that is standing still. If your driving then the moving air keeps the engine cool by itself and running the blower motor is excessive. The thermostat takes not extra energy to operate and the water pump is belt driven and runs all the time anyway.
    – vini_i
    Commented Oct 2, 2015 at 6:57
  • It would depend mostly on what temperature the engine runs optimally at. E.g. the GM Z20LET engine is most efficient at a very toasty 97 degrees celcius. If you bring that temp down significantly, e.g. to 85, your consumption suffers slightly. You also increase wear and tear, which indirectly indicates that the engine has to work harder, and hence uses more fuel.
    – Captain Kenpachi
    Commented Oct 2, 2015 at 9:51
  • @vini_i: Thanks, I think I now understand better. I assumed a car had some sort of condenser to reduce the coolant temperature. If it only relies on air cooling then I agree this cost is not a big deal (though I am surprised this is sufficient).
    – JeffDror
    Commented Oct 2, 2015 at 15:03
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Nope, not one bit. As some others have mentioned, if anything you might need more gas to power the motor to blow the hot air into your car.

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  • As you yourself mention, that answer has already been given.
    – Chenmunka
    Commented Sep 15, 2018 at 10:42

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