How do car heaters work?

Someone told me that car heaters just use waste heat from the engine, and there for it doesn’t use any extra power to have the heater on (unless the fan is on as well). I was under the impression there were some electrical coils involved.

And how does the heat dial work? I mean, does it somehow mix engine heat and ambient air?

Exactly. The engine heat comes from a heat exchanger connected to the cooling system. If you didn’t draw the heat out of the heater, the radiator would have to radiate the heat away. It really is a freebie, other than the power required to turn the fan.

“The inability of science to grasp Quality, as an object of enquiry, makes it impossible for science to provide a scale of values.”
Robert Pirsig

Your heater does indeed work on waste heat from the engine. That’s why it takes a couple miles to get warm air in the winter.

The engine has what is called a water jacket surrounding the block. Water is circulated through it, and thus heated, by the water pump. That water is in turn cooled by running through the radiator.

When you turn on the heater, a valve is opened and that water is diverted through what is called a heater core. It is very similar to the radiator. The electric fan forces air across the heater core and into the passenger compartment. Temperature adjustments are made via the same valve. Opening the valve wider allows more hot water to flow through the heater core, thus increasing the temperature of the air exiting the core.

Some newer, and more expensive, cars will have auxiliary electric heaters. With these, you don’t get that warm-up time lag. You get heat instantly.

“The problem with the world is that everyone is a few drinks behind.” - Humphrey Bogart

Just a few minutes late but a bit more detail.

Incidently, the electric required to power the fan is nearly free also. (As far as anything is free witin the restraints of the Second Law of Therodynamics) The fan consumes electric energy from the battery which is then recharged by the turning of the alternator/generator whenever the engine is running. Assuming of course, the belt is intact.

I feel compelled to note that noneof the above applies to the best-selling car of all time, the VW Type I (or to any air-cooled VW, or to any air-cooled car [that I am aware of], including [until recently] the Porsche 911).
In these cases, there are two tubes called “heat exchangers”, sometimes “heater boxes”, which kind of hang off the sides of the engine. Some of the exhaust system is routed through them (NOT, as commonly believed, any exhaust gases, but exhaust plumbing). When you turn the heater on, a flap opens and heat is shunted into the cabin.
Contrary to popular belief, the heater in a well-maintained VW is more than adequate. It doesn’t have much tolerance for wear, however, as a failure in any component is catastrophic to the system, i.e. rust anywhere along it reduces its effectiveness to near zero. The above should be modified for most Type IIs, where the heater, even in a well-maintained vehicle, is not really up to par (due to the size of the cabin).

Good point whc03grady, if that is your real name. :slight_smile:

One added benefit of pulling the heat out of the cooling system: if you are trapped in stop-and-go traffic and your car starts to overheat, turning on the heater will cool the engine. (Not bad on a 60° day, not good on a 95° day.)

It also can be used as a problem indicator: if you’re driving along in the winter and you suddenly lose heat, your fluid level may have simply gotten too low so that the heat transfer is being starved. (If you have fluid, the next thing to check is the thermostat which is usually less than $10 and is very easy to replace on most cars.)


It’s easy to replace on my car, too, except that you have to remove the radiator and water pump first. Well, you don’t have to but it’s definitely not easy unless you do. I will admit that that’s a design flaw, though, because the thermostat is definitely a replacement item and should be easy to replace.

I wanted to thank UncleBeer for amplifying my short answer. I confess I skipped some detail because I wanted to be first! But he did a better job than I would have even if I weren’t hyper-competitive.

I will quibble about the free power to the fan, though. Charging up the battery to replace the power used by the fan definitely draws real, otherwise available power from the engine. The amount of power involved is very small relative to the power output of the engine, but it’s not free.

“The inability of science to grasp Quality, as an object of enquiry, makes it impossible for science to provide a scale of values.”
Robert Pirsig

  • A question somewhat related to this topic. I’d been wondering about this for a long time, and this topic remided me to ask it.

Some of the Scandinavian car manufacturers supplement the conventional heating system with an electric heating coil. This saves you from freezing to death while waiting for the engine to heat up.

Regarding the VW type 1 heating system. It is more than adequate if you’re moving because the heat is forced into the passenger cabin. At a stop however, there’s virtually no airflow.

Who said that the fan on conventional car heater runs off of the battery? Nothing runs off of the battery once the car is started. It runs off of the altenator (aka generator).

whc03grady, VW heaters get hot air from near the exhaust & if there was an exhaust leak, you could get gas fumes inside. Thus, they don’t make them like that anymore…