Using the AC for heat

Ever since I’ve known him, my husband uses the AC in the car for heat. It’s never made any sense to me, though he says it’s perfectly logical. Does the AC heat up a car any differently than just having hot air coming through the vents? Is he destroying our car’s AC system by using it all year long?

No problem.
He’s NOT using the a/c just directing the air flow through the vents.
The heat and a/c are separate functions from the direction the air flow takes.
By the way, your a/c compressor runs when you select defrost anyway.
No problems.

Yes, it dehumidifies the air. This helps keep the windows from fogging up.
No, it is not harming the car or the AC system in any way.

All the cars that I have had with automatic climate control (including the two I have now) run the AC full time, even in the winter.

It is logical to use the a/c for defrosting or defogging the windows. The air conditioning not only cools the air in the summer, it also dries the air. That’s why you see water dripping under your car in the summertime when the air conditioning is running. It’s removing the moisture from the air. It also makes sense to use the air conditioning along with the heat if your car has a tendency to fog the interior glass. Some cars are worse about that than others, and using the air conditioner to dry the air coming through the vents will prevent that from happening.

In addition, it’s good to use the AC periodically to keep the compressor seals lubricated.

So then, Marriage wise,
even though you don’t care for the warm air flowing right in your face ( I don’t either but my daughter does ) just grin and bear it :slight_smile:

OP could you clarify?
I thought that you meant that he turns the AC compressor on.
Ken believes that you meant that he uses the dash register outlet setting.
Of course, it could be that he is doing both.

Either way, he is doing no harm.

It will do no harm turning the AC on for faster heat in the vehicle. I’ve been doing it for years. It sure beats waiting for the coolant to get hot enough before you get heat out of the vents.


You’re correct, he hits the AC button.

These responses are helpful. Thanks!

The reasons why the car mfgs make the ac kick on when using defrost is twofold One to dehumidify the air and Two…to keep the ac seals lubricated to prevent them from drying out and leaking.

NOW using it while you have the heat on in the winter all the time is foolish and wasteful…whats the idea here? You want to dehumidify the already ARID winter air? NOT logical…and why do you want the extra drag on your engine and the sapping of your fuel economy as well? DOesnt make sense…the only area it makes sense is if you have a fogged up windshield…it will dry the glass and clear it up faster…other than that this is WASTEFUL and ILLOGICAL behavior…

It would appear you don’t have an understanding of how a refrigeration system functions.

When the compressor compresses the refrigerant it passes thru the condenser converting the refrigerant into a high pressure liquid. This high pressure liquid then passes thru some sort orifice or metering device. This causes the high pressure liquid to convert back into a low pressure gas which absorbs heat. This low pressure gas can only reach a certain temperature once it passes thru the orifice tube or metering device. This temperature can be anywhere from 40?-50?. And that’s a fixed temperature

So if you set the vent system for heat at the floor or out the vents, turn the fan on high, set for fresh air, and turn the AC on, the air passing thru the evaporator will be heated to that 40?-50? heating the interior.

And you don’t run the AC all the time. You turn it off once the coolant gets hot enough where heat is supplied thru the heater core.


Never heard of this before. I’m curious, do you still leave the heat setting on high or turn it down to get heat faster through the a/c?? I wan’t to give it a try next cold snap. (Which sounds like could be as soon as Sunday unfortunately.) :frowning:

OK, just so I’m clear, should my husband turn off the AC once the car has heated up? Or does it matter?

I’d leave it on to keep the windows defogged.

I turn the AC off once the vehicle is heated up. No use in running the AC when it’s no longer needed.


Um, slight clarification:
When the compressor compresses the refridgerant, it heats up…as does all matter when compressed. The condensor removes the heat, bringing the compressed refridgerant back to roughly ambient. The compressed, ambient temperature, refridgerant is then put through an orafice and allowed to rapidly expand into the coils, whereupon it absorbs heat from the cabin air. That refridgerant is then recompressed, the heat again removed, and it goes back through the same process.

The moisture is removed because when the heat is removed from the cabin air by the coils it can no longer hold the same amount of humidity and the humidity is deposited on the coils as condensation, which is then drained away.

We may be saying the same thing in different ways. If so, please accept my apologies.

"This causes the high pressure liquid to convert back into a low pressure gas which absorbs heat. "

But where does the heat come from if it’s a sub zero morning? Doesn’t seem like you would be able to get that a/c air temp. up any warmer than what it already was inside the car??

The heat comes from your cabin air. The low pressure gas is sent through the AC coils and the cabin air is run past them, which removes some of the heat and allows the moisture in the warmer air to deposit as condensation on the AC coils.

The AC does remove some heat. Without the AC on, the heating system will put more heat out the heater vents.

This actually is a pet peeve of mine. When the windshield is icy here in NH it was always my practice to blow hot air st the windshield through the defrost vents to melt the underlying layer of the ice and assist removal. My current car turns the AC on in the background when I turn the defroster on, and it doesn’t heat the windshield as effectively. It removes condensation, but doesn’t help the ice removal as much. I’d prefer being able to push only heat through the defrost vents without the AC in the background.

Having said all that, and remembering the original post, it does absolutely no harm to be running both at the same time. Your hubby probably learned his practice based on the “remove the condensation on the inside of the windshield” goal. He’s actually doing the correct thing, my preference aside.

The heat comes from inside the evaporator where the refrigerant is converted into a low pressure gas. When this occurs, that gas reaches a certain temperature when contained in the evaporator. Usually between 40? to 50?. In the summer when it’s hot, this gas will absorb heat out of the air cooling it. When the temperature is cold, this gas still reaches a temperature of between 40? to 50? in the evaporator. So if you pass cold air thru the evaporator it heats the air. So when it’s -10? outside, and there’s 40? air coming out of the vents, it feels pretty darn good.