# Car air conditioning on when heating the car? **URL:** \ **Category:** Factual Questions\ **Created:** [June 1, 2016, 2:44pm UTC](https://boards.straightdope.com/t/car-air-conditioning-on-when-heating-the-car/756238 "2016-06-01T14:44:50Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![Loach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/loach/32/350_2.png) [@Loach](https://boards.straightdope.com/u/Loach)\ **Post date:** [June 1, 2016, 2:44pm UTC](https://boards.straightdope.com/t/car-air-conditioning-on-when-heating-the-car/756238/1 "2016-06-01T14:44:50Z")
This is a question that as a 49 year old man I should know. My ex-girlfriend (this is not why we broke up) would always leave the air conditioner on in the car. In the summer she would turn the temperature dial to cold. In the winter she would run the air conditioner on hot. I of course did it the sane way. When it is hot I turn on the air conditioner. When it’s cold I turn on the air vent with the temperature up high. So my question is does it matter? With the temperature turned up does the air conditioner do anything? Does keeping it on at high temperatures make the AC work or is it basically off with just the vent working? Or is it different from car to car? ---
**Author:** ![Kent\_Clark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kent_clark/32/105_2.png) [@Kent\_Clark](https://boards.straightdope.com/u/Kent_Clark)\ **Post date:** [June 1, 2016, 2:50pm UTC](https://boards.straightdope.com/t/car-air-conditioning-on-when-heating-the-car/756238/2 "2016-06-01T14:50:19Z")
An air conditioner running with the heater on is a defroster. If your ex-girlfriend was not having problems with the windows fogging up or frosting over, then it wasn’t necesssary. Some cars regulate the AC temp by cycling the compressor on and off. Others just leave the compressor running all the time and add or subtract heat, so it isn’t unheard of to have the AC and heat working simultaneously. ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [June 1, 2016, 3:00pm UTC](https://boards.straightdope.com/t/car-air-conditioning-on-when-heating-the-car/756238/3 "2016-06-01T15:00:31Z")
Some card will ‘’ run the A/C " for you when defrosting/defogging the front window (windshield) when that vent is selected. If you don’t need to defrost or remove moisture from the air (fogged/iced/frosted windows/windshield), but you have manually selected “A/C”, and you just want warm air, your wearing down your A/C parts and wasting fuel and losing engine power. Some cars will make all these decisions for you, if you have very modern climate control, but even then, they are probably running the A/C when it’s not needed. You still need to make the right selections (like not blowing air onto the windshield, when not needed, as that implies to the climate control that you need to A/C running – and the associate costs/wear/power loss). Be a smart human: **Opt out** of A/C when it’s not needed (climate control), **or don’t select** it if it’s not needed (manual controls). And if you’re blowing air onto the windshield, that might trigger the A/C system… and you might want to turn it off or deselect the windshield vent/fan if not needed. . ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [June 1, 2016, 3:05pm UTC](https://boards.straightdope.com/t/car-air-conditioning-on-when-heating-the-car/756238/4 "2016-06-01T15:05:02Z")
> [@Loach](#): > > My ex-girlfriend (this is not why we broke up) would always leave the air conditioner on in the car. In the summer she would turn the temperature dial to cold. In the winter she would run the air conditioner on hot. I of course did it the **sane** way. I first read that **n** as an **m** , and was worried about you. ---
**Author:** ![Slash1972](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/slash1972/32/6461_2.png) [@Slash1972](https://boards.straightdope.com/u/Slash1972)\ **Post date:** [June 1, 2016, 3:28pm UTC](https://boards.straightdope.com/t/car-air-conditioning-on-when-heating-the-car/756238/5 "2016-06-01T15:28:37Z")
I run the A/C when using the heater, because the heater blows out warm, moist air and I don’t like that. So, I use the A/C with temperature on hot in order to get drier warm air. ---
**Author:** ![Fotheringay-Phipps](https://avatars.discourse-cdn.com/v4/letter/f/d9b06d/32.png) [@Fotheringay-Phipps](https://boards.straightdope.com/u/Fotheringay-Phipps)\ **Post date:** [June 1, 2016, 5:52pm UTC](https://boards.straightdope.com/t/car-air-conditioning-on-when-heating-the-car/756238/6 "2016-06-01T17:52:08Z")
> [@Loach](#): > > My ex-girlfriend (this is not why we broke up) would always leave the air conditioner on in the car. In the summer she would turn the temperature dial to cold. In the winter she would run the air conditioner on hot. I of course did it the sane way. When it is hot I turn on the air conditioner. When it’s cold I turn on the air vent with the temperature up high. > > So my question is does it matter? With the temperature turned up does the air conditioner do anything? Does keeping it on at high temperatures make the AC work or is it basically off with just the vent working? Or is it different from car to car? It’s recommended that you turn the a/c on from time to time in the winter. It apparently helps prevent leaks. See e.g. [http://acprocold.com/blog/car-ac-stopped-working-winter/](http://acprocold.com/blog/car-ac-stopped-working-winter/) ---
**Author:** ![Loach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/loach/32/350_2.png) [@Loach](https://boards.straightdope.com/u/Loach)\ **Post date:** [June 1, 2016, 6:56pm UTC](https://boards.straightdope.com/t/car-air-conditioning-on-when-heating-the-car/756238/7 "2016-06-01T18:56:27Z")
> [@Philster](#): > > Some card will ‘’ run the A/C " for you when defrosting/defogging the front window (windshield) when that vent is selected. > > If you don’t need to defrost or remove moisture from the air (fogged/iced/frosted windows/windshield), but you have manually selected “A/C”, and you just want warm air, your wearing down your A/C parts and wasting fuel and losing engine power. > > Some cars will make all these decisions for you, if you have very modern climate control, but even then, they are probably running the A/C when it’s not needed. You still need to make the right selections (like not blowing air onto the windshield, when not needed, as that implies to the climate control that you need to A/C running – and the associate costs/wear/power loss). > > Be a smart human: **Opt out** of A/C when it’s not needed (climate control), **or don’t select** it if it’s not needed (manual controls). And if you’re blowing air onto the windshield, that might trigger the A/C system… and you might want to turn it off or deselect the windshield vent/fan if not needed. > . Thanks that was what I was thinking but didn’t know for sure. ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [June 1, 2016, 7:15pm UTC](https://boards.straightdope.com/t/car-air-conditioning-on-when-heating-the-car/756238/8 "2016-06-01T19:15:23Z")
When my windows fog in the winter, turning the air conditioner on clears it almost immediately. But then I turn it off. ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [June 2, 2016, 5:46pm UTC](https://boards.straightdope.com/t/car-air-conditioning-on-when-heating-the-car/756238/9 "2016-06-02T17:46:18Z")
> [@Slash1972](#): > > I run the A/C when using the heater, because the heater blows out warm, moist air and I don’t like that. So, I use the A/C with temperature on hot in order to get drier warm air. Air that is excessively dry OR damp can be unhealthy. I’d venture a guess and say that always pumping in dry warm air (via running the A/C unit when calling for warm air) can be worse than just running the heat if the humidity is acceptable. (Wanted to add: Good lawd my last post in this thread contained a lot of typos and grammatical errors! Jeesh!) . ---
**Author:** ![Muffin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/muffin/32/4341_2.png) [@Muffin](https://boards.straightdope.com/u/Muffin)\ **Post date:** [June 2, 2016, 6:45pm UTC](https://boards.straightdope.com/t/car-air-conditioning-on-when-heating-the-car/756238/10 "2016-06-02T18:45:27Z")
The air conditioner button should be renamed the “clear the windows” button. Although it might seem counter-intuitive at first, I use my air conditioner consistently during the winter, and only occasionally in the summer. It reduces the humidity, which reduces icing from human breath in the winter and fogging while driving in the rain on hot summer days. ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [June 2, 2016, 7:12pm UTC](https://boards.straightdope.com/t/car-air-conditioning-on-when-heating-the-car/756238/11 "2016-06-02T19:12:14Z")
> [@Muffin](#): > > The air conditioner button should be renamed the “clear the windows” button. But isn’t already a DEFOG button/setting? > [@](#): > > Although it might seem counter-intuitive at first, I use my air conditioner consistently during the winter, and only occasionally in the summer. It reduces the humidity, which reduces icing from human breath in the winter and fogging while driving in the rain on hot summer days. Do you live where it doesn’t get very hot or humid in the summer? ---
**Author:** ![Muffin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/muffin/32/4341_2.png) [@Muffin](https://boards.straightdope.com/u/Muffin)\ **Post date:** [June 2, 2016, 7:55pm UTC](https://boards.straightdope.com/t/car-air-conditioning-on-when-heating-the-car/756238/12 "2016-06-02T19:55:00Z")
The defog (in my case the mix and defrost – both of which add a little air conditioning to the flow and direct it to feet+dash or just the dash) do not do the job. I live were it usually does not get hot in the summer, and seldom gets humid (the north shore of Lake Superior in On-tar-i-ar-i-ar-i-o). Most of the time in the summer I drive with part or all of the roof off and do not use the air conditioning. On the occasional hot, humid, rainy day, it requires the full air conditioner (not just the mix or defrost) to keep the windshield clear of fog. In the winter it frequently is below -20 F, which results in the windshield icing up when I exhale. The heater and the air conditioner both at full blast take care of this. ---
**Author:** ![Slash1972](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/slash1972/32/6461_2.png) [@Slash1972](https://boards.straightdope.com/u/Slash1972)\ **Post date:** [June 3, 2016, 12:31pm UTC](https://boards.straightdope.com/t/car-air-conditioning-on-when-heating-the-car/756238/13 "2016-06-03T12:31:57Z")
> [@Philster](#): > > Air that is excessively dry OR damp can be unhealthy. > > I’d venture a guess and say that always pumping in dry warm air (via running the A/C unit when calling for warm air) can be worse than just running the heat if the humidity is acceptable. Well, I’m not trying for healthy, I’m going for comfort.