# Why does my car heater/AC work this way? **URL:** \ **Category:** Factual Questions\ **Created:** [January 7, 2014, 4:12am UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109 "2014-01-07T04:12:14Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Boyo\_Jim](https://avatars.discourse-cdn.com/v4/letter/b/87869e/32.png) [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)\ **Post date:** [January 7, 2014, 4:12am UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/1 "2014-01-07T04:12:14Z")
My car has a dial so you can direct the airflow from the ac/heater through vents around my legs, or through the dash, or up against the window. There’s also a button to control of the air input – one position brings outside air into the unit, and the other position recirculates the air in the passenger compartment. In the latter position, you can heat or chill the air quite a bit faster than in the former. So why is it that the recirculate button is disabled when the airflow is directed against the windshield? So on really cold days like now, if I don’t direct the flow against windshield it starts fogging up. Which means I have to stop recirculating the air, put the flow against the windshield, and the car stays cold for considerably longer. What’s the deal? ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [January 7, 2014, 4:24am UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/2 "2014-01-07T04:24:03Z")
When you’re defogging the windshield you want dry air. If you have a bunch of people in the car (breathing) it’s just going to condensing on the cold windows. The car is trying to bring in fresh air from the outside, pass it over the AC coils (to dehumidify it, that’s why your AC light turns on), pass it over the heater core to heat it up, across the windshield to heat the window and then it exits near the rear of the car taking any humidity with it. Also, every car has it’s quirks, I’m sure in some cars you can set it to blow across the windshield and recirculate. ---
**Author:** ![Grrr](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/grrr/32/146_2.png) [@Grrr](https://boards.straightdope.com/u/Grrr)\ **Post date:** [January 7, 2014, 4:24am UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/3 "2014-01-07T04:24:40Z")
I’m not sure I follow you. I hate having to use the defrost because all of that HOT airflow hits the window and then is deflected right in my face, causing my lips to get chapped. I also haven’t noticed the defrost taking longer to warm up than the recycle mode. The heater gets it’s warmth through the water in your radiator. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [January 7, 2014, 4:26am UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/4 "2014-01-07T04:26:48Z")
**Joey** is correct. The idea is the prevent humid human-breath from condensing on the cold windshield. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [January 7, 2014, 4:45am UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/5 "2014-01-07T04:45:36Z")
Joey got it in one. Most (many) systems force fresh air on defrost and command AC on when in defrost. For the nit pickers: just because AC compressor is commanded on that does not always mean the compressor will turn on. If the temp is too cold there will not be enough pressure in the AC system to trip the AC pressure switch to engage the compressor. So if you are somewhere on the snow-chi-min trail tonight and you start your car cold don’t yell at me if the compressor doesn’t engage, it isn’t supposed to at those temps. ---
**Author:** ![Boyo\_Jim](https://avatars.discourse-cdn.com/v4/letter/b/87869e/32.png) [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)\ **Post date:** [January 7, 2014, 4:48am UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/6 "2014-01-07T04:48:11Z")
> [@Joey\_P](#): > > When you’re defogging the windshield you want dry air. If you have a bunch of people in the car (breathing) it’s just going to condensing on the cold windows. The car is trying to bring in fresh air from the outside, pass it over the AC coils (to dehumidify it, that’s why your AC light turns on), pass it over the heater core to heat it up, across the windshield to heat the window and then it exits near the rear of the car taking any humidity with it. > > Also, every car has it’s quirks, I’m sure in some cars you can set it to blow across the windshield and recirculate. My car’s AC light does not turn on unless I turn it on manually, and while it makes sense to be dehumidifying the air in defrost I’m not sure how that would work unless the heater itself also has a dehumidifier built in. ---
**Author:** ![Boyo\_Jim](https://avatars.discourse-cdn.com/v4/letter/b/87869e/32.png) [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)\ **Post date:** [January 7, 2014, 4:54am UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/7 "2014-01-07T04:54:13Z")
> [@Shakes](#): > > I’m not sure I follow you. I hate having to use the defrost because all of that HOT airflow hits the window and then is deflected right in my face, causing my lips to get chapped. > > I also haven’t noticed the defrost taking longer to warm up than the recycle mode. The heater gets it’s warmth through the water in your radiator. If the air from the heater was already hot, then I wouldn’t care what happened. But when it’s 20 below and I’m starting the car to go to work, the air coming out of the heater is damn cold, and if I want to drive AND see out the window, I have to switch to defrost, which kills the recirculate option, which slows down the heating of the air. ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [January 7, 2014, 4:55am UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/8 "2014-01-07T04:55:22Z")
> [@Boyo\_Jim](#): > > My car’s AC light does not turn on unless I turn it on manually, and while it makes sense to be dehumidifying the air in defrost I’m not sure how that would work unless the heater itself also has a dehumidifier built in. With the blower in defrost mode, both the AC and heater turn on. The air _always_ passes over the AC and Heater coils. If you call for heat, there’s hot water in the heater coils, if you call for cold, there’s refrigerant cycling through the AC coils. When you defrost your windshield you get both. The AC dries the air, the heater heats the cold air up. ---
**Author:** ![Boyo\_Jim](https://avatars.discourse-cdn.com/v4/letter/b/87869e/32.png) [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)\ **Post date:** [January 7, 2014, 4:59am UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/9 "2014-01-07T04:59:43Z")
> [@Joey\_P](#): > > With the blower in defrost mode, both the AC and heater turn on. The air _always_ passes over the AC and Heater coils. If you call for heat, there’s hot water in the heater coils, if you call for cold, there’s refrigerant cycling through the AC coils. When you defrost your windshield you get both. The AC dries the air, the heater heats the cold air up. Ok, this makes sense, but it doesn’t explain why I can’t use the recirculate option, which does make a significant difference in how fast the air heats up. FYI, if it matters, I have a 2002 Mazda Protege 5. ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [January 7, 2014, 5:09am UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/10 "2014-01-07T05:09:15Z")
> [@Boyo\_Jim](#): > > Ok, this makes sense, but it doesn’t explain why I can’t use the recirculate option, which does make a significant difference in how fast the air heats up. > > FYI, if it matters, I have a 2002 Mazda Protege 5. 1)Quirk of your car, I’m sure you can in some cars 2)If the AC can’t remove the moisture inside the cabin faster then it’s being produced (mostly by people breathing, but anything else, dogs, wet laundry, baked goods that are still hot) it’s all going to condense on the windshield and ice over and you won’t be able to see. It’s going to be dangerous and/or you’re going to blame the car maker for having a crappy defroster. With the way it’s set up now, all the moisture is just forced out the back ---
**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:** [January 7, 2014, 5:21am UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/11 "2014-01-07T05:21:25Z")
> [@Boyo\_Jim](#): > > FYI, if it matters, I have a 2002 Mazda Protege 5. I found a copy of your owner’s manual online ([here](http://en.wikibooks.org/wiki/Mazda_Guide/Protege_5)). It includes the following Warning on p. 6-7: > [@](#): > > The Recirculated Air Position: > Using the [recirculated air] position in cold weather will cause the glass to fog up. Your vision will be hampered, which could lead to a serious accident. Do not use the [recirculated air] position in cold weather. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [January 7, 2014, 5:31am UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/12 "2014-01-07T05:31:17Z")
> [@Boyo\_Jim](#): > > My car’s AC light does not turn on unless I turn it on manually, and while it makes sense to be dehumidifying the air in defrost I’m not sure how that would work unless the heater itself also has a dehumidifier built in. The AC system is the dehumidifier. Just because the light does not turn on, that does not mean the compressor is not being commanded on. Some cars will turn on the AC light, some don’t. > [@Boyo\_Jim](#): > > Ok, this makes sense, but it doesn’t explain why I can’t use the recirculate option, which does make a significant difference in how fast the air heats up. > > FYI, if it matters, I have a 2002 Mazda Protege 5. because recirculated air will be more humid than fresh air. With or without AC, fresh air will be less humid. ETA: you can set the system to floor/defrost and run in recirculation if you want. But don’t blame me if the windshield starts to fog. ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [January 7, 2014, 5:35am UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/13 "2014-01-07T05:35:43Z")
> [@Rick](#): > > ETA: you can set the system to floor/defrost and run in recirculation if you want. But don’t blame me if the windshield starts to fog. Not in his car. According to the linked manual, recirc is blocked when defrost or floor/defrost is selected. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [January 7, 2014, 6:02am UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/14 "2014-01-07T06:02:16Z")
> [@Joey\_P](#): > > Not in his car. According to the linked manual, recirc is blocked when defrost or floor/defrost is selected. Never seen that one before, I stand corrected. These Mazda guys are serious about your windows not fogging. ---
**Author:** ![crazyjoe](https://avatars.discourse-cdn.com/v4/letter/c/f14d63/32.png) [@crazyjoe](https://boards.straightdope.com/u/crazyjoe)\ **Post date:** [January 7, 2014, 2:32pm UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/15 "2014-01-07T14:32:43Z")
I don’t get it…I use the AC in recirc mode all the time during the summer…obviously the car is capable of passing recirc air through the AC system for dehumidification, or I wouldn’t be able to use it in recirc mode. Why would it be any different when using it for defrosting? ---
**Author:** ![obbn](https://avatars.discourse-cdn.com/v4/letter/o/d6d6ee/32.png) [@obbn](https://boards.straightdope.com/u/obbn)\ **Post date:** [January 7, 2014, 2:39pm UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/16 "2014-01-07T14:39:58Z")
> [@Boyo\_Jim](#): > > Ok, this makes sense, but it doesn’t explain why I can’t use the recirculate option, which does make a significant difference in how fast the air heats up. > > FYI, if it matters, I have a 2002 Mazda Protege 5. Of topic, but I also had a Protege 5, we traded in for a Mazda 9. I miss that car, man it was a lot of fun to drive. Zoom Zoom indeed! My sister has a Mazda 3 (it replaced the 5 in the lineup) and sadly it isn’t nearly as fun a car. ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [January 7, 2014, 2:47pm UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/17 "2014-01-07T14:47:15Z")
> [@crazyjoe](#): > > I don’t get it…I use the AC in recirc mode all the time during the summer…obviously the car is capable of passing recirc air through the AC system for dehumidification, or I wouldn’t be able to use it in recirc mode. Why would it be any different when using it for defrosting? In summer the glass isn’t freezing cold so your breath (and other humidity in the cabin) isn’t going to condensate and freeze on it. ---
**Author:** ![enipla](https://avatars.discourse-cdn.com/v4/letter/e/54ee81/32.png) [@enipla](https://boards.straightdope.com/u/enipla)\ **Post date:** [January 7, 2014, 3:50pm UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/18 "2014-01-07T15:50:31Z")
I here you Boyo, but what everyone has said here is correct re defrost in the winter and fogging windows. It is also often the case that passengers will bring in snow and moisture on their clothes and boots - Creating an even more humid environment. In my car, the floor mats often start out as a skating rink, and get thawed and frozen every day. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [January 7, 2014, 4:07pm UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/19 "2014-01-07T16:07:34Z")
> [@crazyjoe](#): > > I don’t get it…I use the AC in recirc mode all the time during the summer…obviously the car is capable of passing recirc air through the AC system for dehumidification, or I wouldn’t be able to use it in recirc mode. Why would it be any different when using it for defrosting? You use defrost/defog when the windshield is cold and the air in the car is humid, such that frost/fog is forming on the inside surface of windshield. To get rid of that frost/fog, you need to hit the windshield with DRY air. The air outside of your car is typically less humid than the air inside your car, because the air inside your car contains the same moisture as the outside air PLUS whatever moisture you’ve added by breathing. This is why defrost typically does not allow you to use the recirculation mode: if it did, the air hitting the windshield would not be as dry, and so your windshield would not dry up as quickly as you might like, which is a safety concern. > [@Boyo Jim](#): > > If the air from the heater was already hot, then I wouldn’t care what happened. But when it’s 20 below and I’m starting the car to go to work, the air coming out of the heater is damn cold, and if I want to drive AND see out the window, I have to switch to defrost, which kills the recirculate option, which slows down the heating of the air. It sucks that you have to choose between rapidly warming the car and being able to see where you’re going; this is a classic first-world problem. To enable recirc to be used together with defrost would require a more powerful A/C system to get rid of the larger amounts of moisture effectively, which means your car would be more expensive. Your friends, neighbors and fellow countrymen have already voted with their wallets: nobody wants their cars to be more expensive, so this is what we get. ---
**Author:** ![SpyOne](https://avatars.discourse-cdn.com/v4/letter/s/82dd89/32.png) [@SpyOne](https://boards.straightdope.com/u/SpyOne)\ **Post date:** [January 7, 2014, 4:09pm UTC](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109/20 "2014-01-07T16:09:10Z")
> [@crazyjoe](#): > > I don’t get it…I use the AC in recirc mode all the time during the summer…obviously the car is capable of passing recirc air through the AC system for dehumidification, or I wouldn’t be able to use it in recirc mode. Why would it be any different when using it for defrosting? It isn’t a physical limitation, but rather a design limitation: they designers could have allowed you to use recirc air for defrost/defog, but they decided not to. The reason for that has been stated several times above: fresh air will be drier air, and thus better at defogging than recirculated air. And the owner’s manual even includes a warning that using recirc air (with non-defog settings one assumes) when it is cold out can lead to dangerous window fogging. So they’d like you not to combine recirc with heat, but left that up to you. But they prevented you from combining recirc with defog, because it would be less effective, and they feel it would be dangerous. [Next page](https://boards.straightdope.com/t/why-does-my-car-heater-ac-work-this-way/678109.md?page=2)