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  1. #1
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    AC in vehicles - does it help with humidity when on defrost?

    Hey guys. I was told that when you have your defront on in a vehicle equiped with ac that it will help lower the humidity thus clearing the windows even faster then a car without ac. To me this seems to be true, my vehicle with ac clears the windows fast, and also the compressor cycles on and off in defrost and obveiously ac mode.

    My question is how does it suck the moisture out of the air?

    The inside coil does not change temp as far as i know on defrost. In order for it to lower the humidity the coil would have to get cold so the hot air could condense on it correct?

    Yet the compressor runs.... any ideas? Thanks.

  2. #2
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    The compressor does not constantly run like it would when the a/c is on. The heater coil is after the evaporator. This would be a typical reheat type setup.
    When I get wet, I turn on max a/c (which just means it recirculates instead of brining in outside air) and turn the heat up!

    Stop, step back, relax and have another go at it.
    -----Stop, step back, relax and have another go at it.-----

  3. #3
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    Thread Starter
    So your saying the evap does get cold on defrost but just not as cold as when on full ac correct? When on full ac the compressor runs full time and on defrost the compressor runs to a lesser degree correct?

    I woulnt think the air could get as hot as it does after being cooled to that degree but hey im only iny first month of hvac school

  4. #4
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    Quote Originally Posted by cleanplanets View Post
    Hey guys. I was told that when you have your defront on in a vehicle equiped with ac that it will help lower the humidity thus clearing the windows even faster then a car without ac. To me this seems to be true, my vehicle with ac clears the windows fast, and also the compressor cycles on and off in defrost and obveiously ac mode.

    My question is how does it suck the moisture out of the air?

    The inside coil does not change temp as far as i know on defrost. In order for it to lower the humidity the coil would have to get cold so the hot air could condense on it correct?

    Yet the compressor runs.... any ideas? Thanks.
    Yes, On vehicles with factory air, for many many years now, the A/C compressor energizes when the dash switches are set to defrost mode. The reason for this, is that moisture in the air is condensed, while passing thru the evaporator, because it is below the cabin air temp. dewpoint. If you have an add on A/C unit, you must manually push the A/C button.

  5. #5
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    So we meet again, lzenglish! Didn't know how far to go, not knowing he's a student, probably would have went deeper... pretty much what you posted.

    As for cleanplanets, this is a wonderful place for all levels. Just note that there may be a lot of back and forth on certain topics. Look at the core principal, a lot of it is nitpicking. Many of us are more than happy to answer questions. Keep it up!

    Stop, step back, relax and have another go at it.
    Last edited by BennyD; 03-10-2017 at 09:39 PM. Reason: typo
    -----Stop, step back, relax and have another go at it.-----

  6. #6
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    Hello BennyD, You getting any work done, LOL ! i was going to post to your thread on sabatage, but did not want to cause a stink. As long as you are here, i have seen it a handful of times, be it a prelude to a union strike, or disgruntled workers in gerneral. I could tell you some stories, but maybe later. Take Care!

  7. #7
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    As has been said, the evap does get cold and it does squeeze some moisture out of the air, after that the heater warms it back up and it's really dry.
    I find my GM vehicles won't turn the compressor on until about 32-34F ambient. I assume most are like this. My 1987 Pontiac Grand Prix did this as well. I think American car companies have been doing it forever.

    I believe besides helping defog windows, it also helps keep the compressor seal lubricated.

    It certainly does make a difference without a doubt, I had A\C issues in my truck around 2008 and went one winter without it and it sucked at times. That summer I rebuilt it and was glad by the next winter. Not as glad as I was that summer stuck in traffic, but glad none the less.


    Funny story that's partially related but mostly not. A few weeks ago I went somewhere with the wife and she had the heat on recirculate when it was in the teens out. She stopped and ran into a store during which I turned recirculate off. She got back in the car and said "did you wash the windows or something?" I laughed, and explained what I did and why.

    You can run recirculate for a little while during such conditions, but once it actually gets warm you'll fog up. There's no compressor action going on then, but the cold outside air is very dry once it's warmed. The compressor is more for those 30s-50s humid days.

    Also, most if not all cars will not allow recirculate when the defrosters or defrost + floor is used. You can turn it on, but no dampers move.

  8. #8
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    Most vehicles have a low pressure switch that controls the compressor. Some have a thermistor stuck into the evap.

    They keep the evap from frosting over. Gm pressure range about 25# to stop compressor, 44# engages.

    I keep a Gm switch with me, they have a problem with the contacts.

  9. #9
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    So I read this yesterday and I am cant stop thinking about it. Why would I care about the level of moisture being put out of my heating when I am trying to melt frost on the outside of my window?

    Since its cold enough to be frost the moisture in the air being blown across the windshield could freeze?

    Or because the defrost also acts as a defogger when its cold outside but you just finished at the gym so it hot and moist inside causing the windows to fog up?

  10. #10
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    Most cars won't start the comp below about freezing.

    But for the OP, the evap runs the same temp as when you are in A/C mode. The system doesn't run any different between cooling and defrost/defog.

    Good story Chris!

  11. #11
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    Thread Starter
    Quote Originally Posted by BaldLoonie View Post
    Most cars won't start the comp below about freezing.

    But for the OP, the evap runs the same temp as when you are in A/C mode. The system doesn't run any different between cooling and defrost/defog.

    Good story Chris!

    are you sure it doesn't run any different? in defog my comp seems to cycle on and off where as in AC it runs steady, and most likely producing a colder evaporator.

  12. #12
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    Nov 2014
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    removing moisture from the air = removing heat the AC should be sufficient the defrost basically circulates the air a different way thru your vent system unless you have a coil defrost for your rear windows where there is electric heat and is hurting your cooling effect although at a minimum rate.
    ---------------
    Steve
    HVAC Chicago

  13. #13
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    Most cars cycle the compressor in cooling. Some of the GM cars are variable capacity and don't cycle but the vans and about everyone else cycle them. Now, if you are comparing a cool day with defrost on vs a hot day with A/C on, you might not see it cycle until the car has cooled down inside.

  14. Likes ga-hvac-tech, mnbvcxz liked this post
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