Running automobile air conditioner in winter

[Benj107]

Does a car's air conditioner do any dehumidifying in winter?

I entered these words in a search engine: winter car air conditioning

Several websites say something similar to: Anything that helps to take moisture out of the equation – ie. air-con – will clear fogged glass.

I would not expect the evaporator in the car's air conditioner to be below the wintertime dew point temperature of the air in the passenger compartment.

If the outside air temperature is below freezing, I would expect there to be enough ventilation in the passenger compartment to accomplish the needed dehumidification.

Does it do any practical good to run a car's air conditioner in winter (at below-freezing outside air temperatures)?

 

[Compositepro]

The dehumidification setting on the a/c control panel will automatically turn on the compressor and set the air intake to outside air, for simplicity.

If the outside air is below freezing it is not necessary to run the compressor, but it does not hurt. Simply heating the air lowers its relative humidity.

 

[LittleInch]

Yes, but the min air temp coming in needs to be adove 6C. That's the temp the ac doesn't work to prevent freezing of the evaporator.

Below that the ac simply won't turn on regardless of what the burton on the dash says

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Also: If you get a response it's polite to respond to it.

 

[GT-EGR]

It’s probably not going to do much good for getting fog off the window. Your best method is bringing dry outside air in, second best method is blowing warm air across the glass to pick up the moisture and also heat the glass surface, so blowing cold air seems counterproductive.

There is plenty of dry air in the compartment, but your breath is carrying itself direct to the windshield surface so you have to counteract that somehow.

 

[IRstuff]

Your car's defroster uses the A/C to dry the air and the coolant loop to heat it back up.

The point isn't to dry the air in the passenger compartment, it's to dry the air blowing on the windshield, which needs to be dry and heated enough to warm the condensation/frost and be swept away by the dry air.

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[Benj107]

The point isn't to dry the air in the passenger compartment

Agreed. But part of the air passing through the defroster is from the passenger compartment. If the evaporator temperature isn't lower than the dew point temperature in the passenger compartment, then the air conditioner is doing no dehumidification when it is freezing outside.

The inside surface temperature of the windshield needs to be raised above the dew point temperature where exhaled, moist air contacts the windshield. Running the air conditioner doesn't do that, unless it is operating in heat pump mode to boost the temperature of the air in the defroster until the engine coolant temperature is hot enough to do that.

After the engine is warm enough for the defroster to work as intended, running the air conditioner seems to do nothing but increase wear and tear, making it more likely to need repair by mechanics.

I believe claims about the air conditioner taking moisture out of the equation are wrong if it is freezing cold outside.

 

[IRstuff]

The compartment air is saturated with moisture is why it's depositing on the cold windows; that means the dew point is not as low as you think. After all, the windows aren't THAT cold

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[itsmoked]

The air in the compartment is saturated because a couple of dripping wet, slushy, or snow covered people just got in. All that water blasts into the warm compartment.

Yes, the A/C greatly speeds the defrosting of the windshield.

So much so that the morons who build and put out cars always have the A/C turn on whenever defrost is commanded.

I absolutely hate that nanny state crap. I most definitely do not want the A/C on during defrost 90% of the time. Instead it goes on, often without even signifying it's wasting your fuel, and that results in a bunch of condensate piling up on the evaporator. Four hours later you get into a car that would have no fogged up window problems and instantly your windows fog over because of all the water already sitting on the evaporator. Then you have to run the defrost and that runs the A/C so you can end up with this vicious circle going for days on end.

My Camry will literally build ice on the inside of the windshield if you run the defrost without heat.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Benj107]

Yes, the A/C greatly speeds the defrosting of the windshield.

When I start my car that has cooled down to 20°F overnight, how can running the A/C greatly speed defrosting of the windshield? Is the evaporator in the A/C going to get anywhere near cold enough to condense moisture out of 20°F air? And if it did, how to manage the ice that would form on the evaporator?

 

[itsmoked]

Any refrigeration dehydrates. Less moisture available means there's less water available to condense/steam-up your windshield. Once the engine heat shows up the dehumidifying effectiveness climbs markedly.

Yes, the moment you get in your premise is fairly valid but as soon as the car starts to warm up (even because you're in it) it will shift the temp away from 20°F.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[chicopee]

It is unlikely the AC in your car will work. An AAC compressor in any vehicle have a magnetic clutch which is triggered by the controls on your car dashboard. In the winter time the air within the vehicle compartment is not hot enough for the magnetic clutch to engage the compressor. So suppose the magnetic clutch engages the AC compressor, there is a good chance that the temperature in the condenser will be below the dew point (even in the winter time) of ambient air causing a deposit on ice in the condenser fins and damaging the condenser. There is also the possibility that you could also damage the compressor in the event the magnetic clutch engages it. Now think about this, there is a reason for crankcase heaters in residential and commercial AC units and that is to remove traces of refrigerant that contaminate refrigerant oil in compressors. Contaminated refrigerant oil will damage compressors and in the winter time refrigerant could contaminate the refrigerant oil of your vehicle compressor reducing its life expectancy.
So about the effectiveness of cold air removing a foggy or icy windshield, well that is discussed by the participant above. We know cold ambient air has less moisture than what you may have on your windshield, so know we are in the realm of mass transfer, a topic covered in heat transfer which you should study as it is a fascinating topic. Sure in due time, a windshield could clear up unless the vehicle occupant(s) generate more moisture than is being removed.

 

[LittleInch]

A lot depends on whether you're operating the ventilation system in recirculation mode or direct outside air input mode.

When I had cars with "manual" A/C buttons, then there were two "shoulder" months in the UK where it was very useful to use the A/C to clear the window until the car warmed up and warm air would do the job. However in the rare occasions we get weather below 6C, it had no impact on demisting the inside.

If you have ICE or frost on the inside or outside then A/C won't help. You are using the terms de-misting and defrosting interchangeably. I think they are different things. But then my ait temp rarely gets below zero C / 32F so frost isn't a big issue, especially as climate change warms everything up.

Both cars I have now are climate control and use A/C as it wants.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IRstuff]

The defrost and A/C use the same compressor, but they have different controls.


Your defrost always turns on; defrost at 17F still works, BTDT.

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[Compositepro]

A car's environmental controls must be capable of operating almost anywhere in the world in winter and summer by people who have no knowledge. Considering that, they work pretty darn well. As long as you have fuel you will be comfortable and dry whether you are in an arctic blizzard or a Saharan heat wave. I think how amazing this is when I see old movies of very rich people riding in fancy horse-drawn carriages under blankets, while the driver stands behind the carriage, exposed to the freezing rain (See picture in link below).

https://www.treehugger.com/traffic-central-london-moves-same-speed-horse-drawn-carriages-4856908

There are only two ways to remove moisture from the cabin. And keep in mind that the goal is about 50% RH inside, not just below 100% RH. 90% RH is not comfortable, does not allow drying, and encourages mold growth.
1. Bring in outside air with a lower dewpoint than inside. This often requires a/c dehumidification, particularly when it is warm outside.
2. Use recirculated air, which always requires a/c dehumidification.

As I said before, selecting dehumidify on the control panel will automatically select outside air and a/c compressor on. On my car, this will be indicated on the control panel. I can then manually select recirculated air and manually switch the compressor off, if I wish. This will result in fogging in most cases. In freezing weather I might turn off the a/c in outside air mode, since it is not doing anything.

The evaporator in car a/c is pressure controlled, so that the evaporator will never drop below freezing, which would result in it freezing solid with ice. The load the compressor puts on the engine is not constant, but varies with heat load. In cold, dry conditions the compressor does not use much energy so the energy savings of switching it off is not that great.