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Old 02-04-2013, 10:55 AM   #1 (permalink)
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Default A/C compressor kicks in when heat is turned on, why?

Recently, I opened the hood and checked the grinding noise from the belt.
The heat was turned on with temperature knob turned all the way to the end of red zone. I inadvertently noticed that the A/C compressor was engaged and disengaged periodically. I was very confused about this since the ambient temperature is only about 20F outside. By the way, the heat was not functioning since two months ago and I figured out it might be due to the blend door actuator. I pulled one from junk yard and now the heat is very strong. I know the blend door is used to mix hot air and cold air to reach certain setting temperature. But does it really need to turn the compressor on to blow in cold air to blend given that the ambient air is freezing in new england area. Or is there any sensor that is acting in the system?

Thank you very much.
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Old 02-04-2013, 11:04 AM   #2 (permalink)
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Another question that could have been resolved by actually reading the owners manual.

Defrosting your windows is a two-fold job. Not only does heat help to dissipate the fog, but you need to dry the air. The evaporator (part of the AC system) gets cold, which causes humidity to condense on it then drain away, effectively removing excess moisture from the air. So the AC automatically turns on even in the winter, to help dry the air so that the windows don't fog up.

Secondly, the reliability of your AC system would be very poor if it sat for 3-6 months at a time without running. It relies on the pump running to circulate lubricant around the system. If the system sits too long without running, then the lubricant drains off the seals and other parts. When the system gets turned on again, the seals will run for a few seconds without any lubrication. This can destroy the seals and cause the system to start leaking. Also, AC systems are not completely airtight. There are tiny pores that the refrigerant can seep through. The oil molecules are too large to pass through these pores, so the lubricant can actually seal the pores. If the system sits too long, the oil will drain away from those parts too, allowing the refrigerant to escape.

Probably more information than you wanted to know, but now you know why your AC does and should run in the winter.
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Old 02-04-2013, 11:06 AM   #3 (permalink)
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That's why owners manuals typically tell you to run the AC for a few minutes every now and then in the winter. Not a problem for me here (I typically need it at least once a week), but I could see it being a bit of a stretch in colder climates.
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Old 02-04-2013, 11:27 AM   #4 (permalink)
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Thanks for your explanation and it is very informative. How about cars from other manufacturers? Do they use the same design for the in-car climate control subsystem?

My only concern is the extra gas consumption when A/C compressor kicks in. In my opinion, the heat in the car should be free because it is simply a byproduct from combustion in engine. If you don't use it, it will be just released into the surrounding. By allowing A/C to kick in to remove moisture in air, more gas will be consumed to drive the compressor.

By the way, one thing I'm still not clear is when the temperature knob was turned to the end of red zone, the blend door should also go all the way to the point that air passing through A/C evaporator will be completely blocked and air passing through heater core will be allowed to blow in. If this is the case, how can A/C work to remove the moisture?


Quote:
Originally Posted by KhanTyranitar View Post
Another question that could have been resolved by actually reading the owners manual.

Defrosting your windows is a two-fold job. Not only does heat help to dissipate the fog, but you need to dry the air. The evaporator (part of the AC system) gets cold, which causes humidity to condense on it then drain away, effectively removing excess moisture from the air. So the AC automatically turns on even in the winter, to help dry the air so that the windows don't fog up.

Secondly, the reliability of your AC system would be very poor if it sat for 3-6 months at a time without running. It relies on the pump running to circulate lubricant around the system. If the system sits too long without running, then the lubricant drains off the seals and other parts. When the system gets turned on again, the seals will run for a few seconds without any lubrication. This can destroy the seals and cause the system to start leaking. Also, AC systems are not completely airtight. There are tiny pores that the refrigerant can seep through. The oil molecules are too large to pass through these pores, so the lubricant can actually seal the pores. If the system sits too long, the oil will drain away from those parts too, allowing the refrigerant to escape.

Probably more information than you wanted to know, but now you know why your AC does and should run in the winter.
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Old 02-04-2013, 11:31 AM   #5 (permalink)
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In my experience, and from other people I've talked to who deal with corporate truck fleets, the gas consumption of the A/C compressor is minimal.

When I did a few experiments in my F-250, driving it slow with no AC got me like 7.2 MPG, where as driving it in a 'fun' manner with AC on got me about 7. That was in a 1996 model.

Older cars, yeah, it was a real drag, but newer cars are engineered to handle it better, IMO.
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Old 02-04-2013, 11:52 AM   #6 (permalink)
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Again from manual - the ONLY position that you don't have the A/C working automatically is on VENT.
As for consumption I think that, to defrost the windshield, I can pay an extra 1 cent per mile...
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Old 02-04-2013, 11:53 AM   #7 (permalink)
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All incoming air passes thru the evaporator core. The blend door just diverts air either thru or around the heater core.
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Old 02-04-2013, 12:54 PM   #8 (permalink)
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Quote:
Originally Posted by SoNic67 View Post
Again from manual - the ONLY position that you don't have the A/C working automatically is on VENT.
As for consumption I think that, to defrost the windshield, I can pay an extra 1 cent per mile...

VENT, FLOOR, AND OFF, are the positions with no A/C engagement.
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Old 02-04-2013, 12:56 PM   #9 (permalink)
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Quote:
Originally Posted by SoNic67 View Post
Again from manual - the ONLY position that you don't have the A/C working automatically is on VENT.
As for consumption I think that, to defrost the windshield, I can pay an extra 1 cent per mile...
If I shut it off, my windows get fogged up to the point where I can't really see within 2 or 3 minutes, I'd pay the extra cent so I don't end up killing myself or someone else.

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Originally Posted by KhanTyranitar
Another question that could have been resolved by actually reading the owners manual.
Another question that could have been resolved without a snotty remark. That's what I'm thinking.
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Old 02-04-2013, 01:36 PM   #10 (permalink)
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I read somewhere on this forum that the A/C doesn't run below a certain outside temp when in defrost. As far as having to have dry air to defog a window....that's a bunch of BS. I've been driving since 1965 when there was no AC hooked to the defrost position on cars then and the window stayed just a clear and cleared just as quick as today with "dry air".

Last edited by Bob R; 02-04-2013 at 01:38 PM.
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