Does The A/c Work As Heat Pump? - BMW Forum - BimmerWerkz.com
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#1 (permalink) Old 01-31-2005, 01:42 PM
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I mean, can it work like the A/Cs we have in our homes and offices and blow hot air during cold days? A/Cs are mostly used for cooling purposes but they can also blow warm air. My question is, does BMW's A/C work this way, too?

I turn my car's A/C on even during winter and move the selector to the red area. (Manual A/C control). Nice warm air blows in but where does it come from. I'm not sure if the A/C is working and pumping hot air in or if it's the conventional engine heat that's being blown in.

Could you please fill me in about this matter?

BMW 2005 E46 here.

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#2 (permalink) Old 01-31-2005, 02:28 PM
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i use mine during the winter,,the manual tells me it works,,it just drys the air and stops the screen from fogging,,,,but it is still air conditioning...whether it is hot or cold

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#3 (permalink) Old 01-31-2005, 03:37 PM
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Quote:
Originally posted by AIO@Jan 31 2005, 08:42 PM
I mean, can it work like the A/Cs we have in our homes and offices and blow hot air during cold days? A/Cs are mostly used for cooling purposes but they can also blow warm air. My question is, does BMW's A/C work this way, too?

I turn my car's A/C on even during winter and move the selector to the red area. (Manual A/C control). Nice warm air blows in but where does it come from. I'm not sure if the A/C is working and pumping hot air in or if it's the conventional engine heat that's being blown in.

Could you please fill me in about this matter?

BMW 2005 E46 here.
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Yes it can. They work on the same HVAC principle/s in that if the Compressor does NOT kick in (either in home/car) no freon will be circulated in the system; the outcome will be a blower blowing room temp air and/or outside air minus cold air. Cold air is the responsibility of the compressor+freon.

On your BMW when the button with the snow symbol is enganged it means compressor ON/ freon circulating= cold air. Thus if you getting this hot air it means either your compressor is NOT working or the system had a leak/low freon/no freon.

And Storme when you use the AC as windshield defoger you are using a working system (Cold AIR) foggy winshield is a consequence of warmer temp in cabin relative to the outside temp (in winter) as such by engaging the AC in cabin; you will be equalizing both temps thus defog.
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#4 (permalink) Old 01-31-2005, 04:07 PM
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Quote:
Originally posted by billB@Jan 31 2005, 03:37 PM
On your BMW when the button with the snow symbol is enganged it means compressor ON/ freon circulating= cold air. Thus if you getting this hot air it means either your compressor is NOT working or the system had a leak/low freon/no freon.
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Sorry, but conventionals HVACs can do more than that and compression working does not necessarily mean cold air blowing in.

Take common split A/Cs in a room. When it's cold outside you I turn it on to heat, the compressor works, the freon circulates and air inside is sucked in, heated and blown back into the room.

The heat pump does the exact reverse of what it is doing when it's cooling the room. Heat is being transferred from the outside into the room. My question is, does BMW's A/C work this way?

The compressor can comress as well as expand the gas in circulation. One transfers the heat to the inside, one to the outside. When it's cooling, heat is being pumped outside, and moisture turn into liquid into liquid (water) and flows out through a hose. Hence less or no humidity inside. When the A/C is warming the inside room, heat from the outside is pumped in. So, basically it's just a heat transfer.

Gas only acts as an agent. When the gas compresses or expands, heat is absorbed or given away (don't remember which causes which). That's how HVACs work, and I was wondering if my car A/C was sort of transfering heat inside and not just blowing the engine heat in.

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#5 (permalink) Old 01-31-2005, 04:09 PM
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One thing I want to emphasize is that manual heat dial was on red, not blue when I experimented with it. That's why I don't get cold air blowing in. I don't think anything is wrong with my A/C. When I turn the dial to blue, I get cold well air-conditioned air. So, it's working fine.

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#6 (permalink) Old 01-31-2005, 05:15 PM
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This is your original question:
I mean, can it work like the A/Cs we have in our homes and offices and blow hot air during cold days? A/Cs are mostly used for cooling purposes but they can also blow warm air. My question is, does BMW's A/C work this way, too?
I thought you were asking about WARM air NOT HOT too different things; so I misunderstood your original question....so much for that and I'll try to clarify below:

And yes you are right in your second post (reply) about the AC home units. While you were explaining the central air conditioning unit (heat furnace attached to air handlers plus an outside compressor pumping freon/suction thus hot/cold and heat transfer; I was talking about a pure AC unit without furnace--(this is the part which caused the confusion). Cars DO NOT work like those at all--in fact there is NO relationship between cars/homes from an HVAC pointview. So observe the following:

A. In cars the AC unit is the compressor (belt driven) plus freon for COLD ONLY, the heater is the HEATING CORE plus your HOT ANTI FREEZE (water pump driven)controlled by heater valves and directed by hoses towards the heating core under you dash.

B. Now Freon has no business in transfering heat like homes in this context. But as a complete unit they are related as far as the air ducts and blower are concerned.

C. The blower will blow whatever air is available to it in the duct at any particular moment; needless to say if you choose neither cold/hot on your dial, you will end up with pure OUTSIDE AIR coming out of your cabin vents.

I trust the above will answer your question

regards
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#7 (permalink) Old 01-31-2005, 05:32 PM
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Quote:
Originally posted by AIO@Jan 31 2005, 11:09 PM
One thing I want to emphasize is that manual heat dial was on red, not blue when I experimented with it. That's why I don't get cold air blowing in. I don't think anything is wrong with my A/C. When I turn the dial to blue, I get cold well air-conditioned air. So, it's working fine.
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Yeb your system is doing exactly what it is supposed to do. That is to say you're flooding your duct with mixed cold:AC stuff + hot:Antifreeze stuff= very honest blower blowing whatever air it has at this particular moment. Blue+snow button kicks in compressor while Red opens valves/antifreeze rushing to heating core. Again remember unlike home units it is mixing two different elements not transfering via freon...See explanation above.

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#8 (permalink) Old 01-31-2005, 05:34 PM
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you are correct about the physics.

the car has a separate radiator inside the vent system to provide warm air thru circulating hot engine coolant. the ac cannot be run in reverse thermodynamically (unlike household heat pumps)

on using ac in cars to defog: ac lowers the temp ONLY. as a consequence though moisture comes out (you can see the car pee it out on the road thru the drainage tube), cos absolute humidity is much less at low temp. then the heat just jacks up to your desired comfort temperature and presto:

heat with no humidity to defog.
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#9 (permalink) Old 01-31-2005, 05:35 PM
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crap, somoene just described it while i was distracted. sorry for repeating this...
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#10 (permalink) Old 01-31-2005, 05:45 PM
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Quote:
Originally posted by peterkulesza@Feb 1 2005, 12:34 AM
you are correct about the physics.

the car has a separate radiator inside the vent system to provide warm air thru circulating hot engine coolant.* the ac cannot be run in reverse thermodynamically (unlike household heat pumps)

on using ac in cars to defog:* ac lowers the temp ONLY. as a consequence though moisture comes out (you can see the car pee it out on the road thru the drainage tube), cos absolute humidity is much less at low temp.* then the heat just jacks up to your desired comfort temperature and presto:

heat with no humidity to defog.
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Makes perfect sense Peter and if I may add this:

A. Heat with no humidity to defo--as you did explain is rightly so.
B. Both cars and home share the agony of moisture and the evaporator deals with it. Homes get 3/4"-1" PVC pipes for drainage attached to the evaporator too.
This is my 2 cents on this issue:
C. Also you can accomplish defog via the AC/cold air. In fact it is way much faster than using heat itself. Again as I did argue above; by engaging the AC while directing the air flow towards windshield; I'll be equalizing the temp. on both side of the glass/winshield. Think blowing from your mouth on cold windows (homes/cars) will fog it in winter but the same CAN NOT be accomplished in the summer--well unless someone has a fever--LOL. Needless to say engaging the AC will have a circulating function; smokers love it when it sucks some of the smoke out of the cabin via the return ducts. Heat with a fan ONLY blows it right back at you!!

Important Note: when engaging the winshield defog; AC compressor does KICK IN/ENGAGED whether your dial/electronic controls are HOT or COLD. Thus a circulating effect is taking place in the cabin; and more important your Freon gets its only chance to circulate through your system during winter; since you don't use it in cold weather!!!
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#11 (permalink) Old 01-31-2005, 06:17 PM
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Hi billB and peterkulesza,

Many thanks for your explanations. Yes, my question has been answered and it's all clear now. The car A/C does not work as 'reverse heat pump', it's only there for cooling and not heating... which is totally diffferent than the A/Cs we have at home/work/etc... It's been a very good and informative thread.

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#12 (permalink) Old 01-31-2005, 06:29 PM
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Quote:
Originally posted by peterkulesza@Feb 1 2005, 12:35 AM
crap, somoene just described it while i was distracted. sorry for repeating this...
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Peter: please allow me to disagree...you did contribute in a very positive manner towards explaining the "defog" section.......

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