A/C compressor when using defrost?

it happens as a means to dehumidify the air, so you can see.

As hissin alludes to - re 'de-mist' your windshield, you need warm, low humidity air. The heater heats it, the a/c dehumidifies it. The heater by itself (without the a/c) isn't nearly as effective at de-misting the glass.

I always wondered why it went on...lol

As a side note, it's good to run it once in a while with the heat anyway, it not only is a 'nice' heat, but it also keeps seals lubricated and from drying out over the winter.

Ditto 89Mustang's great advice - only takes a minute or two every few weeks or so - keeps everything moving around in the system.

does that mean me thinking about removing my ac is a bad idea? sorry to hijack like this but it's just a quick question.

FYI, the only settings that the A/C is NOT on, is Vent, and Floor. Mix and Defrost use the a/c, and as does the a/c settings, obviously.

StreetDreamsGT said:

does that mean me thinking about removing my ac is a bad idea? sorry to hijack like this but it's just a quick question.

Click to expand...

No, you can remove it without hurting a thing. Cars are offered without ac from the factory.

I thought that the only reason for the ac coming on with the heat and defrost was for compressor maintenance. When you turn the temp control to heat it diverts air across the heater core instead of the evaporator, correct? If it came across both, the ac would be cooling your warm air right? My heater air is always dry, it really needs a humidifier instead. Hot dry air feels uncomfortable to most folks. Thats the reason so many have humidifiers in the house for winter use. I learned that in AC tech school BTW.

No - it significantly helps dry the demisting air Jerry. Many Japanese cars in the 80's required manually turning on the A/C with the defroster. It was amazing the difference - you could run the heat on high defrost - and just clear a portion of the windshield; almost the instant you hit the a/c compressor button, the rest of the fog on the windshield would begin to disappear. You don't have NEAR the problem with it in FL that folks further north do.

Removing it is a decision only you can make. Cars were offered for years with no a/c - very few are offered that way in the states any longer. Just because they're offered that way doesn't necessarily mean that their defrosters work well during certain times of the year. I grew up driving a 55 Chevy in high school. No a/c. And I ALWAYS carried a rag in the front seat during spring/fall/winter to help 'de-fog' the windshield because the heater/defroster wouldn't do it quickly enough or thoroughly enough. You have to decide for yourself.

As for the heat exchanger battle, the heater will win by a long shot. Temps inside the car are about 70F during the winter. Both evaporator coil and heater core are about the same size (mine occupy the same 'box'). Freon in the coil is at about 36-40F - about a 30F temp. difference with recirculating air. Coolant entering the htr core is at 180-195F depending on t'stat in the car - about a 110-120F difference. With the a/c on, the heated air isn't as warm, but it's still plenty warm to heat the car, and plenty dry to demist the windshield.

By the way - what significantly humidifies the air inside the car are the people in it. With every breath we dump a load of water vapor into the air. And the space inside the car is relatively small - doesn't take much time to load the car up with humidity - especially if 3 or 4 people are in the car. IN a house, the ratio of air volume to people is SIGNIFICANTLY different.