A/C compressor turning on when heat on?
 

I lifted up the hood while the truck was idling today and when I turned the heat on I heard what seemed to be the A/C compressor turn on. The usual click noise that is heard when A/C is turned on. I turned the heat off and then on again and I could hear it turn off and on. I also felt a couple of the A/C steel lines and they were getting colder. I thought this was odd as the heat was turned on and blowing warm air in the cab. So, the truck heat and A/C works fine but my question is this: When you turn the heat on is it normal to have the A/C compressor become active or kick in? I never noticed it before but I never had the hood open when turning on the heat so perhaps I never heard it before. I know there are some A/C pros here. Just want to make sure I am not putting wear and tear on the A/C compressor if it is not supposed to become active when the heat is turned on. Thanks for the help.

Normal operation.

The theory behind it is that the cold evaporator coil de-humidifies the air and speeds the defrost.

Pop

Thanks Pop. I take it that the compressor normally operates in any of the heat selections as well (defrost selection, under dash, dash only etc)?

The a/c compressor runs in every position except heat on floor, vent and off.

Here is a thread where we discussed it and I found the relevant information in the service manual.

https://www.ford-trucks.com/forums/9...ompressor.html

Perfect. I gave it a read and I think I got it. It appears that in all heat settings except "floor" the compressor kicks on WHEN outside air temp is above 43 degrees. My model year is a 2000 but I would imagine it is the same?