2007 Hyundai Santa Fe 6cyl FWD, not really a problem but just something I find strange. When changing through heat modes (floor, regular vents, etc.), it often turns the AC on even though I didn't hit the AC button. Makes no sense as I don't even have the temp on cool - it's usually all the way over on the heat side. Why would I want "hot" AC?? Anyway, I'd rather not have the compressor fail due to all these false starts. I've read that defrost automatically turns AC on, but it seems like it happens more often than that. Also, changing modes seems to make it default to outside air rather than inside. Wouldn't it be more efficient to keep circulating the air inside the car, which is already warmer than the outside air??

Also, any remedy for the "moldy" smell coming from the vents when using heat or defrost in wet weather? I've had the $80 "new cabin filter" service from the dealer and it didn't seem to do the trick. Wondering if I can just pour some kind of gentle cleaning fluid at the base of the windshield and let it seep in, if that doesn't sound too crazy. Thanks so much for your time.

 

Welcome to the forum. It sounds like you don't understand how a vehicle climate control system is designed to work. There are a lot of things that need explaining so I will do my best to clarify how it works. I will do this one by one.

Quote:

Originally Posted by dan_j

2007 Hyundai Santa Fe 6cyl FWD, not really a problem but just something I find strange. When changing through heat modes (floor, regular vents, etc.), it often turns the AC on even though I didn't hit the AC button.

This is normal. The A/C is tuned on anytime the defroster is turned on. This is because A/C dehumidifies the air and clears the windshield quicker. Also, turning the dial to MAX A/C turns on the A/C. Hyundai’s also have some logic that keeps the A/C on When going from MAX A/C to the "face" level vents.

Quote:

Originally Posted by dan_j

Makes no sense as I don't even have the temp on cool - it's usually all the way over on the heat side. Why would I want "hot" AC??

Because you want hot air to clear the windshield and you want the A/C on to dehumidify the air.

Quote:

Originally Posted by dan_j

Anyway, I'd rather not have the compressor fail due to all these false starts. I've read that defrost automatically turns AC on, but it seems like it happens more often than that.

Not a problem to worry about. I'd worry more about the seals in the A/C system drying out and leaking from lack of compressor usage to circulate the refrigerant.

Quote:

Originally Posted by dan_j

Also, changing modes seems to make it default to outside air rather than inside. Wouldn't it be more efficient to keep circulating the air inside the car, which is already warmer than the outside air??

Yes, the system will automatically switch between "fresh air" and "recirculate" depending on the setting. You want recirculate when doing MAX A/C to recirculate the cooler inside air however, you never want to keep it on recirculate because the air gets stale and humidity will build up. The system switches to fresh whenever you put the defroster on because recirculated air is more humid and you want to dehumidify the air to clear the windshield. That's also why the A/C goes on.

Quote:

Originally Posted by dan_j

Also, any remedy for the "moldy" smell coming from the vents when using heat or defrost in wet weather?

Yes, use the A/C and "fresh air" setting to remove the humidity in the car. It's that humid air that is causing the "moldy" smell.

Quote:

Originally Posted by dan_j

I've had the $80 "new cabin filter" service from the dealer and it didn't seem to do the trick. Wondering if I can just pour some kind of gentle cleaning fluid at the base of the windshield and let it seep in, if that doesn't sound too crazy. Thanks so much for your time.

A cabin air filter needs to be replaced every 10K to 20K mileage. It depends on how quickly it gets dirty where you live. $80 seems way high to me. I buy mine online for about $15 and replace them myself. Do not pour anything in the vents or you will damage the system and void your warranty.

I'd suggest you read your owner’s manual and learn more about how your climate control system is designed to work.