There seems to be a lot of complaints lately about heaters not putting out great heat and I think I've discovered a good reason. My WJ hasn't put out great heat since the weather got near 0 lately. Just for grins, I flipped my control to recirculate and got blasted with the hottest heat I've had in weeks! I think the outside air is so cold that it doesn't have time to warm up coming thru the core, but when inside air goes back thru, it does. The only problem is, I can't get it to blow this nice hot air on the windshield! I have to keep switching back to defrost now and then. Right now, I only have high blower, but slowing it down may help warm it. JMHO.

 

My heater wasn't working (only been in 40 degree weather, not 0 though) and I found that I needed to replace the thermostat. It's super hot now.

 

Yeah, I've only got a 195* stat in it now, but I'm going to 210* as soon as it gets warm enough to work in my driveway. Then I won't need it, will I?

 

Probably is because of the extreme cold. Poor airflow across the heater core is another possibility, especially if it still has the factory heater and evaporator cores. Due to a leak in my evaporator core I decided to replace it and the heater core while I had the HVAC box out. Both were really plugged up after 15yrs. I now get awesome airflow and have to turn the heat down when its single digits out. Take it for what its worth I'm a residential hvac guy.

 

Also good point about slowing airflow down across the core with the blower control ****. This allows more time for the heat transfer process