This question is for anyone else that has this option. When using the heater do you notice a huge difference in temperature between 89 and 90 (the maximum setting). 90 really cooks me but when I set it to 89 it's almost too cool.

BTW I am not using the automatic setting for heat. It runs the AC compressor way too much when it is not needed.

I haven't noticed that, but I do know that when heading down the highway when it's fairly cold out my feet are cooking while my upper body is cold. It won't blow the hot air through the vents as hot as it's going to the floor.

I'm not overly thrilled with these new HVAC units. The AC sucks and the heat isn't as good as it was in older trucks. Granted, it's a diesel, but after a couple hours of driving there should be better control over where you want the heat.

I've noted the following on my '04 Denali (I haven't got my D/A yet) which should be the same system, that you can use the Auto mode and disable the compressor by just pressing the snowflake button. I'd guess that when you turn the heat to max, it is recirculating the air, making it much warmer than the outside source.

On that same note, if you select re-circ, the compressor does come on even though the snowflake LED does not illuminate.

Overall I've been impressed with the HVAC system on the Denali XL. Here in Phx., it cools down the inside pronto. Now that being said, I wish it had a bit more intelligence in monitoring the outside temp and humidity and disabling the compressor when it truely isn't needed. From that standpoint, it is no more advanced than the late '60's Caddy/Olds Comfortrons.

The system takes some getting used to if your not used to an automatic system. In the Winter I was used to getting that blast of cold air followed by warm air then finally hot air. The auto system kinda sneaks up on yea with the heat. Overall I find the floor a bit cold and the upper area a bit warm. Normally I can fix that by remembering to it the dual zone button when using defrost.

the heater in my 05 duramax does seem to cook the feet. I did not notice this on my 03 8.1/Allison truck. I could set it at 70 and be comfortable from floor to roof in the summer and winter.

Are you sure it disables the compressor when you hit the snowflake button? I could swear that I can hear the compressor cycling with just the auto mode on and no snowflake.

I've noted the following on my '04 Denali (I haven't got my D/A yet) which should be the same system, that you can use the Auto mode and disable the compressor by just pressing the snowflake button. I'd guess that when you turn the heat to max, it is recirculating the air, making it much warmer than the outside source.

On that same note, if you select re-circ, the compressor does come on even though the snowflake LED does not illuminate.

Overall I've been impressed with the HVAC system on the Denali XL. Here in Phx., it cools down the inside pronto. Now that being said, I wish it had a bit more intelligence in monitoring the outside temp and humidity and disabling the compressor when it truely isn't needed. From that standpoint, it is no more advanced than the late '60's Caddy/Olds Comfortrons.

Well, again, this is on an '04 Denali XL, but the compressor does stop. It is real noticible on the gassers as it is so quite inside you can here the freon in the system. So noticible is this there is actually a TSB:

HVAC Hiss Noise with AC On - kw GMT800 GMT805 GMT806 GMT820 GMT830 idle blower compressor line hose #PI02148 - (Jun 21, 2004)

HVAC Hiss Noise with AC On

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When servicing the vehicle with the VIN you entered, the following diagnosis might be helpful if the vehicle exhibits the described symptoms.

<A href="http://service.gm.com/servlets/BlobShtml?ShtmlFile=1528380&pubid=564&evc=sm#ss1-1528380">Condition/Concern:

Customers may comment of a hiss noise coming from the dash area with the A/C on. The noise can come and go or change/ intensity depending on A/C system loads (outside temp, engine RPMs, humidity, etc.)

Recommendation/Instructions:

The noise is normally only heard while stopped with blower on low and radio off. The noise is coming from the A/C orifice tube and is normal. No repairs should be made.

.

Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance. This diagnostic approach was developed for the vehicle with the VIN you entered and should not be automatically be used for other vehicles with similar symptoms. (http://service.gm.com/servlets/BlobShtml?ShtmlFile=1528380&pubid=564&evc=sm#ss2-1528380)

This is how I noticed that the compressor does run when you select the re-circ button. Eventhough there is no snowflake, I can once again hear the hiss, and feel the cycle of the compressor when idling.

To really verify, check your compressor clutch with the hood up...
Are you sure it disables the compressor when you hit the snowflake button? I could swear that I can hear the compressor cycling with just the auto mode on and no snowflake.

When you set the temp to 90 you get full unregulated heat. The temp control door stays full open regardless of the cabin temp. Set at 89 the system tries to maintain the set temp. It will do the same with the AC set at 60, full unregulated cold.

The compressor will run anytime the controls are on recirc or the defrost is selected.

That's the answer I was looking for. If that's what it's supposed to do then that's fine. I just wanted to be sure there wasn't a problem. Thanks!

When you set the temp to 90 you get full unregulated heat. The temp control door stays full open regardless of the cabin temp. Set at 89 the system tries to maintain the set temp. It will do the same with the AC set at 60, full unregulated cold.

The mystery I haven't sorted out yet is how do you regulate the temperature for the rear seat passengers? My wife and I can get the front seats to a mutually agreeable temp, but the kids are pretty much on their own in the back. So far no complaints, but we're about to head into real cold weather instead of this Northern Cal wimpy cold that we've had to deal with so far.

To really verify, check your compressor clutch with the hood up...
I'll do that when I get a chance.

Here's another thing I just noticed. In automatic mode with heat it will only allow the floor vents to be on. As soon as you change it to use both the floor and upper vents the automatic mode turns off. Sometimes it's nice to have heat out of the upper vents, like when you want to warm your hands.

John

Sometimes it's nice to have heat out of the upper vents, like when you want to warm your hands.

John
That's what I was trying to convey above. When it's real cold out and traveling at highway speeds, you're feet are cooking and it's cool above from the heat lost through the glass. I've been just using manual mode in the winter. I think this auto control is a joke.

As soon as you make fan speed or outlet changes, your out of Auto mode.

I specifically looked for a truck without the auto controls. After riding in dad's which has them and mine that doesn't, I'm glad I did. Seems like there are times it's doing sneaky things with that auto that you don't want it to....

I really liked the auto controls during the summer. It stinks during the winter.

I specifically looked for a truck without the auto controls. After riding in dad's which has them and mine that doesn't, I'm glad I did. Seems like there are times it's doing sneaky things with that auto that you don't want it to....

I have never had any issues with the system on Auto. I just set it between 70 and 75 and let it do its thing. I like that it only turns the blower on slowly as the warm coolant becomes available and then adjust everything automatically to keep it at that temp. You can change the mode to make air come out the vents and the floor and still let the computer control the fan and temperature. I just set it and forget it - Its a lot better than manual controls that had to be fine tuned every 10 or 15 minutes during a road trip. It was actually a serious positive when I purchased my truck over other brands.
Steve