Hey everyone

I'm putting the air ducts (defroster, A/C, ...) back together.
I put some new felt on for the ball vents, and new foam strips on the duct ends.

I also saw that the centre defroster duct isn't connected properly with the 'source duct' that's under it. Would it be bad for the system just to 'duct-tape' the gap between the two ducts? I think it would give the system a much better air flow...
Was there a reason for the gap between the two ducts?
Here is a picture of what I mean:

 

I sure don't remember that gap on my 77's. Something feels like it has shifted up or down here to give you this gap. To get air where it was intended I would certainly close that gap, but will listen for others comments here as well. Does the AIM show a gap?

Curious and engaged.
David

 

 

I believe that gap must be intentional. My suspicion is that it allows air onto the very lowest part of the windshield in the windshield wiper well.

 

Just read this thread and had to comment. I am currently removing the HVAC under dash components on a 85 C10 pick up in order to replace the heater core. Strangely enough, those same two components, which are held together with a tab and screw, had a space almost 1/4 wide between them. The bottom part has residue on it suggesting it had foam or tape on it, but the upper defrost duct shows no sign. I noticed it before taking it apart and thought I would just duct tape it as some have suggested. Although I can't figure why, I wonder now if it was intentional. Chip.

 

The A.I.M. shows very little detail about that junction between the defroster duct and the heater duct. But, it does not appear to have a seal between them or have that joint sealed in any way. Maybe it was intended to be somewhat loose to allow some warm air to cycle under the dash. (???) But, with that large a gap, I would be taping the joint (duct tape or foil tape) to have a good flow of defrost air onto the windshield area.

 

Mine was the same way, Just not as big of gap. Maybe 2" I just had my dash all apart on my 75 to redo the entire wire harness and someone used gray duct tape on all the gaps. At the very least they did a poor job. I went to Auto Zone and bought the alum backed foil tape for 7 bucks. It is made for Car duct work. Its very sticky and it conforms to all the odd angles. I used it on all my duct connections and it looks like the factory did it.

 

Duct Tape, Couldn't resist that.

 

I used the same aluminum tape (it actually IS made and acceptable to use for HVAC ducts) to seal up my '79.

After reading about that gap in the defroster connection and finding that it's on all late C3s I came to the conclusion that it must be intentional. Again, I believe it is there to allow hot air behind the dash at the lowest part of the windshield to aid in removing snow and ice from the windshield wiper well. I cannot think of another reason for the gap and have a hard time believing that it is not intentional. I know that C3s were well-known for assembly problems but there is utterly no way that an assembly error resulted in a gap that large.

 

You may be right - . when i first took mine apart i noticed the gap on the bottom. Also the one that was in the car, on the flange that goes to the dash was busted up so i bought a used OEM one and it fit the same way. That might be what the gap is for but i didn't want any hot air blowing in the cabin so i sealed it up.

 

Since the one-piece dash used '77+ is nicely sealed at the top, center and sides the air that "leaks" through that gap would first go to the top rear of the dash (the very bottom of the windshield below the top of the dash), spread out, cool a bit and enter the cabin via the floor where you want heat to come from anyway. This is yet another reason I suspect the gap is intentional as I don't believe the system is capable of dividing warm air flow between the floor and defrost like more modern cars.

 

I had the same gap on my 1980 corvette and I used aluminum tape wrapped several times to seal it up. I don't think the gap is intentional, just poor fitment.

Richard

 

That's exactly what I thought the first time I removed the dash/console/etc. in my '79. I used fabric "duct" tape to seal the gap then.

When I removed it again a couple years later I was reminded why so-called "duct" tape is NOT approved for sealing HVAC ductwork! Despite extremely little use in defrost and nice cleaning before installation it had come loose. I also had more experience, more knowledge and more time and determined that it was truly impossible for those two pieces to join. While the gauge pack and center console in my car had been removed before (and poorly replaced) before I'm 99.9% certain that I was the first to remove the dash itself and that the ducting was completely original.

Despite the above I STILL sealed it with the true aluminum duct tape that works even on flue pipes.

After more time and more reports here of that very same gap (important that there is ZERO sign of any foam seal present in every other duct joint) for multiple production years I had no choice but to conclude that it is intentional.

 

If that gap was intentionally designed into the HVAC system, it would be nice to know WHY? Has anyone who has sealed that gap ever identified any significant fault because it was sealed?

 

 

Fun Fact of the day - it would be nice to know WHY? As 71Tvette asked. Hope someone can answer the question

 

I was putting my 1976 back together this afternoon after replacing the heater core and noticed the same gap... looks almost identical to the picture. I stopped working and came in to do a search to see if I was missing a piece. after reading this thread, I am convinced that it was designed that way... I have no first hand knowledge of why, but am willing to offer up a scientific wild @$$ guess. I believe that it is possible that the designer did not want to dump a lot of forced max hot air on a frozen windshield. A reduced flow of conditioned and/or heated air will do the job a little more gently, and the overflow will improve the comfort of the cabin... I will definitely admit that this is not a necessarily elegant solution... or maybe it is because of simplicity. Again that is just a guess... but it seems plausible enough to convince me to put it back like I found it (i.e. to not tape it up)... Besides, I have owned the car for 41 years and do not remember having any issues with the defrost before... the brakes on the other hand... Roll Tide!

 

I thought something along the same lines yet believe the gap is mainly there to allow warm air to pool at the very base of the windshield inside that huge wiper bay that's almost completely open to the weather after the automatic wiper door cover was eliminated around '73 or so. My '79 has never been outside in winter weather but I hate to imagine the hideous mess when it's filled with snow/ice and melt coming down from the upper windshield. Can you say 30+ minute warmup to ever free the wipers?