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Roof AC ductwork


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I am in the process of re-sealing, metal duct taping, the AC intake and output sides to make them more efficient.  On the side where the air comes from the fan and into the Ducts, I see 2 square open spaces on mine which are not taped.  They appear to be open spaces between the foam roof panel and the opening is approximately 2x2”.  
Is there any reason to leave these 2 openings untapped, since I think that air is lost into the attic space? Both units have those 2 openings untapped.

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Can you attach a picture?

On mine the duct work is basically centered in the 14X14" opening that the AC rests on.  On either side of the duct it is open as this the the air return. 

FWIW, I replaced both my AC's last year and decided to take the time to do a duct inspection with a camera which I purchased.  Since the AC's were out and I had good access to the inside of the duct it was pretty easy, just used a flexible pole, taped the camera to one and and slide it through the duct watching the camera view on my laptop.  I was expecting minor problems but found I had several issues. 

I found several holes, one fairly large, from mice.  When I was building our house, our lot was heavily wooded, I lived in the RV and had a bout of mice that I eventually got rid of but it was not easy, so I half way expected to find some problems.  At both ends of the duct work I had a couple small holes but I could not reach to fix but what I decided to do was cut a piece of styrofoam at a 45 degree angle and place them right at the edge of the last vent, I was then able to seal/tape them in place.  This actually would help airflow out of these vents since it would cut down on turbulence. 

I found one large mouse hole but luckily it was right above one of the lights and I was able to fix by removing the light and taping over the outside of the hole.

The biggest problem I had is where two duct mated together.  It looked like there was never any tape applied to the joint, there was ~1/2 gap all the way around (see picture),  This was too far from an existing vent to fix.  So I decided to just install a vent at that location which was in the middle of the coach between the kitchen and bath.  So I cut the hole and had good access to tape the gap from the inside and then installed the vent.  No one would suspect the vent didn't belong there and it actually provides additional air flow.

 

Duct Gap.jpg

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18 minutes ago, jacwjames said:

Can you attach a picture?

On mine the duct work is basically centered in the 14X14" opening that the AC rests on.  On either side of the duct it is open as this the the air return. 

FWIW, I replaced both my AC's last year and decided to take the time to do a duct inspection with a camera which I purchased.  Since the AC's were out and I had good access to the inside of the duct it was pretty easy, just used a flexible pole, taped the camera to one and and slide it through the duct watching the camera view on my laptop.  I was expecting minor problems but found I had several issues. 

I found several holes, one fairly large, from mice.  When I was building our house, our lot was heavily wooded, I lived in the RV and had a bout of mice that I eventually got rid of but it was not easy, so I half way expected to find some problems.  At both ends of the duct work I had a couple small holes but I could not reach to fix but what I decided to do was cut a piece of styrofoam at a 45 degree angle and place them right at the edge of the last vent, I was then able to seal/tape them in place.  This actually would help airflow out of these vents since it would cut down on turbulence. 

I found one large mouse hole but luckily it was right above one of the lights and I was able to fix by removing the light and taping over the outside of the hole.

The biggest problem I had is where two duct mated together.  It looked like there was never any tape applied to the joint, there was ~1/2 gap all the way around (see picture),  This was too far from an existing vent to fix.  So I decided to just install a vent at that location which was in the middle of the coach between the kitchen and bath.  So I cut the hole and had good access to tape the gap from the inside and then installed the vent.  No one would suspect the vent didn't belong there and it actually provides additional air flow.

 

Duct Gap.jpg

I went ahead and closed off all areas, except the 2 ducts.  I can’t get a photo now because I created a totally new, sealed distribution chamber using direction I saw on YouTube, link below.  I now have more air flow and it’s quieter.

These openings were on the opposing 2 sides from the 2 duct sides. They are square openings into the ceiling space between two pieces of insulation foam.  Mine are now taped over.

https://youtu.be/TusVLTNm7_s


 

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My duct work ran front to back center of the coach with all the vents installed right down the middle.  The video is different then mine and may not be possible to do.

The changed I made made a pretty good difference including going form a 13.5K BTU unit to a 15K BTU high capacity unit.  When I was in TX last year I could keep the coach cool with one AC running when outside temps were in the high 80's!

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36 minutes ago, jacwjames said:

My duct work ran front to back center of the coach with all the vents installed right down the middle.  The video is different then mine and may not be possible to do.

The changed I made made a pretty good difference including going form a 13.5K BTU unit to a 15K BTU high capacity unit.  When I was in TX last year I could keep the coach cool with one AC running when outside temps were in the high 80's!

I have 2 units and each has 2 ducts running each side.  Since I did the modification, I feel cold ac air coming through the rear ducts, which I’ve never felt before. Also, the rest sends a little air to the front. Thanks 

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Posted (edited)
1 hour ago, vito.a said:

 

 

1 hour ago, vito.a said:

Picture of the outlet on the bottom of a Dometic A/C.  Then they should have a sheet metal adapter that screws to the A/C and connects to the duct system.  

AC bottom outlet duct opening 2.jpg

That is correct, the metal piece is a Divider between the intake and output sides, but in my case there were leaks.  I’m just re-sealing both sides to get the most cool air, and in the process I took out the metal piece and built an airtight plenum on the output side which feeds the vents, as shown in the YouTube video. My flow increased from the vents.

Edited by David White
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19 hours ago, vito.a said:

Picture of the outlet on the bottom of a Dometic A/C.  Then they should have a sheet metal adapter that screws to the A/C and connects to the duct system.  

AC bottom outlet duct opening 2.jpg

Hello Vito, I have started the process of changing out one of my air conditioners on my 95 signature and I have the center run duct like David. I can partially see what looks like a short duct connecting to the underside of the AC unit and going in to the main duct. I can see it has a screw, as you describe fastening this vertical duct to the AC underside. So my question is, does the screw come out and the short vertical duct stay with the main duct when lifting the old AC unit up or does this short duct stay with the old unit and then transferred to the new unit after removal?

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David, I guess I should have asked you this

22 minutes ago, gofastnow said:

Hello Vito, I have started the process of changing out one of my air conditioners on my 95 signature and I have the center run duct like David. I can partially see what looks like a short duct connecting to the underside of the AC unit and going in to the main duct. I can see it has a screw, as you describe fastening this vertical duct to the AC underside. So my question is, does the screw come out and the short vertical duct stay with the main duct when lifting the old AC unit up or does this short duct stay with the old unit and then transferred to the new unit after removal?

David, could you comment also, since your unit has the same center run ducting as mine does?

Sorry, it is actually Jim that has the center run duct, my bad.

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Two years ago I changed both sets of gaskets on the AC units.  Occasionally I would get a leak in the bedroom AC, dripping from the air intake/filter.  Then I had a bad bout with the front so I figured it was time.  That's when I found how the AC's were plumbed/connected to the duct work.  I did the one first and after removing the 4 hold down bolts lifted the AC off the roof.  It came off pretty easy and I could tell it had been removed before but whoever did it just set the AC back down on the duct work partially crushing it and not sealing it. 

When I looked closely I could see that the duct work had a cut out on the bottom side accessible from the vent/filter return. 

So the best way to access the metal slip joint is to cut a an opening in the duct work on 3 sides, pull the flap down and get access to the slip joint.

So this last year I changed both my AC's to the 15K BTU high capacity units and did just that.  The metal slip joint comes in 2 parts so you can adjust the height from the duct to the AC unit.  Best way to do it is get the AC lined up and pulled down with the 4 hold down bolt and then push the lower slip piece of metal up until it contacts the duct.  Then use a good quality duct/foil tape to seal it up.  Then seal the access hole/flap back up. 

Unfortunately I didn't take any pictures.  But if you have any questions feel free to post/PM me. 

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Great Jim! You have answered the mystery. I would not have thought to just cut into the duct and access it from the inside. Thanks for the response.

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The duct adapter is screwed onto the bottom of the A/C and then it has HVAC tape attaching it to the duct work.  I removed the HVAC tape from underneath along with the four attachment bolts.  Then disconnect the wires. There are both 120v and 12v wires along with the RJ11 telephone cord.  

The A/C unit will lift off the roof with the duct adapter attached.  Sometimes the foam gasket is glued on and you have the scrape it loose.  

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Thanks Vito, I know have a better understanding of what I cannot see. I confident I can change it out now without destroying the duct work. Thank you for your response, you guys make keeping up the "old girl" a lot less painful.

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32 minutes ago, Ray Davis said:

There was a thread a couple of yrs ago about A/C ducts and Walter posted some pictures of his.  This pict shows how his A/C connects to his ducts.   

My 03 Windsor has the double duct system so my arrangement is more like the A/C in this video. 

 https://www.youtube.com/watch?v=TusVLTNm7_s

Dynasty_Duct.jpg

 

 

  

As they say a picture is worth thousand words. 

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My duct setup is nothing like Rays picture.  My AC just dumps air into the ceiling area, with no metal, or any, connection.  Mine had a taped in partition inserted between the intake and output openings in the bottom of the AC. That gave very poor control of the air.

if you go back and look at the YouTube video link I posted earlier, you can see what I had to work with. I had to create something to control the cold air and direct it into the 2 ducts. That’s the reason I constructed a plenum as shown in the video.  I was get free mixing of intake and cold air as it was originally installed.

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