Ask Dave: How do I know if my RV’s A/C ducts have collapsed?

Dear Dave,
There is a chance that the ducts for my RV’s two A/C units have collapsed. They are made of some type of duct board. The inside ceiling has a soft vinyl fabric covering. How would I repair them from inside the unit? There is a one-piece fiberglass sheet top outside. —Jesse, 2006 Tiffin 40QSH Phaeton

Jesse, 
First, what makes you think the A/C ducts have collapsed? Is it insufficient airflow to the ceiling vents? If that is the case, I would use an airflow meter to verify what is coming out of the unit and then to the individual vents. They are very inexpensive on Amazon here.

This will help verify that you actually have good airflow coming out of the unit and maybe the first vent. Then if the flow decreases after that it will help identify the problem area.

Also, if there is a duct collapse, I would believe there should be a dip in the roof or at least a soft spot.

Check the ducting system

I am not sure what type of air conditioning ducting system Tiffin uses. However, you should be able to tell by taking off the return air covering and looking at the raceways coming out of it. Here is one from a Thor we just worked on. The ductwork is made of the same hardboard material with foil wrap as the divider.

This was the bedroom air conditioner and you can see the insulated divider panel in the center. The ductwork was on each side and went to the back ceiling vents. Winnebago uses a CNC router to cut the raceways or ducts in the 3” block foam rather than using actual ductwork as it allows them to design sweeping curves rather than hard angles. You should be able to look into the ducts coming off the unit to see what type of ducting is being used.

I would suggest getting an endoscope, which is a small camera that attaches to your phone, and push it through the ductwork to verify what has actually happened and if the A/C ducts collapsed. You can find one here.

I have used mine for more than 10 years looking into A/C ductwork, furnace ducting, going above the belly plastic on trailers looking for leaks in drains, and other hard-to-access places. Since the cable is flexible, I usually use either an electrician “snake” or, for this case, I use the 4’ fiberglass wire running kit from Harbor Freight or Amazon, since the ductwork should be straight. This will identify the type of ductwork and let you know if it has collapsed, and where.

Insufficient airflow to the vents

My initial thought is the ductwork has not collapsed, but that you have insufficient airflow to the vents. If you do the troubleshooting mentioned earlier, you can verify that.

I would first look at the diverter in the main unit as there have been issues with this sealing tightly and even breaking loose. If this is the case, the air comes out of the unit and just circulates within the cavity. Even if it has not dislodged, there is a procedure to add more efficient airflow with additional diverters and HVAC tape to seal the edges.

Fixing A/C collapsed ductwork

If you do find collapsed ductwork, it will be very difficult to replace as the one-piece fiberglass skin on the outside is one continuous piece with lauan paneling and block foam insulation sandwiched. You would have some major fiberglass repair to do if you approached it from the outside.

This would be similar to the inside, as you have the one-piece fabric material glued to 4’x8’ sheets of lauan paneling with the ductwork laid in the block foam. Pulling the fabric off and cutting the lauan would be a major undertaking, as well.

If you can see the collapse with the endoscope, you might try fashioning a series of braces that you could push in from the main unit and try to wedge the collapse up. Something made of wood like this:

Or even a box just slightly smaller than the original ductwork:

In either case, if they are not too long, you should be able to force them in and push up the top of the ductwork. You might need a few pushed back. I have done this with residential ductwork that started to sag a little and created an oilcan thumping effect.

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Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

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