RV Ducted AC System Service - Airflow Balance and Duct Leak Repair

How Slide-Out Flex Duct Connections Fail in Fifth Wheels and Travel Trailers

Modern fifth wheels and travel trailers route conditioned air to slide-out rooms through flexible 6-inch diameter insulated duct that must accommodate 24-36 inches of travel each time the slide extends or retracts. The connection between the fixed ceiling duct trunk and the slide-out branch uses a slip collar with a snap ring designed to maintain a seal through repeated cycles. Jayco, Grand Design, and Keystone systems typically use Deflecto-brand 6-inch flex duct (model PDFW6 or equivalent) terminated with a 22-gauge galvanized snap ring collar. After 200-400 slide cycles (roughly 1-3 years of active camping use), the flex duct's foil outer jacket fatigues at the collar connection point, causing the duct to gradually retract from the snap ring. Once separated by more than 2 inches, the branch loses most of its airflow - the remaining conditioned air dumps into the ceiling cavity instead of the slide-out room. The fix - re-seating the duct onto the collar and securing with a stainless worm-drive clamp plus foil tape overwrap - takes 20 minutes and costs under $30 in materials.

Plenum Box Gasket Deterioration and Its Effect on Cooling Output

Every rooftop RV AC unit connects to the interior ceiling distribution system through a rectangular foam gasket that seals the plenum box to the supply air trunk. Coleman Mach 15 units use a 14x16-inch gasket rated for 10 years, but Florida UV exposure and thermal cycling cause the open-cell foam to harden, crack, and lose its compression seal in 4-7 years in southern climates. A cracked plenum gasket leaks conditioned air directly into the ceiling cavity - air that was cooled to 55-60 degrees F is lost before reaching any register. Technicians use a Fieldpiece SDMN6 differential manometer to measure static pressure in the supply duct with all registers blocked. A healthy system reads 0.30-0.38 inches WC for a single 15,000 BTU unit. A system with a failed plenum gasket typically reads 0.18-0.25 inches WC, representing a 25-35% pressure loss.

Duct Balancing Methods for Multi-Zone Class A and Class C Motorhomes

Larger Class A and Class C motorhomes often route a single 15,000 BTU rooftop AC through 4-6 supply outlets via a branched duct trunk system. Balancing this system requires more than just measuring supply temperatures - it requires understanding the static pressure distribution and adjusting manual dampers or adding register boosters. Newmar, Tiffin, and Entegra coaches use adjustable louvered dampers in the trunk branches, accessible through the overhead cabinet space. A supply air temperature differential of more than 8 degrees F between the warmest and coolest zone indicates a balancing problem. Target differential after service is under 5 degrees F. We use a Dwyer 400-5 flow hood placed over each register to measure cubic feet per minute at each outlet before and after adjustment.

Also see: AC & Heating Services

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Case Study: Duct gasket between the Coleman Mach 15 plenum box and the main supply trunk had a 2.5-inch circumferential split, and the flex-duct branch to the bedroom slide-out had separated 4 inches from the trunk collar due to the slide-out cycling stress over 3 years

1. Duct Static Pressure Baseline: Blocked all supply registers with foam plugs and measured system static pressure at the plenum outlet using a Fieldpiece SDMN6 manometer. Reading: 0.23 inches WC. Expected sealed-system value for Coleman Mach 15 at this duct diameter: 0.30-0.38 inches WC. Pressure deficit confirmed significant duct leakage.
2. Smoke Pencil Leak Location: With AC running and registers still blocked, used a smoke pencil around the plenum box perimeter. Visible smoke draw at the 2.5-inch cracked section of the foam gasket between the plenum and supply trunk confirmed primary leak location.
3. Slide-Out Duct Inspection: Extended and retracted the bedroom slide-out twice while watching the transition duct collar in the ceiling cavity. Confirmed complete separation of 6-inch diameter flex duct from the trunk snap ring - the duct had retracted 4 inches away from the collar opening during slide-out cycling.
4. Plenum Gasket Replacement: Removed the Coleman Mach 15 interior ceiling assembly (12 Phillips screws) to access the plenum gasket. Cut new foam gasket from 1/4-inch open-cell self-adhesive foam tape (3M part 4008) to match the 14x16-inch plenum perimeter. Cleaned old adhesive from both surfaces with IPA wipe and pressed new gasket in place with 60 seconds of hand pressure per linear foot.
5. Flex Duct Reconnection and Support: Re-seated the 6-inch flex duct onto the trunk snap ring and secured with a stainless steel worm-drive clamp (4-inch diameter, #52 hose clamp) over the duct collar. Added a 3M Foil tape (part 3340) wrap over the joint for air seal. Installed a nylon suspension strap from the ceiling joist to support the flex duct midspan and prevent gravity pull from loosening the connection during future slide-out cycles.
6. Post-Repair Static Pressure Verification: Repeated blocked-register static pressure test. New reading: 0.35 inches WC - within the expected range. Removed foam plugs from all registers and confirmed airflow at all four supply outlets.
7. Supply Temperature Survey: Measured supply air temperature at all registers with a contact thermometer after 15 minutes of operation. Front lounge: 59 degrees F. Bedroom: 62 degrees F. Bathroom: 64 degrees F. Slide-out bedroom: 61 degrees F. Temperature differential reduced from 18 degrees F to 5 degrees F across zones.