Fix for 2720sd furnace poor installation
Cold air side: I replaced the 4" round return air register with a 8 by 10 inch brass colored return air register from Home Depot. It fit nicely and clears the table etc in the dinette.
Hot air side: Rather than replace the output ducts I took the ends off the hot air registers and stretched the hoses out until the kinks and accordion pleats smoothed out. I left a bit of slack for movement and refastened the ends in place. I also used aluminum tape to seal off gaps left by TM in the connectors.
Result: No more cycling on the 180 degree limit switch. Interior TM temperature went right up from 40 degrees until the thermostat said 72. The hottest the output I got was 169 degrees F with a cold air temp of 66 degrees. This is a TR (Temperature rise) of 103 degrees which is within limits for the NT20s furnace.
There is a change in noise from the cold air return...it is a bit louder but is now more white noiseish.
This good enough for me. Now I can go winter dry camping.
Possible improvements: Suburban ideally wants a TR of 90 degrees and this could be accomplished by rearranging the hot air output. The way TM taps the hot air from the furnace is not very well done. Two 4 inch ducts side by side on the front of the furnace would yield more heat than putting the ducts one above the other. The hottest air is near the top of the furnace and that duct is running 10 to 15 degrees hotter than the one on the bottom. Moving the bottom duct up to the top beside the other on would be an improvement but not something I want to do right now.
The attached documents helped me to correct the defect.
Peter
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