Suburban furnace min. air return grill size?

Both my trailers with furnaces have had really poor ducting. Fixing the return air grill size was pretty straitforward.

But they also try to force the hot air into much smaller ducts than specified by Suburban.

The ducting was really bad in our old Trailmanor; only a 3" dia round duct to the whole living area. But it was also relatively easy to fix; just cut open the duct plenum and let the air blast out into the living area through a 3X12" duct (it was a small trailer). Didn't have to move or disconnect the furnace.

The heating in our new trailer (Timber Ridge 240RKS) works better than that, but still seems weaker than it shoould. And the ducting seems too small. According to Suburban the 35K BTU furnace needs 56sqin of bottom duct (or 48sqin of side duct) instead of the existing 35sqin. Changeing this will be very difficult though as I think the existing ducts all run through the ~3" thick wood floor (~2.5"x14"=~35sqin ducting).

Not sure how the underbelly is heated. It might just be radiant heat from the Al ducting running through the 3" wood floor. The ducting runs straight up the middle of the trailer from the rear to the bedroom in front, running over all the underbelly tanks. This would actually make sense as the tanks are located in two compartments, one fore and one aft of the relatively open axle/slide underbelly area.

One of these days I'll open up the underbelly and take a look.

And good luck in your investigations.

Chris Bryant wrote:
I have it in this pdf file naturally, the specs are on a page I scanned upside down, but for all but the over 40k btu/hr models it is 55 square inches. It gives ducting, as well.
That is for older models, but I cannot imagine they have changed that much.

Got it. Model NT20 = 55 sq. in. for return air. Thank you! Now to see if I can get it to work out.

Installing a blower in the return duct can allow you to use a smaller opening. Someone who specifies forced air cooling and heating systems can help you figure it out. The key is the cubic feet of air per minute going into the heater.

I have it in this pdf file naturally, the specs are on a page I scanned upside down, but for all but the over 40k btu/hr models it is 55 square inches. It gives ducting, as well.
That is for older models, but I cannot imagine they have changed that much.

I seem to remember there was a square inches spec in the install manual.