Suburban SF-30FQ furnance

[Mr Lars]

in our 2018 Forest River barely moves air thru the floor duct work. Removed the front cover today, to discover the furnace has plenty of cold air return, but the warm air traveling thru the heat exchanger only exits the furnace thru the bottom(floor) duct. With the front cover in place, the air is almost (90%) blocked from reaching the exit duct. Has anybody else had this issue with this model furnace ducted to only floor vents?

 

[Kirk]

The air must be going somewhere, as the sail switch is operated by the inside air movement. Are you sure that there isn't a leak in the ductwork from the heat exchanger?

 

[Mr Lars]

The air must be going somewhere, as the sail switch is operated by the inside air movement. Are you sure that there isn't a leak in the ductwork from the heat exchanger?

I have not gotten to exploring the ductwork yet. Was hoping someone has the same furnace installed the same way, in the same model travel trailer. I want to eliminate or confirm if the furnace was installed correctly.

 

[Isaac-1]

I have the 1 size larger SF40 series furnace, which has a similar issue, as I recall the installation manual says that at least 3 of the output duct connectors must be used, it sounds like your RV manufacturer also did not read the installation manual, I added an addition duct out the front that using a flange adapter and a short length of Silicone (4 inch) hose, which blows out with enough force to minimize it being sucked back in the air return. see https://www.amazon.com/gp/product/B003G9FMR8/ (wow price has went up almost 3X since 2018) and similar to https://www.amazon.com/KARCK-Silicone-Straight-Universal-Reinforced/dp/B0CQSTWWMV/

 

[Kirk]

I want to eliminate or confirm if the furnace was installed correctly.

I had a problem much like this yours with the furnace in an RV or ours a long time ago and eventually discovered that one of the ducts was torn only a foot or so from the furnace and so most of the heat was blowing into the back of the kitchen cabinets. Those ducts are just a cloth like material so they can be damaged. Like your situation, we were not the first owners of the RV so I never knew how long it had been that way.

 

[Mr Lars]

The manual for this furnace does show the bottom duct knockout as a usable option. My manual does not indicate a minimum # of knockouts to use. Unfortunately this model travel trailer design, the furnace is at the very front, in the kitchen underneath the stove/oven. The floor duct in the far bedroom does not get any noticeable warm air.

 

[Mr Lars]

Done some investigation today, but still no resolution. Pulled the furnace out, the bottom knockout feeds to a plenum that has 5 flexible ducts, one for each floor vent. I can the furnace on a bench, with the blower running very little air comes out that bottom knockout, take the front cover off, and all sorts of air blows at you.
With the furnace re-installed and front cover back on, start the blower, and not enough air comes out the floor vents to barely blow off a paper towel laying on each floor vent.

 

[Ray-IN]

Hum, it appears you have your own version of Rubits cube.
Like Kirk said, the sail switch determines if air flow is adequate and proves that is true in your furnace because it allows the gas valve to open.
Suburban RV Furnace Service and Training Manual