Furnace Ducts - Replacement Necessary

[JaycoLovers]

Hello. I have a project I'm in the middle of on our 2022 Jayco 25RB. In October, we had a hostile takeover by some squirrels looking for a place to store their bounty of walnuts. I found where they were entering the underbelly...they were getting in through the hollow a-frame member of the hitch. I addressed that with some expanding foam. Unfortunately, the damage was done. After getting access into the underbelly, they took over the furnace ducts and got access to the area under the kitchen counter by chewing their way through the flimsy ducts. In less than 2 weeks, they stored over 40 lbs of walnuts inside the cabinets and drawers of our camper. Nearly all the ductwork was compromised. I had to remove the underbelly, where I found more nuts, and ended up removing all of the destroyed ductwork.

I'm in the process of planning for replacing the ducts. I hope to use semi-rigid duct hose rather than the flimsy stuff the manufacturer used, but I have an issue. The locations in the underbelly where the ductwork was routed won't accommodate a 4" semi-rigid duct. In fact, the flimsy stuff barely fit those areas and a lot of it was crushed when installed through frame members or crushed between the underbelly and holding tanks (don't get me started on the build quality in this area of the RV...it is a mess). Even if I attempted to the 4" flimsy stuff, I wouldn't be able to route it through some areas without tearing and/or crushing it.

So here is my question, can I reduce the duct to 3" without causing any issues to the furnace? I realize that the airflow through a 3" vs. a 4" duct will change...but will it cause any issues with the furnace itself? Will the furnace fan be able to overcome the restriction? Will the furnace overheat from a reduced airflow? Anyone ever do this?

 

[Gary RV_Wizard]

The answer is "maybe". The furnace is very sensitive to adequate air flow, but there are usually multiple outlet ducts so a modest reduction in one isn't necessarily a problem. And you could probably add another outlet if necessary - most furnaces have a couple unused knock-out ports that could be used. Depending on where the furnace is physically located, you might even be able to just vent some extra air in that immediate area.

Speaking of duct quality brings back a memory. I bought a new National RV Dolphin LX motorhome in 2002 and one of the many issues it had was very poor heating from the ducting. I was able to arrange for the factory techs to inspect it at an NRV National Rally and after a brief test they really tore into the flex ducting. In an hour they removed over 10 feet of excess hose, cutaway multiple badly crushed sections, and replaced two duct outlets that were so damaged during installation that hardly any air passed thru. National's Director of Service was there watching and he just shook his head in despair. His only excuse was that there must have been a substitute worker on the assembly line that day. Later I took that coach to the factory in southern CA and they spent two days fixing things that were messed up since Day One. It was a pretty decent coach after that!