I have two Duo-Therm 90125.001 furnaces on my AS. The front one
starts up and works great. The rear one would never light off. I
tried dozens of times. So I figured that I needed to pull it out.
The manual says to pull the external inlet/exhaust plate first. It is
however caulked in place. After cutting the caulk with a putty knife
the plate came loose but would not pull out, the exhaust pipe was
stuck. With much wiggling and pulling it came loose. It looked
Not good, a hole had burned or rusted through the pipe. With
the outside plate removed I started on the furnace itself. The
manual says to remove the one screw at the bottom center
in the front of the furnace. Well I did that and it would not
come out. I found another screw to the right of that screw
that held the furnace housing to the floor. Removing that then
the entire furnace was loose.
Next I had to remove the electrical and gas connection. It looks
I disconnected the gas line and marked the wires and disconnected
them. Marking them is important since the colors do not match
their mating connections.
I also disconnected all the air ducting. The fittings are held in by
two tabs which are easily pushed in to release the matting fitting.
Then I could pull the whole thing out.
Then the furnace could be pulled out of it's housing. It should not
have been necessary to remove the housing and disconnect the air
ducts but it did not work for me.
Upon inspecting the furnace I found this;
It looks like a mud wasps nest and is likely the reason that the
furnace would not light off, restricted airflow.
Further inspecting the furnace I found this;
The exhaust piping is formed by two inner pipes that fit one within
the other and have a packing ring for a seal. And two outer pipes
that protect the motorhome from the hot inner pipes. The packing
seal can been seen in the upper pipe here;
Since both pipes were burned through exhaust could have leaked
inside the motorhome. This is BAD!
The other part of the exhaust piping is two outer pipes that also
slide together but do not have a packing seal. One is shown as the
lower pipe in the above picture. Since the outer pipes do not
have a seal they will not prevent exhaust from leaking if the inner
pipes have holes in them.
The aluminum flex tubing in the above picture is the air inlet to
the combustion chamber.
I am going to try to find replacement exhaust pipes. I don't
know if they are available.
Now I am concerned about my other furnace. Does it have
burned through exhaust pipes? Was the burn-thru caused by
the restricted air flow at the inlet?
It seems that you should be able to inspect for burn through
using one of those flexible inspection cameras that are pretty
cheap these days. The entire exhaust pipe length is only about
12 inches and it is perfectly straight. I will try that on my good