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Old 11-19-2017, 08:44 AM   #1
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1986 34.5' Airstream 345
Londonderry , New Hampshire
Join Date: Mar 2017
Posts: 59
Fixing my furnace.

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
like this;
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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
like this;
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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.
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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.
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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.
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Upon inspecting the furnace I found this;
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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;
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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;
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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
furnace.

Pete.
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Old 11-19-2017, 10:52 AM   #2
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1984 34.5' Airstream 345
Foothill Ranch , California
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Nice work, and great write up!
I put some little SS mesh covers over my exhaust and inlet pipes to keep critters out.
Keep us updated!
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Old 11-19-2017, 11:08 AM   #3
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1984 34.5' Airstream 345
Franklin Park , Illinois
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Looks like your doing a GREAT job
It too bad you have those burn through holes but a good thing you found them!
The nice thing about pulling these things out is you can clean them up really good
On my 345 I had a bad blower motor on my front heater and a bad circuit board on the rear. I forgot where I got the blower motor from but I was able to get a circuit board from a please called dinosaur boards
Good luck In your repairs, and great pictures! I’m sure this post will help others in the future
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Old 11-19-2017, 06:32 PM   #4
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1984 34.5' Airstream 345
Foothill Ranch , California
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I know I have a spare piece of pipe in my MH that I am pretty sure is that piece that you have a hole in.

Let me dig thru my pics, cos I know I have one of it.

I searched for that part online and failed to find it, so if what I have works, its yours!
Failing that, the rest of your furnace looks in good shape, so maybe make a new one?
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Old 11-19-2017, 07:27 PM   #5
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Menomonee Falls , Wisconsin
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Posts: 62
Hey,
Could you tell me what wires go to what connection on the furnace. I took mine out too, but now it does not fire up unless I jump one of the wires. Then the darn thing stays on until I disconnect the jumped wire. The heater is not on, I'm just getting the blower to run. I think I have the thermostat wires going to the wrong terminals on the furnace. What is it with mud wasps. Mine too had them in the pipes. Thanks dj
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Old 11-19-2017, 07:59 PM   #6
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1986 34.5' Airstream 345
Londonderry , New Hampshire
Join Date: Mar 2017
Posts: 59
Quote:
Originally Posted by davejay View Post
Hey,
Could you tell me what wires go to what connection on the furnace. I took mine out too, but now it does not fire up unless I jump one of the wires. Then the darn thing stays on until I disconnect the jumped wire. The heater is not on, I'm just getting the blower to run. I think I have the thermostat wires going to the wrong terminals on the furnace. What is it with mud wasps. Mine too had them in the pipes. Thanks dj
This shows the schematic that is stuck to the furnace.

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The red wire goes to the +12 volt supply. The yellow wire goes
to ground. The thermostat connects to the two blue wires and
the polarity does not matter.

In my AS the red wire from the furnace goes to the brown wire
from the MH wiring. The yellow wire from the furnace connects
to the white wire from the MH wiring. The wires from the
thermostat are both blue with a white stripe and they connect
to the blue wires from the furnace with polarity not important.
This might be a better picture.

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If you mix up the thermostat wires with the power wires you may
damage the thermostat. If you get the polarity wrong on the
power wires (12 volt) you may blow out the controller board.

The controller board is in the bottom of the furnace as shown
here:
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Hope this helps,
Pete.
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Old 11-19-2017, 09:46 PM   #7
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1936 20' Clipper
1947 22' Liner
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1989 37' Airstream 370
marshfield , Massachusetts
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Given that these heaters are somewhere around 30 years old, and were never exactly built like cast iron boilers in the first place, how much more do they have to give? There are a lot of ways these old Classics can leave you stuck on the roadside, but only a few ways that they can actually kill you, and these heaters fall into the latter category.
Without getting into too long a story, I wasn't overly concerned that my front furnace was reluctant to fire up when I bought my 370 a couple of years ago, since I barely had time to use the coach anyway, and the rear didn't work at all. I'd also confess that I didn't fully understand the entire starting sequence with blower check prior to ignition, etc, etc.
The "Get together" at Pennwood last year was my first outing with it (and we actually had snow!) so I purchased an electric unit to survive the weekend. Fortunately, I never asked any of the more seasoned campers for advice about getting the furnace to fire up. When I got back home after that chilly weekend, I dug into it a bit more and got it to fire up quite nicely. To my surprise, as I laid on the floor looking into the darkness under the cabinet, I could clearly see the raw flame in the combustion chamber almost blowing through a continuous open crack where it attaches to the heat exchanger!
I don't know exactly how these two components are fastened together, but it was a potentially lethal situation that could have either started a fire, or killed me as I slept. I sleep much better now with two new furnaces.
Just saying.
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Old 11-19-2017, 10:11 PM   #8
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I agree with you Charly. There are lots of things you can fix that could bite you, but I believe an old RV trailer is at the top of the list.

We replaced our heater six or seven years ago, and I can sure say I rest a whole lot easier knowing there's a relatively new unit in our TT.
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Old 11-19-2017, 10:51 PM   #9
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1984 34.5' Airstream 345
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I agree with the above comments.
Mine were both replaced and therefore not the originals, however I don’t know when yet.
The trick is assessment.
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Old 11-19-2017, 11:12 PM   #10
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Those furnaces were a safety hazard when they were manufactured, that"s why they are no longer in business....they had problems with cracks in the combustion chamber and I'm almost certain that there were liability cases involved... Definitely something not to experiment with. Replace them with an Atwood or Surburban and be done with it. Regards, Bob
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Old 11-20-2017, 03:49 PM   #11
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1982 31' Airstream 310
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Quote:
Originally Posted by bobmiller1 View Post
Those furnaces were a safety hazard when they were manufactured, that"s why they are no longer in business....they had problems with cracks in the combustion chamber and I'm almost certain that there were liability cases involved... Definitely something not to experiment with. Replace them with an Atwood or Surburban and be done with it. Regards, Bob
Yup. In the long run you're safer and better off just replacing the unit...believe me, I tried like heck to save my original and in hindsight I would have been dollars and time ahead if I would have just bit the bullet and bought a new one to start with. I'm all for trying to save and repair but in this case, do what Bob says, buy new and be done.
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Old 11-20-2017, 04:09 PM   #12
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1984 34.5' Airstream 345
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Ok, they are probably the originals... I dont know much about them,
Are the intake and exhaust horizontal?

Im not sure if that style is still available, because the Suburban replacements on mine were vertical, and whoever did that job, hacked it...
Looks like I will be making some patches.
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Old 11-20-2017, 06:38 PM   #13
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Menomonee Falls , Wisconsin
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Hey,
Thanks for the advice and wire configuration . This unit didn't t have a front furnace. So that one will be new. I think y'all are right on replacing the rear one too
. Does Atwood or suburban make a little unit for the rear. Anybody got a part number for same. I'm also going to replace the water heater. What do you think about a tankless unit I ve read mixed reviews. Thanks Dj
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Old 11-27-2017, 02:44 PM   #14
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1974 31' Sovereign
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Quote:
Originally Posted by davejay View Post
Hey,
Thanks for the advice and wire configuration . This unit didn't t have a front furnace. So that one will be new. I think y'all are right on replacing the rear one too
. Does Atwood or suburban make a little unit for the rear. Anybody got a part number for same. I'm also going to replace the water heater. What do you think about a tankless unit I ve read mixed reviews. Thanks Dj
What size trailer do you have Dave? AS only put front and back furnaces in the 34-footer AFAIK. There is steel ductwork leading to the front and even to the area around the waste tanks, so the heat "gets around". Spring summer, or fall, one does nicely in our 31' Sov.

The only reason I can think of to have two is if winter camping in Menominee is on your mind. And if it is, I would put in better-than-stock factory type of heater... Jeez two heaters rumbling on and off all night? Hmmm... Don't think I need that.
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