suburban furnace tripping on high temp

[xtal_01]

Hello,

I have a Bounder RV with a year old Suburban SF-35Q furnace ... about a weeks run time on it.

A few weeks ago I replaced the propane regulator.

At the time, I noticed the unit was tripping out ... I suspected on high temp.

I was also having an ignition problem (would not lit after ignition).

So, I bough a Dinosaur board. This fixed the ignition problem.

Now I can say for certain, the safety circuit is tripping (board has an indicator).

I tested the sail switch ... seems fine.

I replaced the high temp switch ... we are still tripping out.

So, I pulled the ducting off the rear port on the furnace .... ran it with no line ducting on it ... just dumped the warm air into the basement ... worked. Ran for an hour with no trip.

So, I put hard piping (house type galvanized steel) onto the furnace ... just some straight and 4 bends ... this is the line that comes across the RV and into the washroom. there is a small Tee to bleed air below ... I left inline as it was.

Trips out again on high temp!

Tried pulling two of the duct lines off the plenum coming off the top of the furnace ... still trips out

Tried opening one of the other side ports on the furnace ... still trips out.

I did put a manometer on the regular ...

old ... 12" wc static ... 10 - 11 while running (just bleed some off as it is in the shop and not on the furnace).

New 15" WC static ... 13" when the furnace is on.

motor is a constant 6.5 amps

It even tripped with the outside cover off so I now it is not the return air.

OK, I am stumped ... I have been at this for days standing in the snow.

I did everything I can think off ... I checked with the factory. They said it should not be tripping the high temp switch.

I am getting desperate ... suppose to be leaving on a trip this weekend.

Any suggestions ?????

Thanks .... Mike

[Bruce_Brown]

Any chance its just a bad switch?

[garyemunson]

I added 2 extra registers to mine and solved the problem. Luckily my duct was rectangular metal right under the floor and I was able to just cut down into it.

[xtal_01]

I should have added that this is a 20 year old (1999) RV so I am a bit surprised to have this issue "pop up".

No, I did not take any temperatures ... guess I should do that tomorrow.

I guess that for the past 10 years (that is how long I have owned the RV ... even lived in it for three years full time) I could have missed it but I think this problem just started .. hard to tell since the furnace does not shut down, fan keep going ... re-starts after the high temp switch cools off.

I though for sure that the longest duct had just failed after this many years and replacing it would do the trick.

Now with the longest duct connected in hard smooth pipe ... two open 2" holes in the top plenum and a side port (never opened before) now open you would think this problem would have gone away.

Maybe opening up the back port a bit (the one near the high temp switch ... the middle one is used now which is almost in line with the high temp switch) would let more air by it.

Even with just the 2" bleed Tee open in the basement, it is actually much warmer down there now than upstairs in the cabin.

Thanks .... Mike

[time2roll]

Can you add a couple of additional 4" ducts? This what fixed mine.

[MountainAir05]

size of ducting coming out. Heating can be large, Ac is the one you have to stay close to.

Did you get a output temp with the duct and return
did you get a return air temp with the duct and without when dump into basement. Just sound like you are not getting enough air thru the system duct. Which would be maybe a loose fan on the shaft.