Do I have to remove Furnace?

TXiceman,
Yes that was the first thing I replaced. I even tryed 2 different new high limit switches and got the same symtoms.
 
I haven't seen RV Wizard involved in this discussion will short cut your problem down to RV SYS & APPLIANCES stay tuned.
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I have come across a couple of furnaces in coaches that have had this very same problem. They have been due to the fact that there was not enough air flow across the heat exhanger to make the unit run hot and the high limit thermostat would cycle the burner on and off. On an Alpine I found that WRV had not followed the installation instructions well enough and we had to cut in another vent to all allow sufficient air flow. Another was that the return air ductwork did not line up well to the opening into the furnace cabinet. This is something that is not usually as easily found as one might suspect. It could be a combination of a few things adding up to it "short cycling" on high temperatures.
 
Just a wild guess but is the fan running in the right direction?
 
RV Wizard, I have been working the same thought as yours not enough airflow, I even put a web cam on my laptop and put it through my ducts to look for blockage. there was none.
I then took the furnace out of the MH and ran it on a bench (set up on saw horses right behind the MH so I could use the MH propane supply) It would still cycle off, it took a little longer but it still turn the burner off.

Last night I built a Manometer (from http://rverscorner.com/manometer.html) and tested the propane pressure, I thought it might be high. But when I measure it it turned out to be within specks, 13 inches water with nothing running, 11.5 in of water with stove and Fridge running. So now I'm at a loss still,
I'm beginning to think burner or propane valve problem.

And yes the fan is spinning in the right direction, checked that 1st
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I would set the lp regulator at 11" WC with the stove and furnace load. Then find the installation instructions for your furnace. Look to see that you have at least the minimum # of 4" ducts they require for your BTU rating. Do no count 2" ducts if you have any. Then make certain that none have excessive # of turns and all registers are open. Look at the sizing of the return vent; make sure it is correct. Also look at the position of the blower wheel in relationship to the housing. Make sure both (combustion & recirculating) are close to the inlet side so as to pull the most air it can. Let me know what you find and also include the make and model so I can help better.
 
RV Wizard,
The furnace is a Surburban sf35, installed in a 2001 itasca sunrise 30w. I checked the return air sq inch and it is more then is required. The heat feeds into a plenum and then is distrubuted throught the mh though floor ducting. There are no 4 inch ducts. just the whole backside of the furnace is open.

Also earlier I had removed the furnace and tested on saw horses behind the MH, I got a 24 inch section of flexiable gas tubing for a dryier to hook it to the propane in the MH. It ran for a little longer but then it still shut off on the high temperature switch. All the fans look clean
 
rvcarpenter

Do you have the Aux Coach Heater option on your coach?

If so, the problem may be with the door located between the heater output and the ductwork not opening all the way.

My coach has this option, and there are 2 doors, one at each end of the floor ductwork. They are hinged on top, and are supposed to open when the fans are operating.

These doors maintain pressure in the ductwork, depending on which heat source you are using.
 
I wonder if by chance you have a 40,000 BTU orfice installed in the 35,000 BTU furnace. THis would be too much heat for the size of the heat exchanger. Look at the area where the fan discharges into the heat exchanger to be sure that there is good clearance and sufficient air flow. If it looks ok then I would investigate the orfice size to insure it is correct. I wish I was there, I would like to be able to find & correct it for you.
 
Dancing campers,
No I don't have the AUX heater, although I did check the "damper" onmy installation, it's working fine.

RV Wizard,
Thanks for you help but I have now cried uncle and pull the furnace and took it into my local dealer for him to look at. I droped it off and talked to the tech for 15 20 min's going over everything I have done. they will look at it and get back to me. I'll let you know what they find out.

I hate giving up but with work and kids I just don't have the time to spend on it right now.

Thanks for everyone's help.
 
OK I took the furnace out of my rig and took it to my local dealer, dropped it off and let them look at it. They got back to the next day and said the fan motor needed to be replaced, that it was not producing the RPM's that it should. So they got a new fan motor and installed it, and it works on the bench! yea. I'll pick it up and reinstall it in my MH on Wednesday, I'm hoping once it's installed in the MH it will stay fixed, as a side note I've always thought there wasn't enough airflow out the frount heat regestars in the coach, maybe the fan motor was slow all the time.
 
Good deal, rvcarpenter! Thanks for taking the time to keep us posted. -And we're glad the problem is solved.
By any chance, did they say what the rpms should be, versus what it was running?
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Troth,
I have yet had the time to go pick it up, and the person you talk to on the phone is just a receptionest, so when I pick it up tomorrow I'll talk to the tech and find out what the rpm's were and how he checked the rpm's. I had an idea the fan might be a little slow but couldn't figure out how to test the rpms.
 
Troth,
I pick up the furnace yesterday (have yet to reinstall it) and asked about the motor RPM's. They said they found that the motor was bad by how many amp's it was drawing as opposed to the rpm's. The tech said that the motor was drawing 2x as many amps as it should have been, that's why they said it needed to be replaced. He said they ran it for more then a 1/2 hour on the bench with the new motor and it worked fine. So I'm hoping all is well when I put it back in.

Hope this answers your question.
 
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