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Old 11-05-2012, 10:03 AM   #1
Junior Member
Join Date: Nov 2012
Posts: 2
Suburban Furnace Overheating/Cutting off

Hello all, I am new to the forum and also new to Travel Trailer use. I'd like to thank in advance the the availability of such forums as they have assisted me greatly with repairing not only the TT I currently use but also my father's '10 Wildcat 5th wheel. What I have is an 06 FR Cardinal TT. It actually belongs to my mother in law . They bough it lightly used in 07 and hardly used it. I decided to take it over, repair all the issues, use it a little and eventually sell it for them. It is in excellent shape but has been sitting for several years and we all know how bad that is on the systems. Anyway, I would like to discuss the furnace. When I first started working on it it would not fire. After removing it, I determined that there was no spark and concluded it needed a new control board. I purchased a Dinosaur Fan 50 and installed it and while bench testing saw that I now had fire. I reinstalled the furnace and to my relief, (albeit temporary) it fired right up. I didn't have time to play with it at that time so it went a week or two before I actually could fully test it. This weekend I was working on it some more to determine how well it worked and realized that I now have another issue. On cold mornings (40s) I could start the furnace and it would run "fine". After the coach would get into the 60s the furnace would cut off. (not satisfying the thermostat) I was able to turn it back on by either shutting the furnace off at the stat and flipping it back on or by lowering the stat til I heard the click at the furnace and then raising it back. However once it started back up it would only run for about 5 - 8 minutes before it would cut back off. I read through several forums and troubleshooting guides and thought that it may be overheating and the limit switch would cut it off. My concerns for overheating would be from measuring TR or temperature rise as noted in one of the manuals. Temperature going in would be about 65 ish (even though coach wasn't that warm) but temp at the registers topped out at almost 200 degrees! THe limit switch is supposed to cut it off at 130 so I know I was right at that level. 90% of what I read stated an airflow issue. The coach initially came with only 3 registers which according to the furnace manual was not enough. (Its a 32' trailer) The existing 3 had very good flow. I easily added a fourth register in the back of the trailer in the kitchen floor. (basically the back 12' of the trailer had none) Unfortuneately, this did not help. Temps at the new register which is approx 5' from the furnace were roughly 200 degrees. (that would definitely burn some toes stepping on that metal register!) And of course furnace would cut off. So now I am back to square one. And to make matters worse, the furnace doesn't always cut back on when I try it. Like I said before, all I would have to do to restart it was turn stat off or temp down and then back on or temp up and it would turn on. However about half the time it would only turn the fan on for about 20 seconds and then shut down. Sometimes I would have to try that 5 times before it would actually fire up. That makes me think the sail switch is flaky. So to sum things up, here are my questions: Is 200 degree temps at the register considered an overheating furnace? I was considering getting a new sail switch and Limit switch since they are cheap and I do have intermittent start up issues. Would a bad sail switch shut down the furnace after it was running? Could this be a gas problem? IE too much pressure equalling a bigger than normal flame? A new regulator was my next step. As additional information: My furnace is a Suburban SF-35F. It is clean as a whistle. Like I said the coach was hardly used and when I had it out I cleaned what ever needed cleaning. It is also located far enough from any registers to prevent warm air recirculation. And the thermostat is also located in an area where its not affected. Any help with this would be greatly appreciated!!

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Old 11-05-2012, 07:51 PM   #2
Junior Member
Join Date: Nov 2012
Posts: 2
Expect the unexpected I guess! I have read about this and didn't think it can possibly happen to me! I went back to do more troubleshooting today and to make the long story short I decided to use my little flashlight and camera to actually see if there was anything wrong with the ductwork. As you can see from the pictures, crushed ductwork. Not sure how that can possibly happen and I am pretty sure it was that way since it left the factory. I am guessing someone probably stepped on it before the subfloor was installed and just left it. Of course the paperthin ducting doesn't help either! To relieve the back pressure causing the overheating, I added a "temporary" Duct hose from one of the knockouts on the furnace frame and laid it out across the floor of the living area. Low and behold the furnace was now able to run up to the desired temp and shut down appropriately. So, any body have any ideas how I can open up that ductwork without ripping up the floor??
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