Another LP Furnace problem...

[Moder2]

I have been having troubles with my main furnace. Last week, we woke to a rather chilly MH, as the fan was running, but the burner wouldn't light. I reset it, and the burner lit, and would run, then would drop out with the fan still running. The burner would not relight until I turned it off and back on.

Not knowing a whole lot about Suburban, and being a bit nervous because I needed this solved, I called the warranty company, they said call someone to take care of it, it was a covered problem. Fine. I called a local RV repair company, and they sent out a tech.

The tech was first convinced it was low gas pressure. Fine, he check it, said it was a couple inches low, and adjusted it. I am glad I called them just so I could witness how that procedure is actually completed, but I still wasn't convinced that was the problem. We restarted the furnace and it lit, then for lack of a better term "Short Cycled" several times before the thermostat was satisfied.

The tech really didn't want to pull the furnace, but I kept insisting that I had never noticed this doing this before. The furnace used to literally cook you out if you put the stat to high. He suspected a bad high limit switch.

They supposedly took it back to the shop, cleaned the burner, and installed a new high limit switch. He brought it back out, and installed it, and sorta rushed to get out of here... I am sure he had other jobs lined up, but I was a bit frustrated he wouldn't let it cycle off. In any case, the furnace is still 'Short Cycling'.

The burner seems to run about 5 mins, then off for 3 or 4. Then relights. Blower remains on continuously.

Is this normal? My understanding is the furnace should burn until the stat is satisfied. Mine is clearly not doing that. I assume the high limit switch is tripping it off until it cools, then re-fires.

The first thing the tech said on the phone was "It worked fine on the bench" followed with "each furnace will vary depending on it's installation." Coupled with the hurried state the tech was in when he was here to install it, and the fact he was so sure it was gas pressure, I couldn't help but feel like these guys just wanted to jump to a conclusion to take my money. Then I got the "you must have an obstructed vent" which they are clearly all open, and there is no excess pressure on any of them. It has full access to cold air, and still stands that it did not do this before. When he brought the furnace back, I asked the tech first was was wrong and he went in to explain how dirty the burners were, and they replaced the limit switch. I check the furnace, and there was some dust on the outside of the burn chamber, which I intentionally rubbed my finger through. That mark was still there. Now, I know when I disassemble something to clean it I clean EVERYTHING so it looks NEW. Ok, I understand these are people working on other people's things, and I suppose I can't hold them to my standard... That being said the limit switch looked the same as the old one, and I didn't even see screws that were scratched...but the installing tech of course stripped the heads on every screw in the access panel. I then asked the tech if he was the one that worked on it at the shop and he said no, he was not, that "old Mitch did, cause he doesn't like being out here in the cold." I don't think they did a darn thing to it at the shop, but of course i can't prove anything.

Am I correct in my assumption on how the furnace operates? (burns till satisfied?) Does anyone know what could be the problem? and how would you handle the situation with the repair company? Thanks in advance, and sorry for the long winded post...

John

[Moder2]

I have been having troubles with my main furnace. Last week, we woke to a rather chilly MH, as the fan was running, but the burner wouldn't light. I reset it, and the burner lit, and would run, then would drop out with the fan still running. The burner would not relight until I turned it off and back on.

Not knowing a whole lot about Suburban, and being a bit nervous because I needed this solved, I called the warranty company, they said call someone to take care of it, it was a covered problem. Fine. I called a local RV repair company, and they sent out a tech.

The tech was first convinced it was low gas pressure. Fine, he check it, said it was a couple inches low, and adjusted it. I am glad I called them just so I could witness how that procedure is actually completed, but I still wasn't convinced that was the problem. We restarted the furnace and it lit, then for lack of a better term "Short Cycled" several times before the thermostat was satisfied.

The tech really didn't want to pull the furnace, but I kept insisting that I had never noticed this doing this before. The furnace used to literally cook you out if you put the stat to high. He suspected a bad high limit switch.

They supposedly took it back to the shop, cleaned the burner, and installed a new high limit switch. He brought it back out, and installed it, and sorta rushed to get out of here... I am sure he had other jobs lined up, but I was a bit frustrated he wouldn't let it cycle off. In any case, the furnace is still 'Short Cycling'.

The burner seems to run about 5 mins, then off for 3 or 4. Then relights. Blower remains on continuously.

Is this normal? My understanding is the furnace should burn until the stat is satisfied. Mine is clearly not doing that. I assume the high limit switch is tripping it off until it cools, then re-fires.

The first thing the tech said on the phone was "It worked fine on the bench" followed with "each furnace will vary depending on it's installation." Coupled with the hurried state the tech was in when he was here to install it, and the fact he was so sure it was gas pressure, I couldn't help but feel like these guys just wanted to jump to a conclusion to take my money. Then I got the "you must have an obstructed vent" which they are clearly all open, and there is no excess pressure on any of them. It has full access to cold air, and still stands that it did not do this before. When he brought the furnace back, I asked the tech first was was wrong and he went in to explain how dirty the burners were, and they replaced the limit switch. I check the furnace, and there was some dust on the outside of the burn chamber, which I intentionally rubbed my finger through. That mark was still there. Now, I know when I disassemble something to clean it I clean EVERYTHING so it looks NEW. Ok, I understand these are people working on other people's things, and I suppose I can't hold them to my standard... That being said the limit switch looked the same as the old one, and I didn't even see screws that were scratched...but the installing tech of course stripped the heads on every screw in the access panel. I then asked the tech if he was the one that worked on it at the shop and he said no, he was not, that "old Mitch did, cause he doesn't like being out here in the cold." I don't think they did a darn thing to it at the shop, but of course i can't prove anything.

Am I correct in my assumption on how the furnace operates? (burns till satisfied?) Does anyone know what could be the problem? and how would you handle the situation with the repair company? Thanks in advance, and sorry for the long winded post...

John

[DSouthw524]

When we bought our current MH we had similar furnace problems using it first time in the winter heading south. We stopped at several Winnie dealers and they took furnace out and bench tested it and it ran fine. They reinstalled and off we went only to have no heat. The last time we had them run it while installed and had same problem They removed it and replaced a board and that fixed the problem.

I suggest you go to a dealer and have them test it as installed. I bet it is a board!

[NYMoose "Lenny"]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Moder2:
How would you handle the situation with the repair company? Thanks in advance, and sorry for the long winded post...

John </div></BLOCKQUOTE>
You may have a board issue. Regarding how to handle the repair company, if you are still around them I would call your warranty company and tell them you are not satisifed yet, and the problem is still occuring. I would then call the shop that came and speak to the owner and explain the situation and that your are not satisfied, and see what they say. I would think they would either have you bring it in or send another tech.(I would want to run it in your coach)

[RV Wizard]

If the furnace is short cycling off the high limit switch then it is a result of not enough air flow. I have had to cut in another vent off the furnace and up through the floor to prevent over heating of the furnace and cycling on the high limit switch on an Alpine under warranty. Western RV argued with me about how it was installed but I finally convinced them it was not right acording to Suburban. What I (actually my techs.) did fixed the problem the customer had going on for over a year before bringing it to us.

[Moder2]

Mike,

I would agree with you if this wasn't a new thing. The furnace worked fine up till a couple weeks ago. We did have some chilly weather last winter, and we could always heat the coach to whatever we wanted it to. Now, the furnace won't ever raise the room temp over a degree in two hours. I don't think it is a vent problem, and I don't know for sure that the cycling is due to the limit switch either, that is just what I got from the tech.

Thanks for your advice too, I will ask them to ensure there is enough airflow, but I really don't think that is the problem.

John

[DonavonP]

John, you might be able to pick up an idea on this pdf. http://www.bryantrv.com/docs2/docs/ntseries.pdf

I had a furnace that would short cycle and back fire. It was the computer board.

Maybe these people can help you. http://www.dinosaurelectronics.com/Ignitor_boards.htm

[tderonne]

Are you at the same elevation as you've been before when it's cold? Ours acted up a little when we were at Rocky Mountain National Park for a night. We were only there one night, so I never really got to confirm it was doing anything wrong, but hasn't acted up since. I sort of wrote it off to being at 10,000 feet.

[KIX]

Sometimes the old simpler stuff was better. I know your front furnace works off the True Air t-stat. If the t-stat were an old model it could be the anticipator causing the problem. I don't know if your t-stat even has a traditional anticipator. Maybe you could call RV Products .......I think they supply the t-stat. That's where I would start looking for the cause. I doubt your furnace is the problem.

[Moder2]

An update, but nothing really new...

The repair company sent out another tech today. He listened to the furnace, and saw my complaint right away, it short cycles. We are still not sure what is causing it to do this.

We know for certain it is short cycling due to the high limit switch. We thought it was a bad one, and have replaced it with two different switches now.

We bypassed the high limit switch, and it does not short cycle.

The TR with the limit switch bypassed is about 60 degrees, with Outdoor temp in the 40's and inside about 65, the register closest to the furnace is reading about 125 degrees steady.

With the limit switch, the TR rarely makes it above 40 degrees, at its hottest point. When the furnace cycled off, it would drop to almost the same temp as the intake air. I ran the furnace today from 8:00 to 11:00 again about 40 deg. outside temp and inside started about 64. The furnace did not raise the room temp one degree in three hours.

We know it is not the thermostat. We bypassed it, and the furnace has the same short cycle symptom.

The tech that came out today is the tech that did the bench work the first go around. He seems to know what he is talking about. He felt as I did that there should not be an airflow problem. The ductwork is all open and clear. We did run the furnace with the access door off, which again, it showed the same symptoms and rules out a lack of return air flow.

Other suggestions??

John


We are right now stumped. He took the unit back to the shop again. He said he could test the board, and would call suburban tomorrow.

[KIX]

John, If you had asked I would have bet $$ that the problem was in the t-stat. Sorry but you missed your chance. Hope they find and can fix the problem. I'll keep fingers crossed. Please let us know the outcome. Good luck!

[tomsm]

John, how did the tech explain away the high limit switch bypass test? Regardless of how many times he's replaced that switch, if it short cycles with the switch in and doesn't with the switch bypassed, how can it be anything other than the switch???

I'll be very interested in how this shakes out because I just realized mine is doing the same thing and has since it was new. We don't use the furnace much here in FL, just the heatpump. The few times I used it and it didn't heat the coach very fast, I just thought it was because of the size of the coach (my first big one). My TR is almost identical to yours - input 75, output ~115-120.

Hope they find the fix quickly for you!

[Moder2]

The limit switch I don't think is bad. I after 3 switches, I think it is genuinely doing what is is supposed to do. I started to suspect the the installation as Mike suggested, but even going back to venting it still doesn't make sense for 2 reasons:

1. This is a new problem that surfaced a couple weeks ago. This furnace in the past would heat to whatever i set the stat too. Now, it doesn't, it short cycles.

2. I check the 'Square Inches' of the available vents and cold air return against what suburban recommends. For my furnace 35k BTU they recommend 4 4 inch ducts, or 56 Sq. Inches) I have 3 2x10 and one 3x7 duct, for a total of 75 square inches of ducting. The Cold Air return is 81 Square inches located directly above the unit, they recommend at least 55 SI.

Honestly we are both baffled. We have both looked at all the options, and can't figure this out. I am very curious to see what he finds on the test bench. I am sure on the first go around he wasn't looking specifically for what it is doing since he wasn't the tech that pulled it out.

Back to the TStat, There is a work camper here that also suggested the stat, in fact was very adamant it was the stat. Last night I bypassed it to be sure, and again today with the tech here, he bypassed it. It certainly has nothing to do with it.

Thank god I have the rear furnace, and the two space heaters, it is live able in here, but with the furnace out, I have a big hole to the outside world... I stuffed it with a blanket, but I still feel the draft! Of course it is right under our desk and met feet suffer!

My thought is that there is something wrong with the burner that it is burning too hot (I don't know if this is possible???) or there is something wrong with the fan that it isn't moving the air over the heat exchanger. Again he will know more on the bench tomorrow, and after he talks with Suburban.

John

[tomsm]

I guess that's one possibility I didn't consider - that it actually is an overheat condition. That doesn't seem to match with your low TR though...

I've never seen how a high limit switch physically connects into the system. Is it possible that whatever connects it to the system (jack, socket, ?)is defective and opening up with heat? Low probability, but sometimes it's not the "parts", but the connections to the parts...