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Old 10-04-2015, 09:11 PM   #29
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1979 31' Excella 500
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Quote:
Originally Posted by mayco View Post
As far as not reaching temp, thats probably just the old thermostat being out of calibration, set it at a higher temp till you are comfortable.

I thought the thermostat might have been the problem too (it's a new digital thermostat, details here http://www.airforums.com/forums/1606559-post.html) so I bypassed the thermostat completely and just connected the two blue wires together (which, when a circuit is completed through them via the thermostat turn on the furnace); still had the same behavior.

At this point I'm leaning more toward either a faulty limit switch or a limit switch that's working properly and the unit is getting too hot and shutting off (leaving the blower on to cool it down).
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Old 10-04-2015, 09:14 PM   #30
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Originally Posted by CDONA View Post
What happens when you raise the set point temp, will it burn longer/warmer than the 57* set @ 70* ?

Unfortunately not. No matter how high I set it the heating element always turns off after a few minutes.

Seems not to be a problem with the thermostat telling the furnace to turn off prematurely. This is supported by the fact that when I bypass the thermostat and just connect the two blue wires from the furnace together, I still have the same behavior.
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Old 10-04-2015, 09:43 PM   #31
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Originally Posted by Minno View Post
Why is there silver tape around the propane line going into the furnace? And flexible gas lines should never be used in an an RV. Too much vibration. Where does the other end of the flexible line connect to the gas line? Under the trailer I hope. I highly recommend that the flexible line be replaced before it fails. RV's should only be using soft copper (bendable) for propane and flare fittings. No connections inside the trailer or belly pan except for the final connection to the appliance.

Chris

We opted for Homeflex because it was easier to work with. We ran it under the AS and encapsulated it in rigid PVC to protect it (though it's already well protected just by the nature of its construction, so not sure the PVC was necessary).

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The Homeflex runs to shut offs (also beneath the AS) for each appliance, then the flexible gas line connection goes to the appliance from the shut off.

The silver tape was just there to hold it in place and keep it from moving around too much. It's not trying to keep anything in or out of the unit.

Still was able to use flare fittings.
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Old 10-04-2015, 10:42 PM   #32
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trekerboy ,
Find the data plate attached to your furnace. On the data plate the "Heat Rise" will be listed. This number is the difference of the temp. of the air entering and exiting the furnace. If your reading is higher than the listed (max.) heat rise , it is likely that you are not moving enough air across the heat exchanger. If you don't move enough air the units high temp. limit sensor will shut off the gas supply at the control valve. This can be fixed by reducing the restrictions in the duct system. Restrictions can be duct that is : kinked , blocked , or undersized. The registers may be a source of restriction if not open or blocked . Also make sure that someone didn't add a air filter to the return side of a unit that may have been designed to run without one.
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Old 10-05-2015, 04:13 PM   #33
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Originally Posted by altnbndr View Post
trekerboy ,
Find the data plate attached to your furnace. On the data plate the "Heat Rise" will be listed. This number is the difference of the temp. of the air entering and exiting the furnace. If your reading is higher than the listed (max.) heat rise , it is likely that you are not moving enough air across the heat exchanger. If you don't move enough air the units high temp. limit sensor will shut off the gas supply at the control valve. This can be fixed by reducing the restrictions in the duct system. Restrictions can be duct that is : kinked , blocked , or undersized. The registers may be a source of restriction if not open or blocked . Also make sure that someone didn't add a air filter to the return side of a unit that may have been designed to run without one.
Thanks for the tip! Here's the badge:

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And the Model Badge:

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Now I just need to get a thermometer to be able to measure the temperatures
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Old 10-05-2015, 04:22 PM   #34
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Quote:
Originally Posted by altnbndr View Post
If you don't move enough air the units high temp. limit sensor will shut off the gas supply at the control valve.
I took the access panel off the furnace to observe what happens when I first turn on the thermostat. Here's a video of what happens:

IMG_0954.MOV

... or on YouTube here: https://youtu.be/CPe0vPb776Q

At this point, the furnace had already run for a while, then stopped heating, leaving the fan on. I turned off the thermostat, then turned it on again, and this is the behavior I observed. Sounds like the ignitor tried to light up 3 times.

When the flame goes out, is that the limit switch kicking in?
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Old 10-05-2015, 04:27 PM   #35
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That's a great point; I hadn't thought of that. I'll try it and report back. Thanks Dennis!
So, I tried this (opening up the bed lid to allow the whole area around the furnace to be ventilated. The unit still stopped heating after about 10 minutes.

What's interesting is that when I took off the access panel on the furnace and let the air from the fan blow straight out the back instead of through the attached vents, the furnace stayed on and heated up the interior of the AS until the thermostat temperature was reached.

This leads me to believe the limiter switch is actually functioning properly and the unit itself is just getting too hot for some reason. The unit itself (exterior skin of the furnace, does get too hot to touch, and the vents attached to it also get really hot... seems normal; is it?

Could I have a ventilation issue?
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Old 10-05-2015, 04:46 PM   #36
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If the furnace is making it all the way through the ignition sequence, which yours is, it narrows down the source of your problem. Dont know if you had a chance to take a look at those two links I posted earlier but they give you a good understanding of how these furnaces work and their sequence of operstion. Those limit switches can get weak over time and trip before youve reached any dangerous temps in the furnace. On many of the suburban models you can replace that limit switch withoit too much effort on the front of the furnace without having to pull the unit out. If you google suburban rv furnace limit switch you can see a picture of the switch. There are two screws holding it in place, remove the screws a pull out on the switch, youll see two wires attached with spade connectors. You just pull them off the old switch and place them on the new. The only problem Ive encountered whe replacing one is that the wires are vut awfully short and the switch wont pull out very far to get to the spade connectors. Some long nose plyers and good light to see what youre doing. The switch is pretty inexpensive, lees than. $20 if I remember correctly. Just about all the suburban models use the same switch so you shouldnt have any trouble finding one. its beginning to sound like that may well be your problem. Easy enough to find out by replacing the old with a new one
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Old 10-05-2015, 04:47 PM   #37
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Could I have a ventilation issue?
I like this explanation even more.
Compare the installation manual to what was done and check the ducts for obstructions.
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Old 10-05-2015, 04:47 PM   #38
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Ah ha! Now I Remember!

I had a similar problem in Old #1.
My heat distribution ducting was blocked in two places.
One where some previous owner had crushed it.
And another where a mouse had built a nest and filled the duct with toilet paper and the filling from a quilt.
You have pursued a very logical fault analysis, and I think you are on to something.
If the furnace cannot push enough heat through the ducts it will overheat and shut down.
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Old 10-06-2015, 08:56 AM   #39
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Originally Posted by nrgtrakr View Post
I like this explanation even more.
Compare the installation manual to what was done and check the ducts for obstructions.
There are no duct obstructions (either coming into the furnace or going out of the furnace) but I'm wondering if the long run of the ducts is a problem... When I take off the access panel on the furnace and let the blower run the amount of air that it moves is pretty significant, but the air that eventually comes out of the vents is no where near that.

Would it be in my best interest to try to draw more air into the ducting system from the furnace with something like an in-line blower?

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Old 10-06-2015, 09:06 AM   #40
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Originally Posted by mayco View Post
Don't know if you had a chance to take a look at those two links I posted earlier but they give you a good understanding of how these furnaces work and their sequence of operation.
I did! Thanks so much, these older manuals are not easy to come-by, so I appreciate you posting the links!

The more I try to diagnose this, the more I think the limit switch is working properly and it's actually shutting down the heating part of the furnace because it's getting too hot. Right before the furnace starting blowing cold air, the whole thing is too hot to touch, and that includes the vents coming out of the unit.

The area under the bed where the furnace is located is sealed, and I'm wondering if I need more ventilation under there to help with this issue... There are two slotted openings on the top and side of the furnace, and according to the owner's manual these are for "return air". Am I not allowing enough "return air"?

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Old 10-06-2015, 09:39 AM   #41
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Was your rig modified, furnace moved from its original location? If it is stock, the way it came from the factory, it really should operate properly without additional air flow or ventilation.

But, you say it WILL reach the set temp on the thermostat if you leave the access panel off? Hmmmmmmmmmm.

Since it goes all the way through the cycle till the burner lights, thats a good thing. Sooooo, About the only thing left that would shut it down would be the high limit switch. If you werent getting enough air flow through the furnace then the sail switch would prevent the furnace from lighting. Hmmmm, these old suckers can be frustrating but I wouldnt give up on your furnace.

I may be totally wrong but I still think it is your high limit switch tripping before it should be. Wish I could be more definitive. Good luck, youll get it, just keep at it.

Mike
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Old 10-06-2015, 09:52 AM   #42
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I went through the Suburban Heaters in our '82 Newell Coach a few years back.
It really is a good idea to get a good look inside the unit(s) and do a thorough cleaning. From what I remember the Burner needs to be checked and possibly cleaned annually. I found that some wasps (mud daubers) had gotten in through the vent and nested on the fan blades.
I found that Dinosaur Electronics makes an excellent replacement for the circuit boards. Here is a link, sorry don't remember what circuit board I used but they have a great customer service;
http://www.dinosaurelectronics.com/Ignitor_boards.htm
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