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Old 10-04-2015, 12:02 AM   #15
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Quote:
Originally Posted by drboyd View Post
First: Install a working carbon monoxide detector in your coach.

Second: The thermostat actually starts the burner and fan simultaneously. The burner, in turn, heats up the heat exchanger. As long as the heat exchanger is hot, the fan will continue to run - until the heat exchanger has cooled off. What you're seeing there is IMNSHO perfectly normal.

Save yourself the trouble and expense of a new furnace. Believe me, I installed a new furnace in my 1978, and it was a Serious PITA.
Actually, that is not the furnace starting sequence. On initiation of the thermostat, the blower motor comes on and operates the sail switch, AKA 'air prover switch'. Once the sail switch is fully depressed for a specific time, thus guaranteeing that the blower is operating properly, only then will the gas valve open and the igniter fire to start the flame.

The furnace will try 3 times if ignition isn't seen and will then go into cool down mode, which is approx. 3 minutes. To initiate the sequence again, turn the t/stat off, then on and the furnace will re-set.

I definitely agree that you should not be using a furnace over 15 ears old due to rust and potential CO or LP leakage. When I was still repairing LP appliances (I no longer service LP appliances due to liability issues and the even increasing costs of insurance for LP service work) I would not touch any furnace over 10 years old.

Get a new furnace with DSI (direct spark ignition) and you won't have an more issues for a long time.
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Old 10-04-2015, 09:59 AM   #16
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Originally Posted by Alumaholic View Post
As drboyd has diagnosed you may be low on airflow. There is a device called an air sail safety switch in the air intake of your furnace. If there is impeded air flow, the sail switch breaks the circuit and the gas valve closes.
Thanks for this tip! I checked the air intake, and no obstructions.

I stood by the exhaust vent to check it's behavior. When the unit first turned on, it was HOT, almost too hot to put your hand in front of... It stayed that way for about 7-10 minutes, then gradually became cooler (seems consistent with the idea of a "cool-down"). Only problem with this cool-down at this specific time was that the cabin temp had only raised 6 degrees (51 to 57) and I had set the temp for 19 degrees higher than that (at 70) using the thermostat.

I'm guessing the flame is being extinguished when I start to feel it cool down, but I'm not sure why. Any way to diagnose the issue? I can get a new furnace if need be, just want to make sure there actually is a problem with this one before I do so.

Thanks for your help!
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Old 10-04-2015, 10:12 AM   #17
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Quote:
Originally Posted by lewster View Post
Get a new furnace with DSI (direct spark ignition) and you won't have an more issues for a long time.
Got any recommendations for units you like?

The furnace I have is in really good condition physically (was hardly used by the previous owners, no rush or damage or broken seals), so even though it's old I'm just trying to ascertain if the problem I am experiencing is specific to this furnace, or if maybe there's something wrong with my installation (it's in a 100% renovated AS).

Here's the exhaust on the rear of the AS:

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Here's the installation of the furnace under the read bed:

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And a further away shot for some context as to the installation:

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Old 10-04-2015, 12:39 PM   #18
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Looks like whoever built the bench didn't do you any favors for servicing the furnace. Does the bench come apart at all? No matter what's wrong, you'll need to get the furnace out so you can either replace it or replace parts inside it. If you have to disassemble the bench, I'd replace the furnace rather than try and troubleshoot it.

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.

To answer your question, Vintage Trailer Supply sells direct replacement furnaces.

Chris
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Old 10-04-2015, 01:20 PM   #19
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Can you see what model and manufacturing date there is on the manufacturers plate? The furnace is probably attached to the floor with a couple of screws. Also attached to the wall at the inlet exhaust flange. Hopefully you can then slide it forward for removal.

Last time I checked Palomino RV Supply beats the competition prices by an Airstream mile, but they are short on advice and you have to know exactly what you want.
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Old 10-04-2015, 03:05 PM   #20
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Furnace heating then cold air

I take it that the furnace stops putting out hot air before it should but continues to run and put out cold air. If so, there is an easy to install, overheating sensor inside. If the sensor is becoming defective, it will stop the flow of gas and the furnace will continue to run, putting out cold air. Price for a replacement sensor is around $15 and as noted is easy to install.
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Old 10-04-2015, 03:36 PM   #21
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What Pete said ...
If it has electronic ignition and no combustion chamber leaks or rust inside why replace it?
If it has a flex hose connecting it to the gas line, as someone wrote earlier that should be replaced with the correct size soft copper using flare joints.
Not sure I would want my furnace under the bed ...
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Old 10-04-2015, 03:40 PM   #22
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Perhaps the rather small area the heater is in sets off the overheat switch. Try running the heater with the compartment lid open and see if it stays lit longer.

Dennis
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Old 10-04-2015, 03:47 PM   #23
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The Suburban Dynatrail series furnaces (which is what you have) are designed for the blower to continue to run for a couple minutes after the gas valve has shut and the flame (heat) is out. This is to expel any residual gasses in the combustion chamber. It has nothing to do with the age, condition, or temperature (cool down) of your furnace. They are extremely inefficient but thats what Airstream and most other RV manufacturers have installed for many years. Your furnace blower should run for 2 to 3 minutes after the gas valve closes and the flame is out. Sounds like your furnace is operating as it was designed. 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. You can replace the thermostat or just learn to set it higher than the numbers indicate. The furnace does have a high limit switch that automatically shuts down the gas valve if it senses that the internal temps are too high in your furnace. If the furnace never does reach the temp that is set on the thermostat then you have other problems with the furnace.
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Old 10-04-2015, 03:51 PM   #24
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http://www.gmceast.com/technical/Nic...an_Furnace.pdf

http://www.bdub.net/manuals/Suburban_Service_Manual.pdf
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Old 10-04-2015, 04:06 PM   #25
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What happens when you raise the set point temp, will it burn longer/warmer than the 57* set @ 70* ?
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Old 10-04-2015, 04:10 PM   #26
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On mine, brand new furnace old thermostat, the thermostat is about 5 degrees off so I just set it higher and the furnace will run until it reaches the set temp. Ive just gotten use to it.
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Old 10-04-2015, 09:02 PM   #27
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Quote:
Originally Posted by coloradopete View Post
I take it that the furnace stops putting out hot air before it should but continues to run and put out cold air. If so, there is an easy to install, overheating sensor inside. If the sensor is becoming defective, it will stop the flow of gas and the furnace will continue to run, putting out cold air. Price for a replacement sensor is around $15 and as noted is easy to install.

Thanks for the tip! Do you know how I can tell if the sensor is bad or if it's actually doing its job and turning the flame off if the unit is getting too hot? I guess I could just replace the sensor and see if the behavior changes; that would answer my question...
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Old 10-04-2015, 09:04 PM   #28
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Perhaps the rather small area the heater is in sets off the overheat switch. Try running the heater with the compartment lid open and see if it stays lit longer.

Dennis

That's a great point; I hadn't thought of that. I'll try it and report back. Thanks Dennis!
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