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Old 10-03-2015, 10:07 AM   #1
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1979 31' Excella 500
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Furnace blows hot, then cold. What gives?

Anyone had this happen before? We have a Suburban Furnace and it will blow hot air for a while (maybe 5 minutes) to heat the cabin, then a few minutes later the air is blowing but no longer warm/hot. Thermostat is on the whole time (it's not the one turning off the warm air). I can turn the thermostat off, wait a minute, then back on, then the furnace will again blow warm/hot air for a while then go to cold.

Any clues? Do I need a new furnace?
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Old 10-03-2015, 10:29 AM   #2
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Perhaps There is a "cool down cycle"? Our 1999 will run a bit after "heat" cycle.
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Old 10-03-2015, 10:35 AM   #3
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Your furnace is pretty old. I would consider replacing it with a new furnace if you need to drop a bunch of money into it for repair . It is hard to find anyone willing to work on old furnace for liability reasons. A new furnace would be so much easier to use and efficient.

The problem may not be the furnace at all. It could be the regulator on your tanks. If it also is original it is living on borrowed time. Same thing applies to your thermostat. You could rule this out by disconnecting the wire leads and crossing them. If the furnace now runs OK that is your problem.

Any cheap thermostat will work even it it says 24v. A new regulator will cost you about $50.
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Old 10-03-2015, 10:49 AM   #4
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If it is an original pilot light furnace (not electronic ignition) it is time to replace it anyway. You can play whack a mole with them, fix one thing, another pops up. They rust out, become dangerous, and that era of Suburban furnaces had recall issues. No recall parts are available from Suburban any more.

Even if the furnace has already been replaced with a newer one, often finding the real issue, be it sail switch, temp limit switch, circuit board issues, gas valve issues etc can be a real PITA. Unless you are a dedicated and knowledgable tinkerer and just love to pull furnaces apart, you are often better off just replacing it.

I am such a tinkerer, but over the years and many rigs, have learned my lesson, and now just replace old furnaces with new ones and LIVE happily ever after. LIVE is the operative word here, there is danger lurking in old rusty furnaces.
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Old 10-03-2015, 11:02 AM   #5
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Yes, I agree with idroba, it may be time to replace if it is original. Our '66 Overlander has a furnace from the late 90's with electronic ignition and has a bad overheat sensor. The problem sounds the same. Comes on for a few minutes and the sensor cuts it off. I replaced that and works well now, but I was risking the ' whack a Mole' syndrome. If yours is original, then it is something else most likely.
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Old 10-03-2015, 01:55 PM   #6
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The older furnaces did have a cool-down period. It makes sure everything inside there is cool before the fan shuts down. Safety stuff, old style.
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Old 10-03-2015, 02:05 PM   #7
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Because That is What is Supposed to Do...

There is nothing wrong with your furnace.
At the end of a heating cycle, after the the thermostat signals that desired tenp as been reached, the gas valve closes, the flame goes out, and the blower fan runs until the burner is completely cool.
This does not indicate need for a new furnace.
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Old 10-03-2015, 03:56 PM   #8
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As mentioned, the newer are safer than old. The older tend to develop holes that can allow combustion gasses into Your breathing air yielding a permanent nap. Please have it professionally evaluated or replace yourself if you have skillz..
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Old 10-03-2015, 04:07 PM   #9
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Quote:
Originally Posted by trekerboy View Post
Anyone had this happen before? We have a Suburban Furnace and it will blow hot air for a while (maybe 5 minutes) to heat the cabin, then a few minutes later the air is blowing but no longer warm/hot. Thermostat is on the whole time (it's not the one turning off the warm air). I can turn the thermostat off, wait a minute, then back on, then the furnace will again blow warm/hot air for a while then go to cold.

Any clues? Do I need a new furnace?

How old is your furnace?
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Old 10-03-2015, 05:08 PM   #10
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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.
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Old 10-03-2015, 05:55 PM   #11
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1979 31' Excella 500
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Quote:
Originally Posted by Alumaholic View Post
There is nothing wrong with your furnace.

At the end of a heating cycle, after the the thermostat signals that desired tenp as been reached, the gas valve closes, the flame goes out, and the blower fan runs until the burner is completely cool.

This does not indicate need for a new furnace.

That's what I thought, but the thing is that the desired temperature has NOT been reached... In my case, I set the temp at 70 but the temp never rose above 61.
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Old 10-03-2015, 05:58 PM   #12
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Quote:
Originally Posted by nrgtrakr View Post
How old is your furnace?

I'm not sure. We are the 3rd owners, I'm guessing it's original to this 1979 Excella 500.
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Old 10-03-2015, 06:05 PM   #13
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Try setting the thermostat higher? The temperatures and setting on those old thermostats are pretty unreliable...

The other option is that you're low on airflow, which is allowing the heat exchanger to overheat. This could be due to blockage in ducts or at outlets, or it could be due to wear on the brushes in the furnace blower motor.

In the latter case, you might as well bite the bullet and buy the new furnace.
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Old 10-03-2015, 09:32 PM   #14
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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.
I have seen several instances where the intake was closed, once by physical damage to the opening and once by mud dabbers. Another time the owner had leaned something against the side of his Airstream and blocked the flow of intake air.
Don't be too worried about the age of your furnace. If properly cared for they work for years and years. Few of us use them enough to burn out the heat exchangers and get leakage of combustion gases into the coach. But you must install a carbon monoxide detector as suggested.
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Old 10-03-2015, 11:02 PM   #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, 08:59 AM   #16
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Quote:
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, 09:12 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, 11:39 AM   #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.

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Old 10-04-2015, 12:20 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, 02:05 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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