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-   -   Furnace blows hot, then cold. What gives? (http://www.airforums.com/forums/f287/furnace-blows-hot-then-cold-what-gives-141949.html)

trekerboy 10-03-2015 10:07 AM

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?

cwf 10-03-2015 10:29 AM

Perhaps There is a "cool down cycle"? Our 1999 will run a bit after "heat" cycle.

Lumatic 10-03-2015 10:35 AM

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.

idroba 10-03-2015 10:49 AM

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.

KAYAKAMPERS 10-03-2015 11:02 AM

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.

mimiandrews 10-03-2015 01:55 PM

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.

Alumaholic 10-03-2015 02:05 PM

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.

cwf 10-03-2015 03:56 PM

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..

nrgtrakr 10-03-2015 04:07 PM

Quote:

Originally Posted by trekerboy (Post 1692340)
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?

drboyd 10-03-2015 05:08 PM

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.

trekerboy 10-03-2015 05:55 PM

Quote:

Originally Posted by Alumaholic (Post 1692423)
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.

trekerboy 10-03-2015 05:58 PM

Quote:

Originally Posted by nrgtrakr (Post 1692468)
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.

drboyd 10-03-2015 06:05 PM

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.

Alumaholic 10-03-2015 09:32 PM

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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