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ATSL77 · 03-15-2011, 10:40 PM

I'm having an issue with my Suburban furnace in a 77 Sov. I apologize as I'm not sure as the model number on it. However when I kick my furnace thermostat up the blower kicks in and I have a good flame in the chamber for about 10 seconds then the flame dies out and it blows cold again. I bled the propane system best as I know how and I don't believe it is a supply problem. I'm wondering if it maybe a thermal switch but am not real sure if its located on the inspection plate or further inside the camber to test it. If anyone could give me some insight I'd appreciate much. Thanks.

Corey

tinman54 · 03-16-2011, 05:28 AM

Could be just a cleaning issue. Can you get a picture of the ignition, burner area? There should be some type of flame sensor in there that probably needs cleaning. If you are not comfortable taking this apart or working with gas then you should take to a shop.

mojo · 03-16-2011, 06:37 AM

Sounds like the sail switch.

Lumatic · 03-16-2011, 08:54 AM

First I am not an expert on Suburban furnaces. But do know enough to know that messing with propane furnaces if you don't know what you are doing is not to be taken lightly.

I had similar problems with a burnt out electrode and cracked asbestos gasket. These are located in the front of furnace after you take off the cover. Both parts are relatively cheap and easily accessible.

ATSL77 · 03-16-2011, 07:32 PM

Thanks for the ideas. I will try to clean the old girl up try again and post the results.

HowieE · 03-16-2011, 07:49 PM

10 seconds is a short period of time for the high limit switch. The sail switch has to make before it will light at all. And 10 seconds is a short time for a defective thermostat.

Even give that I would jumper the thermostat and see if it will run longer than 10 seconds. I say this because I have seen thermostats that would cause short cycling. I had to shim mine further open with a piece of match book cover to get it to stay on for a reasonable time.

Lumatic · 03-16-2011, 08:57 PM

Quote:

Originally Posted by HowieE

Even give that I would jumper the thermostat and see if it will run longer than 10 seconds. I say this because I have seen thermostats that would cause short cycling.

Ditto that. Replacing the old thermostat stopped short cycling for me. You can use the cheapest Wally world thermostat you can find, but make sure you can adjust to low and high on/off level. It will probably say 24v but does not matter. Polarity does not matter. Just disconnect the thermostat and cross the 2 thermostat wires and see if she stays on.

ATSL77 · 03-17-2011, 07:20 PM

In your experiences with faulty thermostats did the blower keep running as well? MY blower keeps on truckin' even after the fire goes out in the chamber. I will try to cross up the wires on the stat though.

HowieE · 03-17-2011, 08:59 PM

Quote:

Originally Posted by ATSL77

In your experiences with faulty thermostats did the blower keep running as well? MY blower keeps on truckin' even after the fire goes out in the chamber. I will try to cross up the wires on the stat though.

The control board controls the fan and even after the thermostat opens the fan is kept running for about 30 seconds as a PURGE CYCLE to cool the heat exchanger.

Just as there is a 15 second purge cycle before the flame is lit to prevent an explosion.

dscluchfc · 03-17-2011, 09:16 PM

Could be the regulator valve on the furnace is dirty and providing irregular supply of fuel, and the board may be shutting it down.
Would suggest a complete cleaning and tune up.

**azflycaster** · 03-17-2011, 09:24 PM

A furnace from the 70s does not have a control board, at least mine does not. It has a relay and several switches. The fan runs if the thermostat is turned up and the contacts close. It does not care if there is flame or not.

The sail switch, fan switch or limit switch could be defective. All of these are needed to hold the gas valve open.

wasagachris · 03-17-2011, 09:29 PM

Corey read this thread before spending any time trying to fix your furnace. It should be pulled and inspected first.
http://www.airforums.com/forums/f427...ace-75112.html

ATSL77 · 03-20-2011, 05:14 PM

Just a quick note, I did cross the thermostat wires with no different result. I do believe I will pull it and inspect and clean it. Thanks for all the suggestions and tips.

HowieE · 03-20-2011, 05:19 PM

OK if the thermostat is no the cause of the short cycling check the Proof of Flame system. Something is telling the control board that proof of flame was not seen and thus the gas valve is closed.

KYAirstream · 03-20-2011, 08:37 PM

Just curious if you figured out the problem? Mine has been doing the same thing. It's a new unit so I bypassed the thermostat , but no change. Tried a new limit switch, and again no change. Just to experiment, I flipped the limit switch around so it was facing away from the furnace and the burner worked fine without short cycling. So why does the burner cycle so frequently when the limit switch is facing the proper direction?

HowieE · 03-20-2011, 08:53 PM

If you effectively took the limit switch out of the circuit and the heater worked I would suspect the limit switch was bad. Limit switches are generally KlicksOns, a bi-metallic strip that flips at a set temperature. A cheap check would be to replace the limit switch and chech again.

dscluchfc · 03-20-2011, 10:52 PM

I tried to bust out the BBQ grill today...it would light, and in about 30 seconds was down to nothing and sputtering then out.
So I took the regulator off an old grill. It works great with the old regulator.
Might be a similar situation with your furnace regulator if all your other systems (water heater, stove, etc.) work just fine.
just a thought.

HowieE · 03-21-2011, 07:44 AM

There is no regulator involved in the heater or hot water heater. The regulator is just after the bottles in the gas line.

Yes the spring in the regulator for the smaller portable appliances will loose it's umph in time. Another frequent problem is the presence of earwigs. They love those tight spaces and crawl in for a rest.

KYAirstream · 03-23-2011, 02:33 PM

This is my second limit switch, so I guess unless I got really unlucky it is isn't bad. The unit is clean, so no issues there. Any other suggestions?

KYAirstream · 03-23-2011, 02:41 PM

I also tested the continuity of the limit switch. While the burner is on, there was continuity. When the burner cycled off, there continued to be continuity when the positive lead of the tester contacted the white wire, and the negative lead contacted the black wire. However, when reversed (that is, positive lead on black wire, negative lead on white wire), I did not have continuity. I can't figure out why this would be the case, since this is a "switch" and I wouldn't think the way the leads are applied would matter.

A little more data if it helps.... fan runs the entire time, but the unit will operate with the burner on for approximately 2 minutes, then burner off for approximately 1 minute, back on for 2, off for 1, etc. This cycling is with the front panel of the burner off since I was testing the continuity of the limit switch. When the front panel is installed, the cycling is more frequent. I've got 4 ducts, plus a small duct to the holding tanks. I can't see why things would be overheating, especially when the the front panel to the burner is removed, thereby causing minimal restriction in terms of airflow.