Furnace gremlins? Suburban SF-42

[Bob&Pat]

Furnace gremlins?

Furnace cycled fine in December 2007. Exercised over night at home in cold Michigan with the thermostat set at 56 degrees.

At home the motorhome furnace, Suburban SF-42, worked fine off of the 110 for about three days prior to leaving. Worked fine from Jan 10th. 2008 until last night here in Florida with the thermostat set at 64 degrees for overnight.

Conditions: Temperature outside was 38 degrees. LP tank just under ½ full on the LP tank gauge outside on the tank. Plugged into 110 shore power.

1:00am woke up furnace fan came on and the furnace did not ignited it was 63 degrees, the thermostat was set at 64 degrees. I shut the furnace switch on the thermostat off and back on again and the furnace fan came on and the furnace ignited after three tries of the off and on again.

2:00am woke up furnace fan came on and the furnace did not ignited it was 63 degrees, the thermostat was set at 64 degrees. I shut the furnace switch on the thermostat off and back on again and the furnace fan came on and the furnace ignited after two tries of the off and on again routine.

3:00am woke up furnace fan came on and the furnace did not ignited it was 63 degrees, the thermostat was set at 64 degrees. I shut the furnace switch on the thermostat off and back on again and the furnace fan came on and the furnace ignited after one try of the off and on again.

Also plugged in a auxiliary electric heater. The electric heater however doesn't circulate very well.

5:00am woke up turned the thermostat up to 66 degrees, I shut the furnace switch on the thermostat off and back on again and the furnace fan came on and the furnace ignited after one try. Unplugged the electric heater. The furnace ran fine from 5:00am until 9:30am.

Just hate it when things work sometimes and sometimes not, sure enough if you take it in for service it will work just fine for the service technicians.

Any ideas?

Furnace gremlins?

Bob

[Ptribbey]

Had nearly the same problem. I took the furnace apart and found lint stuck in the sail switch so I cleaned it and worked the switch a few times and it worked. The furnace worked fine for a couple of weeks, then started the same thing again. this time I checked the sail switch with a meter and it was open. When I removed it and checked it again it started working. I replaced the switch (cost about $7.00) and that was the end of the trouble.

Hope this helps.

[Bob&Pat]

Pat, Thank you for your reply.
I will check out the furnace.

Last night was cold again about 38 degrees and the furnace worked just fine.

Bob

[Jackm]

Bob&Pat

I would give tech support at Suburban a call. Since they deal with thousands of problems like this every year, they can often steer you to the right solution. Also, keep in mind, your furnace only runs off your battery, even if you're plugged into shore power. As a result, you can eliminate the power source as a probable factor. Best of luck.

Jack

[Bob&Pat]

Last fall I decided to take a look for myself. When I removed the exhaust Vent Cap the exhaust tube had large pieces (scales for lack of a better word) nearly the size of 1/3 to 1/2 the size of the inside circumference of the exhaust tube. Maybe the rust piece was larger than a quarter but less than a half a dollar. I was thinking could this piece of rust scale sometimes turn or twist to block airflow and cause my problem? So I removed the furnace to inspect it and clean it. I was surprised to see very little dust and lint in the blower.

I had the furnace out of the motor home and on my bench. The response from Suburban Furnace was to send me some specifications on the furnace combustion to hand over when I call a furnace tech.

My last contact with the factory in March on 2008 they suggested it may be a limit switch. The limit switch is to control a over heating situation and my problem was happening when the furnace was cold so common sense tells me it was not the limit switch.

Looking straight down at the mounting flange for the sail switch it seamed to be bent toward the combustion chamber so as to let the arm of the sail switch drag on the blower housing. With cold temps and expansion and contraction of metals I believe it would stop the sail switch from lifting far enough to start the furnace.

Well since the factory response was of no help I bent the sail switch back into what looks to me to be the correct position and not rubbing or dragging on the inside of the blower fan housing as it was.

Reinstalled the furnace and tested it about six to eight times and it worked fine.

However only time would tell if I solved the Furnace problem.

We are now in Florida with this cold spell we have had and the furnace has worked fine for about 45 days in January and February.

[rickwils]

I had a similar problem and after removing the furnace and taking it apart found a small piece of rust/scale wedged into the gas jet. It would let some gas through but sometimes it was not enough to light the burner. Since the gas tube that holds the jet had a bracket welded to it, the inside of the pipe was subject to rust, and I think that was the problem. I had to clean and coat the inside of the tube to fix.

[Roadking]

Several of the above posts described potential problems, and you will need to do more "trouble shooting" to isolate the exact cause. This long, but hopefully will help you determine the root cause of your problem.

You mentioned you cycled the thermostat "on-off" several times to get it to light. This is a possible indication of potential "lock out" and cycling the thermostat results in a reset. First I'll describe the ignition cycle, then list possible problems occuring during this ignition cycle. I'm doing this from memory, so this may not be exact, but I believe it is very close.


OPERATION

1) When the thermostat is closed the suburban SF ignition cycle first runs the blower for about 30 to 60 seconds. This ensures any unburned propane is cleared from the combustion chamber.

2) Then prior to attempting to open the gas valve and apply spark the "sail switch" must be closed and the "over temp" sensor must be closed. If either of these swithces are open then the SF will not open the gas valve nor will it apply spark.

3) If the above conditions are met, then the SF will open the gas valve and apply the ignition spark.

4) When ignition occurs the flame covers the ignition probe and flame on this probe causes a very small "micro-current". The SF senses this "micro-current" and if flame is "sensed" the SF continues combustion until the thermostat temp is reached, or until an error is detected . Errors could be overtemp, or sail switch opening due to low voltage (run down batteries while dry camping).


5) IF a flame is not detected (sensed) the SF will spark again. The SF repeats the spark attempt three times. IF flame is not sensed after the 3rd attempt then the SF goes into "lock out" and shuts down after the "cool down" period.

To reset "lock-out", you simply remove power by cycling the thermostat.

TESTING

Since your problem is intermittent, you may have to listen to the SF cycle several times in different temps and humidity to determine the exact cause. Corroded switches may work once then immediately fail the next time, so listen to the cycle several times prior to making a conclusion of where the problem is. You may also have multiple problems, such as dirt or rust in the chamber, a corroded sail switch, and a sail switch with dust which occassionaly prevents full movement. If you do determine the problem is associated with the sail switch, DO NOT BEND THE SAIL SWITCH or related components.

To determine what the SF is doing, stand outside near the SF and have someone close the thermostat. When the thermostat is closed you can easily hear the fan blower start. Once the initial blower timeout passes (30 to 60 seconds) you should be able to easily hear the gas valve open and the spark being applied. You should also be able to hear when ignition/combustion occurs.

IF you doe not hear the gas valve opening (a click), or the spark then there is a possible problem with the sail switch or the over temp sensor.

If the spark is heard and ignition fails then you may have a gas problem (unlikely) or the chamber is dirty causing the ignition to fail.

If ignition occurs and combustion burns for about 1 second, then goes out then the flame sensor is possibly not sending a signal to the the circut board. If this occurs you will hear the spark being applied again. After the third attempt to ignite the SF will go into "lock out".

Ignition followed by shut down (gas valve closing) could be caused by rust in the chamber (poor burning indicating cleaning is needed) or by contamination on the flame sensor (cleaning or replacement).

By listening to what the furnance is doing during the ignition cycle you should be able to narrow down the possible cause of your problem.

A voltmeter, or 12 volt test light can also be used to determine if power is being aplied thru the sail switch and the over temp switch for the duration of the iginition period.

Most likely you problem is contamination on the flame sensor/sparking probe, or contaminiation in the combustion chamber.

The SF is very easy to disassemble. To remove, disconnect the gas line, unplug the electrical connector, and remove the single screw in the bottom center. You should not have to remove the SF exterior housing from the coach. After removing the single screw, the entire furnace "guts" should slide out from the exterior housing. To clean the combustion chamber you will need to remove the burner assblembly, when doing so make sure the gasket on the burner is not damaged. Likewise there is a gasket on the ignition probe. Gently clean the probe and unless you have a "flame sensing" problem, do not bend the probe.

Good luck.

[Bob&Pat]

Thank you, Roadking for pointing out that many items can cause the furnace problems, your post and suggestions will help out many with furnace problems.

So far so good 57 days and counting. Furnace is still running flawlessly this winter here in Florida. (The furnace was needed 49 out of the 57 nights.)

For clarification of my post above:
“You had to be there to see it and hear it” the mounting flange and or bracket that was bent or out of alignment and the arm was dragging on the blower housing. For clarification of my post above, I did not bend the Sail Switch I bent the “mounting flange” with the Sail Switch attached to it. Was I taking a chance of course I was but it worked for me to solve an ongoing problem.

In 2008 when I call a furnace service tech they said if it is working when they come out they could not fix my problem. It was frustrating for sometime.

It was an ongoing problem since it was new and the first diagnosed solution was to replace the Fan Control Module Board and a certified repair facility replaced it. Worked after that on a test in the summer months when the heat was not required. It was some time before I realized it was doing the same thing.

[Bob&Pat]

Update 4/07/2009
Furnace is still running flawlessly this winter here in Florida. Last night it was 47 and it ran when needed.