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Old 03-06-2009, 05:46 AM   #1
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My furnace has worked great until late last night. When the thermostat calls, the fan will turn on, run for 30 seconds and then shut down. No heat is ever present.

I rebuilt a NT-30 last year, but its problem was that the fan would run continuously and not light. This seems different. Any help appreciated.
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Old 03-06-2009, 11:16 AM   #2
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Give this LINK a try to trouble shoot.
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Old 03-06-2009, 01:45 PM   #3
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Thanks, I looked through the troubleshooting. Most were what I had already considered. The thing is, the fan runs 30 seconds and then shuts down. If I cycle the thermostat off and back to gas heat, the fan will start again and run for 30 seconds. I am not sure why it shuts down at 30 seconds unless the sail switch is not being closed.

I have not pulled the furnace yet. Will do that at the end of March. I just want to have a game plan and some ideas of what y'all would look for.
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Old 03-09-2009, 01:45 PM   #4
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Looks like you are on the right track. On mine the screws on the switch had come loose.
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Old 03-11-2009, 01:24 PM   #5
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Kehardman check the little breaker switch in the outside bay its a small black rocker type switch.

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Old 03-11-2009, 07:22 PM   #6
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The new furnaces will not allow the fan to continue to run if ignition failure occurs. They will shut the fan down so that it will not continue to blow cold air and run the battery bank down. You need to fix the ignition problem and the fan will continue to run normally.
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Old 03-16-2009, 01:02 PM   #7
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Kerdhardman:
You will need to do some "trouble shooting" to isolate exactly what your furnance is doing. This long, but hopefully will help you determine the root cause of your problem.

You mentioned you could not get it to "put out heat" and it runs for 30 secs and shuts down. This is a possible indication the burner is failing to light. THere could be several causes for this, so you need to determine what the furnance is really doing. This is long, but 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. Check the Suburban web site as they have good trouble shooting information.


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

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 do not hear the gas valve opening (a click), or the spark (more clicking) 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 combustion 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.

A common problem is the sail switch not closing, or the switch failing. You can check continuity on the sail switch. Do not bend the sail switch metal arm. Your problem could also be the high temp ECO thermostat, which should have continuity.

If these are good, and no spark is detected then it most likely is the circuit board.

If you do get spark followed be an initial ignition and then the combustion stops then there may be contamination on the flame sensor/sparking probe, or contaminiation in the combustion chamber. If the flame sensor is detecting flame, but the furnance still shuts down then once again it most likely is the circuit board unable to "sense" the flame.

If you get spark, but no ignition, then you potentially have a gas valve problem, or the circuit board is failing to open the gas valve.

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. The Suburban manual describes how to position the flame sensor.

Good luck.
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Old 06-10-2015, 08:57 PM   #8
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I just read your post and found it very helpful. While attempting to find a dead mouse I removed the exterior cover and found a large wasp nest which I removed with a shop vac. Then I noticed another nest in the larger blower wheel and attempted to clean it. I only had removed the combustion blower screws and possibly messed up the sail switch.
Another possibility is I shoved the shop vac into both the in and out of the combustion chamber and may have damaged or bent something. I will tomorrow use your info to completely remove the furnace because todays activitives have done nothing to find the dead stinking mouse and now my furnace does not light which it did before I started my misdeeds. anyway I notice on startup a hint of propane like it is trying to start but eventually times out from several tries to no avail. Any suggestion on what I may have done. If I understand you the sail switch must still be working since it tries to light. Boy did this SF work good before today. Ron
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Old 10-04-2019, 04:29 PM   #9
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I'm trying to troubleshoot my SF42 in my 2002 Horizon 34HD. It has always run top notch but now this fall it will not start. I get fan in the house and combustion fan also on. I think it could be failed sail switch or sail is stuck. How do you access the unit. It is installed above the water heater and below the floor. Seems not access at all.
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Old 10-04-2019, 04:43 PM   #10
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Quote:
Originally Posted by jhanan View Post
I'm trying to troubleshoot my SF42 in my 2002 Horizon 34HD. It has always run top notch but now this fall it will not start. I get fan in the house and combustion fan also on. I think it could be failed sail switch or sail is stuck. How do you access the unit. It is installed above the water heater and below the floor. Seems not access at all.
Note fans run for 4 min and then I hear a click and the off and I do not hear any ignitor clicks
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Old 10-04-2019, 04:53 PM   #11
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Quote:
Originally Posted by 007";20949]Give this [url="http://www.rvtechlibrary.com/heatair/furnace.htm
LINK [/url] a try to trouble shoot.
Link is dead now
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