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Old 03-26-2020, 09:56 PM   #1
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Help Troubleshooting Heater

I have a Suburban NT-12S propane heater that was working ok but making some weird noises (later found a dirt dobber nest was rolling around in the combustion fan area and the bearing on the electric motor is shot).

I removed the heater to clean it up a couple of months ago to try to fix the noise issues mentioned above. I blew out all the road dust, put some high temp paint on some of the rusty areas checked the sail switch was operating smoothly and reinstalled the heater. At this point, I didn't really do much other than clean it. I put it back together and reinstalled it.

This weekend I got my refinished propane tank installed and tried to fire up the heater for the first time since cleaning it and it would not work. The fan clicks on when I turn on the thermostat and turn up the heat but the furnace will not ignite. I tried the following steps to troubleshoot.

1. Verified there was sufficient voltage. This is not a problem. I was plugged into shore power and had 13.6 Volts at the battery. I also verified similar voltage at the unit.

2. Fan clicks on when the thermostat is turned on and the temp turned up.

3. I cycled the thermostat about 6 times per the instructions (steps to do when you fail to get ignition). I was assuming at this point that there was air in the line that just needed to get passed through.

4. Next, I tried firing up the propane water heater which is downstream of the heater. The water heater fired right up so there is propane supply is not the issue.
5. I tried cycling the heater about 6 more times but now I took the back cover off to try and hear what is going on in the heater. There was no clicking sound of the gas valve nor was there a clicking sound coming from the igniter. Still no luck in firing up. Through all the cycling I was not able to detect any propane smell at the unit at all.
6. At this point, I was convinced that it's not a propane supply issue and it is either the high limit switch or the sail switch is bad. I don't have experience with this unit but I was leaning toward this because there was no audible clicking of the gas valve or igniter.

I removed the propane unit to investigate and this is where my skills are lacking I tested the following.

1. Verified there is continuity across the high limit switch. I did this with the wires disconnected. I also checked if 12 volts was passing thru the switch by hooking up a spare battery and it is passing voltage with no voltage loss. It seems the high limit switch is ok unless I am missing something. <<Does the switch work the opposite way where there should be no continuity (or 12 volts) passing through this switch unless it is overheating. If this is the case then problem solved.>>

2. Verified there is continuity through the sail switch when engaged. I also did this with the wires disconnected and I tried to put 12 volts across the switch with no problem or voltage loss.

3. I hooked the entire unit up to a spare battery to bench test it without propane hooked up. When I engaged the thermostat by connecting the 2 blue thermostat wires together the fan clicked on no problem. I listened closely for any clicking sounds from the gas valve or the igniter and there is definitely no clicking from either.

Can anyone verify if I am missing something here? I am thinking that I have a bad ignition control board or I have a bad high limit switch that works the opposite of what I am expecting (it should not have continuity normally and have continuity when overheat is tripped). Seems odd that the control board would suddenly go out just from cleaning up the unit and reinstalling. I don't want to spend $90 on a new board if it is not the issue. Is there a way to test the ignition control board? Could the gas valve be the issue and is there a way to test it? I am assuming that the gas valve is not the problem since the igniter is not firing along with the gas valve not clicking.

I already bought a new sail switch (which operates the same as the old one so it was a waste of money) and bought a new fan motor that should fix the bad bearing squeal. I'm going to buy a new ignitor since it's only $10 and I have it pulled apart but don't want to throw money at a new board if I don't need it.

At the very least I'm learning a lot about the heating system in my van.
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Old 03-27-2020, 12:45 AM   #2
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I had a similar issue with a suburban heater in a 1995 travel trailer. Finally found that some of the connectors in the wiring coupler to the ignition board had become loose over the years and were no longer making a good contact. Fan would run but propane would never fire. This may not be your problem but worth checking.
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Old 03-27-2020, 12:33 PM   #3
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Since you had the tank out some crap could have gotten into the propane lines and plugged up the gas solenoid. The other thing I have seen is the heater igniter mount gets bent and the spark isn't close enough to the gas to light it. Or the flame detector gets bent out of position.
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Old 03-27-2020, 07:14 PM   #4
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Both the gas valve and igniter are not functioning so I am not suspecting a gas blockage but maybe I should be? I pulled the igniter and it looks good. I cleaned it off and reinstalled. I also voltage checked the igniter during the startup and it was not getting anything. I am bringing the ignition board to my local RV service place. They test the board for free I found out today so I will be doing that tomorrow morning. Hopefully, that's the problem. I will update the thread with what I find out.
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Old 03-28-2020, 04:58 PM   #5
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Ok, reporting back on the heater. It is now installed and working flawlessly. I replaced the fan motor and associated gaskets, sail switch (was not necessary), pulled off the combustion chamber door and pulled out the burner and cleaned out the chamber and burner and put a new gasket on the combustion chamber door and igniter. Installed a new ignition control board (this ended up being the reason the gas valve and igniter were not functioning.

After all is said and done, I have a very good understanding of how this unit works. Here are my recommendations for troubleshooting your fan is coming on but you are not getting a spark at the igniter and/or if your gas valve is not opening.

1. Check the high limit switch. There should be continuity across the terminals of this switch. (FYI, the high limit switch is a safey feature that detects an overheating unit and will cut gas and shut the unit off). See photo of where this is located below. You may consider running 12 volts across the high limit switch to make sure there is not resistance. You should read the same voltage you get directly on your battery and when testing across the switch. This may not be necessary but I have read its a good idea to make sure it's good.

2. Check the sail switch. There should be continuity across the terminals when the sail switch is engaged and no continuity when it is not engaged. To test, just pull the wires off the terminals and connect your multimeter wires one on each terminal and push the sail switch as if the fan is blowing it to engage it. If there is no continuity in either position the switch is bad. You should also check for clearance and free movement of the sail on the switch. This could cause it not to function properly.

3. If the above 2 steps pass the next step is to remove the igniter control board and bring it to an RV parts store that has a tester. The can hook it up to a device and let you know if it is good or not. Mine was totally dead and resolved my issue.

Hope this helps someone else troubleshoot the Suburban NT-12S.

In the image below 1 = High Limit Switch 2= where the Ignition Control Board is located (removed in this image).
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Screenshot 2020-03-28 at 3.56.37 PM.jpg  
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Old 03-28-2020, 06:02 PM   #6
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Good stuff CDA... thanks for the detailed info.
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