Another furnace won't light problem

[DiegoWest]

Hello again Wayne,
Just doing some trouble shooting on my furnace in which the fan comes on but the heater never clicks on. I removed the screws on the exterior vent but the grate still doesn't come off. Is it supposed to be able to come off? Don't want to break anything. Just want to see if there's anything I can clean up even though there doesn't seem to be any visible obstructions.
Right now, I no longer hear any clicking sounds in which the igniter even attempts to ignite. Thanks.

It's most likely an air flow issue. The furnace has a "Sail switch" in the air intake and the exhaust outlet. If there is any airflow disturbance when the fan kicks off, the gas valve will shut off immediately.

It could be spider webs, mud dobber nests or a host of other obstructions. Those furnaces are pretty indestructible and very serviceable. They seldom need replacing.

 

[Shane826]

The vent probably has a bead of silicone holding it on.

 

[Bill]

I agree with Wavery - the sail switch is the most likely cause. I have experienced it, and found that it was easily fixed. Get access to your furnace, remove a few sheet metal screws, and lift off the sheet metal cover. Advance the thermostat, observe the switch, and if it doesn't swing all the way over, move it by hand, and see if the furnace fires up. In my case it did, so I was easily able to fix whatever was going on. I don't recall the specific problem, but it was most likely solved with a gentle shot of compressed air to clean out dust, and a small drop of oil in the right place.

Bill

 

[DiegoWest]

Thanks Bill,
Looks like we have close to the same 2720 QS model so I can ask you detailed questions about this. Did you need to pull the exterior vent grate off first? Looks like I'd need to disconnect the propane line before pulling the furnace out, correct? After that it looked like just a couple of screws held the rest of the unit in place before pulling it out.
Thanks in advance!

 

[Bill]

Rob -

I actually did this on an earlier TM, not the present one, but no, there was no need to remove the outside vent. In fact, there was no need to remove the furnace from under the sofa. Everything was accessible from the top.

Bill

 

[Wavery]

Hello again Wayne,
Just doing some trouble shooting on my furnace in which the fan comes on but the heater never clicks on. I removed the screws on the exterior vent but the grate still doesn't come off. Is it supposed to be able to come off? Don't want to break anything. Just want to see if there's anything I can clean up even though there doesn't seem to be any visible obstructions.
Right now, I no longer hear any clicking sounds in which the igniter even attempts to ignite. Thanks..

As Bill said, you don't have to remove the outside exhaust pipe assembly (that's not just a vent cover). You only remove that if you want to permanently remove the entire furnace from the cabinet. The furnace can slide out of the cabinet from inside the camper with the interior grille removed.

Check out this video. There may be better videos for that furnace but I couldn't find one.

Basically, this video will give you a "maybe" fix trick but more importantly, it will show you how the furnace comes out. The exhaust pipe just slides out of the furnace as you pull the furnace out of the cabinet.

I don't know the make or model # of your furnace so this is about as close as I can get.

 

[DiegoWest]

Thanks Wayne,
The furnace in that video did look like mine. I have the suburban NT-20SEQT as you can see from the uploaded picture. If you look at the picture of the front of the furnace with the grill removed, it shows the furnace within it's steel enclosure.

It shows the propane pipe coming in from the right. If you look at the one photo I think I can even see the sail switch on the left. I also uploaded a picture of the top of the unit with the exhaust pipe going out the back. The difficulty is that this steel enclosure is one piece.

It would be very convenient if I could pop the top off to see the insides but that's not possible. It looks like it's only possible to disconnect the propane pipe and then disconnect a bunch of electric wires before then pulling out the entire enclosure. After that I can see or feel some screws on a plate on the back of the unit that would have to come off in order to expose the sail switch behind it.

So it looks like they've made this thing extremely difficult to work on if all you need to do is check or replace the sail switch. Am I missing something?

 

[Wavery]

I think that you have to remove the furnace. It isn't hard.

*shut off the propane at the tanks.
*remove the ground wire from the battery.
*disconnect the propane line from the furnace.
*Either unplug the wiring or mark and cut the wires (to be spliced later)
*Remove the screws that screw the furnace to the floor.
*Pull the furnace straight out.

I think that I see an inspection plate located near the sail switch. Remove that plate to gain access to the switch.

It's been about about a dozen years since I've removed that furnace. I just remember that aligning the exhaust pipe for re-installation was a little tricky but doable.

 

[Bill]

If you go to RV Parts Shop
https://rvpartshop.com/pages/suburban-furnace-12vdc-19-000-btu-product-id-nt20seq-parts-list
you will see exploded parts diagrams. I don't see any page numbers, but go to the end of the last page of the site, then scroll upward to find Item 11 in the diagram. In the parts list, this is called "micro switch" instead of "sail switch", but this is the item in question. Using the diagram, you can see where it is mounted, and by clicking the part number 233100 you can see what it looks like.

My memory - not so trustworthy these days - says I was able to thread a piece of stiff wire into a hole or grate somewhere, and push the sail around. Once I confirmed that the sail was the problem, I was able to put a drop of oil on the end of the wire, and carefully oil the pivot point.

There may be better diagram somewhere - this was just the first I found as I tried to help. But using the term "micro switch" rather than "sail switch" may help you find it.

Bill

 

[DiegoWest]

Thanks Bill and Wayne.
Bill, I'd say the only way to get at this sail switch without pulling out the entire enclosure is by running a stiff wire down the left side of the enclosure after taking the front grate off. With a narrow stick, I had already been able to push a flexible sail back which must have been the sail switch. The location definitely agreed with the diagram you referenced. But I was most likely pushing it in the wrong direction to activate it since the fan would blow air to push it in the opposite direction. This makes me think a stick with a hook on it to pull the sail toward me might activate the switch if that's the problem. Does that sound like what you might have done when you were playing with it? And then I'll try putting a tab of oil on it to see if I can lubricate the spot where it pivots from.

Wayne, if that doesn't work, then I may give it a shot to pull out the entire enclosure following your instructions.

Thanks again guys for the suggestions!

 

[Wavery]

Bill's suggestion is certainly worth a try. I'd suggest a real lite oil, like 3-1 oil.

I would not use WD40.

https://www.amazon.com/3-ONE-10038-...8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1

 

[DiegoWest]

Unfortunately, it's looking like my furnace problem might not be a sail switch issue. When I manually pull the sail back with a pole and a hook while the fan is running the igniter still does not ignite.

With the fan on and the sail pulled back, I checked the voltage at the igniter. It goes from zero volts with the blower off to about .25 volts with the blower on. So it's like the control board tried to do deliver 12 volts to the igniter but came up short. Or at least that's one possibility. I suppose it's still possible that it could be related to the sail switch but visually there was very little dust anywhere, the fan comes on quite strong and the air intake and outflow from the rear vent seemed pretty good too. So I don't think it's an airflow problem.

To do any more troubleshooting it looks like I'll have to pull the furnace out. A control board or limit switch problem are other possibilities. Odd that anything would go bad though if these are supposed to be very reliable in general and my TM is only a 2023.

Tempting to just get a heater buddy at this point as I'd be disappointed to spend alot of money on various parts and still come up short. A new unit would be another possibility but that looks pretty pricey at over $700.

 

[Wavery]

You couldn't pay me enough $ to go back to using that furnace again. We are very happy with Mr Buddy. We have 2- dogs and never have an issue with them getting too close. If our crazy beagle gets crazy and knocks it (never has yet) the heater shuts off and has to be re-lit. It doesn't take much of a bump for it to automatically shut down.

 

[Bill]

When I manually pull the sail back with a pole and a hook while the fan is running ...

So the airflow from the fan is not sufficient to move the sail switch? Odd, but once you have moved it all the way by hand, I'm with you - I don't know how the sail switch could be the problem.

Don't be in a hurry - once the sail switch opens, spark will not begin for 5 or 10 seconds while the gas valve operates. When the igniter starts to operate, I think you should hear a tick-tick-tick of the spark, and I gather you don't hear it. Once you hear the ticking, actual ignition of the flame may be delayed even further while gas flows through the pipes from the propane tank.

That reminds me - this furnace sometimes has a gas valve in the gas line - item 55 in the same diagram. Since you have a fresh-from-the-factory 2023 unit, it might never have been turned on. Has your furnace ever worked?

I assume you have confirmed that the thermostat on the sink apron is actually delivering +12 VDC to the unit? Move the lever all the way to the right.

Bill

 

[Wavery]

So the airflow from the fan is not sufficient to move the sail switch? Odd, but once you have moved it all the way by hand, I'm with you - I don't know how the sail switch could be the problem.

Don't be in a hurry - once the sail switch opens, spark will not begin for 5 or 10 seconds while the gas valve operates. When the igniter starts to operate, I think you should hear a tick-tick-tick of the spark, and I gather you don't hear it. Once you hear the ticking, actual ignition of the flame may be delayed even further while gas flows through the pipes from the propane tank.

That reminds me - this furnace sometimes has a gas valve in the gas line - item 55 in the same diagram. Since you have a fresh-from-the-factory 2023 unit, it might never have been turned on. Has your furnace ever worked?

I assume you have confirmed that the thermostat on the sink apron is actually delivering +12 VDC to the unit? Move the lever all the way to the right.

Bill

Voltage is a big deal. The fan will operate until the battery is flat dead. However, the gas valve won't open unless the furnace motherboard gets a minimum of 11.0V. Even that is iffy. If the battery is at 11.0V and the gas valve tries to open, that will lower the voltage below 11.0V and it will shut off again. (I'm pretty sure it's 11.0.... it may be 11.5. I don't remember exactly.)

Even if the gas is shut off, the gas valve will open (if everything is working right) and the ignitor will click (that clicking is usually pretty loud). It will make 3 attempts and if there is no ignition, the motherboard will shut everything down. The motherboard is dumb and assumes that the gas is flowing when the valve is open. No ignition is the signal that something is wrong.

In order to get it going, you must wait a certain amount of time (maybe 10-15 minutes) before trying again.

If the ignitor isn't clicking at all, it could be the sail switch or motherboard. The sail switch is more common. The only other thing that it could be is obstructed air flow. If something (like a kleenex) got sucked up in the intake, it would wrap itself around the squire-cage blower and stop the air-flow. In that case, you would hear the fan running but little to no air will come out of the front vent of rear exhaust pipe. That's why the sail switch is there. No air flow could equal a gas build-up and eventually an explosion (which never happens).

 

[Bill]

Rob -

If you decide to stop work on the furnace, please let us know so we can close out the effort.

Bill

 

[DiegoWest]

Voltage is a big deal. The fan will operate until the battery is flat dead. However, the gas valve won't open unless the furnace motherboard gets a minimum of 11.0V. Even that is iffy. If the battery is at 11.0V and the gas valve tries to open, that will lower the voltage below 11.0V and it will shut off again. (I'm pretty sure it's 11.0.... it may be 11.5. I don't remember exactly.)

Even if the gas is shut off, the gas valve will open (if everything is working right) and the ignitor will click (that clicking is usually pretty loud). It will make 3 attempts and if there is no ignition, the motherboard will shut everything down. The motherboard is dumb and assumes that the gas is flowing when the valve is open. No ignition is the signal that something is wrong.

In order to get it going, you must wait a certain amount of time (maybe 10-15 minutes) before trying again.

If the ignitor isn't clicking at all, it could be the sail switch or motherboard. The sail switch is more common. The only other thing that it could be is obstructed air flow. If something (like a kleenex) got sucked up in the intake, it would wrap itself around the squire-cage blower and stop the air-flow. In that case, you would hear the fan running but little to no air will come out of the front vent of rear exhaust pipe. That's why the sail switch is there. No air flow could equal a gas build-up and eventually an explosion (which never happens).

Good information there.
When I was testing, I did wait a minute or two holding the sail fan forward before giving up on the clicking sound. I tried that multiple times although not necessarily waiting 10 to 15 minutes between tries.

I think the air flow is probably good just because it comes out the front side real strong. It seems to come out much stronger on the left side though than the right if that means anything. There's air coming out the exterior vent also and a little suction from the bottom of the exterior vent which I believe is the intake area.

There's no voltage issues in general on the RV just because the battery is at 13.25 volts. I can only say that the voltage was at .25 volts at the igniter because that's the only easily reachable spot.

I'm thinking about pulling the furnace out within a few days although I'll have to have a way of reconnecting the 4 wires that I cut to reconnect them with longer lengths to do more troubleshooting.

Thanks for your feedback guys. I haven't given up yet... Take care..

 

[DiegoWest]

So the airflow from the fan is not sufficient to move the sail switch? Odd, but once you have moved it all the way by hand, I'm with you - I don't know how the sail switch could be the problem.

Don't be in a hurry - once the sail switch opens, spark will not begin for 5 or 10 seconds while the gas valve operates. When the igniter starts to operate, I think you should hear a tick-tick-tick of the spark, and I gather you don't hear it. Once you hear the ticking, actual ignition of the flame may be delayed even further while gas flows through the pipes from the propane tank.

That reminds me - this furnace sometimes has a gas valve in the gas line - item 55 in the same diagram. Since you have a fresh-from-the-factory 2023 unit, it might never have been turned on. Has your furnace ever worked?

I assume you have confirmed that the thermostat on the sink apron is actually delivering +12 VDC to the unit? Move the lever all the way to the right.

Bill

Yes, this furnace has worked before and when it did I heard the clicking sound.

One test I did to rule out that it was a thermostat problem is that I connected the white wire to the red one on the thermostat. When I did this it immediately turned on the fan. I waited a couple of minutes after that but still no hot air coming out.

 

[DiegoWest]

Rob -

If you decide to stop work on the furnace, please let us know so we can close out the effort.

Bill

I did some more troubleshooting. I checked and there's definitely 13 volts on the red wire of the thermostat. So the thermostat is fine.

I looked at the furnace more carefully and I could see that the limit switch is actually accessible. See picture. According to troubleshooting guides it's supposed to have 12 volts on each side of the switch where the red and brown wire is going in to if everything is running correctly. It had 12 volts only on one side when I checked but zero on the other. According to the guides that was an indication that the switch could be bad. To test, the guides recommended shorting the brown and red leads going in to it that you can see from the foto but when I tried that it did not cause the furnace to try and ignite.

If you look at the foto carefully though, you can see a red open ended connector near the orange wire. It's not very visible but there's a blue wire attached to the red connector. This sure seems like it must have fallen off from something as it would seem like a rather shoddy design if it's actually supposed to be like that. Just looking and feeling around that area of the board though I couldn't see anything the red connector would plug in to that it would have fallen off of.

In any case, this makes me more tempted to cut the wires outside the furnace enclosure and pull the whole furnace out now. As it looks like with a little more disassembly that that whole area in question would become more visible.

I'll start thinking about this again after the Super Bowl. lol

 

[Bill]

The limit switch (Item 17 in the diagram referenced in post #29 above) is supposed to be closed normally, and opens only in an overheat situation. A quick check is to remove both wires from it, and measure its resistance. The resistance should be very close to zero. If it is very high, it is stuck opened. In this case, give it a couple short sharp bops with a screwdriver handle and see if it closes. That is known as BILL'S UNIVERSAL FIX.

If shorting across it didn't work, that means there are multiple failures. While possible, this seems unlikely, so while you have it shorted, re-check everything carefully, using all the advice about wait-timing above.

I see the empty red connector you mentioned - good eye. I think I see a blue wire, but a better photo of it would help. Are you thinking that the blue wire should be coming out of the empty red connector? Is the end of the blue wire actually not connected to anything, as if it were broken off? Is the end of it stripped, leaving a half inch of bare copper showing, as if it had pulled out of the red connector? Is it long enough to reach the red connector? Bear on mind that an empty crimp connector may be what is intended by the mfr, and does not necessarily indicate a problem. In this case, it could be part of a thermistor circuit, which is not used in this furnace.

Bill