Another furnace won't light problem

[Kidkraz]

Rob, using the model number from your picture, Suburbans site lists this relay for the NT series of furnaces.



https://suburbanrvparts.com/suburban...-nt-qt-models/

[Bill]

Rob -

Be careful of the item in that Amazon ad. I can't tell for sure, but the text of the ad says the thing is powered by 120 VAC, and there is no AC power anywhere in your furnace. Your furnace contains only 12 VDC power. Beyond that, on the left end of the board in the second picture, I see two AC rectifier diodes designated D1 and D5, along with a power supply filter capacitor C2. If I am right, this board is intended for use in a Suburban water heater (such as the SW6-DE in the TM) where AC is available, but not in a furnace where no AC is available.

Bill

[Rob Culver]

Originally Posted by Kidkraz

Rob, using the model number from your picture, Suburbans site lists this relay for the NT series of furnaces.



https://suburbanrvparts.com/suburban...-nt-qt-models/

Hi Kidkraz,
The problem with getting one like this is that you also need another wire kit to go along with it in order to allow it to mount to the board because the size and position of the connector is different. Those wire kits seem to be hard to come by also.

I found the exact correct part on ebay and even though it's used, I may go that way anyway if I can't find the exact correct part new.

[Rob Culver]

Originally Posted by Bill

Rob -

Be careful of the item in that Amazon ad. I can't tell for sure, but the text of the ad says the thing is powered by 120 VAC, and there is no AC connection in your furnace. Beyond that, on the left end of the board in the second picture, I see two AC rectifier diodes designated D1 and D5, along with a power supply filter capacitor C2. If I am right, this board is intended for use in a Suburban water heater (such as the SW-6DE in the TM) where AC is available, but not in a furnace where no AC is available.

Bill

Thanks for pointing that out Bill.

[Bill]

Rob -

Since the fan comes on when it is supposed to, I don't think the time-delay relay is faulty.

The diagram shows an item called a Valve Switch. I'm not sure what it is, but since everything downstream of it seems proper, I don't think it is faulty.

On page 19 of the manual you linked, step 12, there is mention of a neon light that should be flashing as the unit tries to light. Have you checked that?

Bill

[Rob Culver]

Good question. I looked for that light and couldn't find it. I'm not sure if it wasn't there at all or just wasn't flashing.

[Bill]

I think you are approaching the end. Everything around the control board seems to be good, but the control board is not trying to open the gas valve when it should, and it is not generating a spark when it should. Incidentally, it should produce an audible spark regardless of whether the gas valve actually opens. (If sparks are produced but the flame doesn't start, then the flame sensor will signal the board to shut down, and quit sparking. It will reset and retry 3 times, then lock out.)

Unless you have some more ideas, I think you are in for a new board.

Bill

[Rob Culver]

Originally Posted by Bill

I think you are approaching the end. Everything around the control board seems to be good, but the control board is not trying to open the gas valve when it should, and it is not generating a spark when it should. Incidentally, it should produce an audible spark regardless of whether the gas valve actually opens. (If sparks are produced but the flame doesn't start, then the flame sensor will signal the board to shut down, and quit sparking. It will reset and retry 3 times, then lock out.)

Unless you have some more ideas, I think you are in for a new board.

Bill

I agree Bill,
You should hear a spark even if the gas valve doesn't open. That's fortunate for testing. Otherwise I'd have to hook it up to a gas supply just for testing.

Now the challenge will be to find the correct board.

[Rob Culver]

Update:

I got a used control board on ebay. I wanted to get a new one but suburban no longer sells the furnace and doesn't sell parts for it either. They do have a substitute part but that part was not the same size and also required an extra cabling adapter to make it work for another $80. To make matters worse, no one seemed to have that adapter in stock.

But good news! I got the used part for just $50. Plugged it in and it worked! I noticed that it made the clicking sound about 15 seconds after the fan came on when I had it on the work bench. The clicking sound it made sounded like a relay coming from the board. Should have unplugged the wire going to the igniter to verify that the sound was not from the igniter but I forgot to do that.

Then I put the furnace back in the TM and hooked the wires back up. Everything works fine now and the heater comes on. Kind of odd to have a control board problem like that on such a new Trailmanor, (2023), as everyone said the sail switch and limit switch failures are much more common.

Thanks Bill and Wayne and others for weighing in on this project! It helps to get second opinions before diving in on some something like this.

[Bill]

That was quite a process, and I'm glad you stuck with it to a successful end.

Thanks Bill and Wayne and others for weighing in on this project! It helps to get second opinions before diving in on some something like this.

That's what this Forum is all about - no one-and-done answers. Congratulations.

Bill