Furnace Issue - Suburban NT-30 won't light

[vincewarde]

I have a Suburban NT-30 furnace that is giving me fits.  Before coach went into storage, it worked.  Then I got a wasp's nest in the blower.  I cleaned that out and it now spins up fine - but won't light.
So, next I inspected and replaced the igniter (it was badly corroded).  I can hear it firing, but still no flame.
I next checked the gas valve.  I can both hear it opening and a test light shows that both magnets are getting power.
All I can think to do is tear out the plumbing and make sure it is clear - and perhaps check to see that the valve is actually opening.  Yes, there is gas and yes, the valve to the furnace is open
Does anyone else have any ideas?  Thanks!

[DonD]

I put mine on the bench and powered it up. It turned out to be the sail switch wasn't free, was hanging up a bit. I freed that and exercised the gas valves and she woks well now.

[vincewarde]

According to the information in the manual, the sail switch has to be working for the gas valve to be energized.  Therefore, I don't think that is the problem.  I am really stumped.  Hm?

[DonD]

My valves wouldn't pass air when blown into but a few cycles and they freed up. Blow by mouth or very low pressure.

[joanfenn]

I know a lot of guys that have valves that pass air,  usually at the most inconvenient time too. W%

[OldEdBrady]

It's not very nice to make fun of us old folks.   W%

[Oz]

Joan, you said exactly what I was thinking too... LOL!

[audioguyinMI]

You say you can hear the igniter, but  can you see it?  On mine there's an inspection window and I can see it sparking when it's working. 

If you can't SEE yours and the gas is OFF, you could safely remove the igniter, leave it connected and place it on something non conductive and preferably non flammable, and verify that the thing IS actually making sparks, and not just noise.


If it is, then the only thing left is the plumbing and/or the valve itself.

[vincewarde]

Well, I finally recovered enough from most recent back problem to be able to check out the furnace valves.  When I put 12v to the valves with a jumper set up they open right up.  Tubes are clear too.  Next I'll put it back in the coach and see if I can get it to light...... if it doesn't fire up I'll check the igniter for spark.
In any case, I'll post the results......

[DonD]

Further troubles with mine so I benched it and found the grounding wire was bubbling on both sides of the plug. In the interest of safety I replaced it, caught a sale at PPL. EZ swap, no worries now.

I'll be watching with interest to see what you find.

[vincewarde]

Further troubles with mine so I benched it and found the grounding wire was bubbling on both sides of the plug.

I assume you are talking about the connection on the valve - correct?

[DonD]

No, the feed, both sides of the plug so I knew it wasn't a bad connection in the plug.

[DaveVA78Chieftain]

You will not get ignition spark if the sail switch is not engaged so given you are getting spark the sail switch is ok.
Have you purged the air out of your gas lines?  Sitting over the winter you usally end up with air in the gas line.  First thing is to get the burners on the stove working.  This will remove most of the air leaving only the line from the main supply line to the furnace unpurged.  Getting the air out of that line may take 20 or more tries depending the length of the line.

An in-depth description of furnace operation: http://www.classicwinnebagos.com/forum/index.php?topic=2640.msg5117#msg5117

You have a DSI model furnace.

Dave

[vincewarde]

OK, it's been a while, but we finally solved the problem today.

A few weeks ago, I pulled the furnace and took it apart.   I used a battery to cycle the valves a number of times.  They opened just fine.  So, at a total loss to understand what the problem, I put it back together.

Today we put the furnace back into place and hooked it up.  We ran the stove to make sure the gas line was as free of air as we could make it.

Then the big moment:  We fired it up.  It lit on the first try!!!!   

I can only assume that cycling the valves freed them up.

Thanks to everyone who offered advice!   

[moonlitcoyote]

I need to know more about tis cycling the valves part. My furnace is exactly 2 years old and it wont light. We have spark, lots of propane and no mud dobber nests in it. I cant find the problem anywhere.

[Oz]

Hi MJ, good to see you again.
Take a look at the topic,
"Anatomy of a furnace".

https://www.classicwinnebagos.com/forum/index.php?topic=2640.msg5110#msg5110

This should help.

[moonlitcoyote]

I did thank you. I need someone that KNOWS furnaces. What I have found is that I can blow through the gas valve IF I power both sides. BUT if I try to blow through it while the electrode is clicking I cant. It seems to be opening for just a second then closing again.

[moonlitcoyote]

Still no takers on this topic? I changed out the gas valve today after waiting to get it in the mail and still no heat. Everything seems to run perfectly except we get no heat. The fan comes on, the thermocouple starts sparking, no ignition so it goes through the sparking sequence 2 more times and still no ignition. Have run the stove to be sure there is no air in the lines. There is no gas smell from the exhaust. I'm at a total loss as to what to do next.

[Elandan2]

Here is the service manual for the Suburban furnaces.
http://techsupport.pdxrvwholesale.com/wp-content/uploads/2017/09/Suburban-Dynatrail-Furnace-Model-NT34SP-Service-Manual.pdf
Page 11 has the troubleshooting instructions for electronic ignition models. It seems like your problem may be # 5. Also, on the next page, item C might help you track down the problem.

[moonlitcoyote]

Thanks, I have done everything except check the gas pressure and the limit switch. I was told that if the limit switch was bad it would shut off AFTER starting. Mine wont even start.

But I am confused about this step: If sparking is audible approximately 15 seconds after the blower starts, but no ignition,
check the high-tension wire for continuity, grounding and secure connections. Repair if
necessary.

If there was a problem with the wire would it still spark?

[Oz]

If the switch has power and good ground, and does that, then the switch is malfunctioning.

If it has power and good ground and it's not even starting, then it's not functioning at all. 

Check the ground and that it is, in fact, getting power.

[Oz]

Here is the service manual for the Suburban furnaces.
http://techsupport.pdxrvwholesale.com/wp-content/uploads/2017/09/Suburban-Dynatrail-Furnace-Model-NT34SP-Service-Manual.pdf

We have these manuals right here  too...

[moonlitcoyote]

If the switch has power and good ground, and does that, then the switch is malfunctioning.

If it has power and good ground and it's not even starting, then it's not functioning at all. 

Check the ground and that it is, in fact, getting power.

Which switch is this? I still have no heat and it's going to be in the 30's this week.

[Oz]

By following the information referred to several times, and your description, you have gas supplied and you have spark.  But, it sounds like you have no gas to the spark.  This is controlled by the gas valve.  If you know there is gas going to the valve, the ignitor is working, but nothing lights, then there must not be anything to light.

Again, referring to the Anatomy of a furnace topic which covers all of this.
https://www.classicwinnebagos.com/forum/index.php?topic=2640.msg5110#msg5110

Dave laid out the process very well.  Scroll down to "Blower runs but furnace fails to ignite".

[moonlitcoyote]

I replaced the gas valve, working on limit switch now, I went through that post and also a bunch of youtube videos and anything else I could find. Just haven't figured out the problem.