Suburban Furnace NT20S intermittent operation

[KyDan]

Hey all you tech guys (and gals) especially anyone with RV appliance experience!!
Here's my story and a question at the end....

My 2008 Rockwood UltraLite TT has a Suburban NT20S furnace.
You can view a maintenance manual for it here-
https://docs.google.com/viewer?url=h...s/subntnew.pdf

Getting ready to go camping last weekend I flipped on the thermostat
to take the chill off while we were packing at home.
Blower came on immediately- normal
After a few seconds gas valve opened and spark ignited the burner- normal
Spark kept clicking for a few more seconds- NOT normal I think.
Gas valve shut- burner went off

Blower still running but no flame.
After about 10 seconds maybe 15 it starts again-
Gas valve opens
burner lights
igniter keeps clicking even tho the burner is lit
after 5 seconds gas valve shuts

This repeats 3 times then stops-board has faulted apparently.
Blower continues to run for 2 or 3 minutes then shuts off.
IF I cycle the thermostat off and on the scenario starts again.

The burner is lighting successfully each time but the board doesn't
-know- it's lit.
After 3 tries it faults out.

OK now look at the manual- figure 18 on page 21.
That is my schematic.
Notice there is NO flame sensor shown.
Look at the electrode assembly figure 21 on page 22- That is NOT like
mine!! Mine has the spark gap and ground but there is NO flame sensor.
Look at figure 22- that is like my board- there is a high voltage wire to
the electrode but no other wire for any flame sensor probe.
Figure 22 also says that is a direct spark 3 try ignition which is what I have.

I think mine just doesn't have a flame sensor-- WHAT do you think???

[onetonford]

Flame sensor can be the spark electrode itself I have had that on a home system. They can be one in the same.

[KyDan]

MORE of the story--
I have removed the furnace from the TT.
There are no spider webs or wasp nests in it anywhere.
The sail switch is working freely.
The sail switch works because if it didn't the furnace wouldn't try to lite.

I removed the burner and wire brushed it and the spark gap and
blew it off and out with compressed air.
Nothing looks amiss.
I took it back to the TT and hooked up the wires but not the gas
and flipped on the thermostat.
The fan started and ran, the gas valve opened and it started to spark.
OK- that same as before.
I hooked up gas line and it LIT and worked normally once.
I tried it again and it won't even try to light.
I cycled the thermostat and the main battery switch off and on in case
it had faulted.
No go. Now it won't even try to light. Blower runs nothing else happens.

Back to the bench-- Sail switch is free and clicks audibly and checks ok
with a meter. Sail switch can be seen to move fully over when fan is on.

Gotta be the board-- I pull the "card edge" plug off and it looks fine.
I plug and un-plug the card edge plug several times hoping it's just
tarnished and not quite making contact.

I again wire up the furnace with no LP and it again appears to be ready
to light. Gas valve opens- spark sparks.....

Today I am going to place it back in the TT and do a full normal run
test. Hoping it works!!

Still wondering how it's supposed to work -apparently without a flame sensor????

[KyDan]

Originally Posted by onetonford

Flame sensor can be the spark electrode itself I have had that on a home system. They can be one in the same.

Gotta be that I guess.... testing it later this AM. I'll report back.
dino boards makes a replacement for the board if I have to go there.....
Cost just south of $100 incl shipping.
I -think- it may work OK now that I've "cleaned" the contacts on the board
connections. (fingers firmly crossed!)

[gasman6674]

Dan
Jumper the limit switch and see if it runs. It could be weak and trippng due to false overheat.
Good luck

[Witch Doctor]

The first part, you do not have a flame sensor, The electrode go's to ground and turns the spark cycle off when it heats up. You could have a corroded connection at the board. If your limit was opened it would not do anything at all until it reset if it is the disk type, most of them have a little black button on them to reset it. If it cont. I would say the board itself is going out. Your on the right tract in my opinion. About maybe a month ago someone was having the same problem replaced everything and it came out to be the board in the end.

[BarryD0706]

Dan, as stated earlier, the spark electrode is also the flame sensor. It uses a technique called flame rectification (if you want to look it up).

Make sure the electrode is in the flame when it's burning.

Make sure the burner (which is the other side of the circuit for the flame sensing) has a good ground.

Take some emery cloth and clean any corrosion off of the spark electrode.

Sounds like you've already checked the connection of the electrode to the board.

If none if those things fix the problem, replace the board. There is nothing wrong with the sail switch or other components. It's just the flame sensor.

[KyDan]

There is a tiny window so I can see the burner and spark electrode when
the furnace is burning.
The spark is large and the burner lights immediately. The electrode very
quickly turns red/orange hot. It's directly in the blue flame.

Testing after a full re-install in a couple hours..... foggy and cold this AM.
Stay tuned and thanks for the suggestions/info.

[KyDan]

SUCCESS! I guess..... I hate it when you're not really sure what you did
(if anything) that made it work.

I pulled and re-seated the card edge plug several times even tho it was
not loose or unseated to begin with.
I guess that maybe polished off some tarnish and made better connections.

I had a dsi water heater some years back that one day decided not to work.
I opened the access door and saw a sticker on the board that said to make
sure plugs were seated before calling for service. I did the same thing then-
pulled the plugs and re-seated them a few times. The WH fired next try and
worked for years after.

Thanks for listening and for the information!
Happy (Fall) Camping!!

[wmtire]

I appreciate your detailed account of the steps taken, Dan. I have filed this one away, in case I encounter the same problems or read where another member has.

Glad you got it worked out.

[Witch Doctor]

Good for you Dan it sounds like a bad contact, you might want to spay some contact cleaner in the plug end and push it on and pull it off a couple of times. Glad you didn't need a new board....Happy Camping

[Herk7769]

This is why they call it troubleshooting!
Sometimes we shoot in the dark!

[KyDan]

FAILURE -- AGAIN...
Went camping last weekend.
Tried the furnace and it would not work.

As before it lights but keeps on sparking and shuts off.
After 3 times it faults out.
I cycled the power and tried again.
That time it lit and stayed on but I can't trust it to work when we
need it. We had electric available this time but I need to know it's
gonna work when we do need it.

Thanks to Barry for his " flame rectification" post. I googled and now know
how it's supposed to work without a separate flame sensor.
I will remove the burner assembly and carefully polish the spark probe and
the burner and try again. I can do that from the front end and don't have
to remove the furnace.
Since it works sometimes I feel like it's just -so close- that might bring it
back to the land of the living!

It's 31 deg and I'm gonna wait a few days for a little warmer weather
to pull it and try again. I'll report back later. I sure don't want to spend the
$100 on a new board unless absolutely necessary.

[Witch Doctor]

Originally Posted by KyDan

FAILURE -- AGAIN...
Went camping last weekend.
Tried the furnace and it would not work.

As before it lights but keeps on sparking and shuts off.
After 3 times it faults out.
I cycled the power and tried again.
That time it lit and stayed on but I can't trust it to work when we
need it. We had electric available this time but I need to know it's
gonna work when we do need it.

Thanks to Barry for his " flame rectification" post. I googled and now know
how it's supposed to work without a separate flame sensor.
I will remove the burner assembly and carefully polish the spark probe and
the burner and try again. I can do that from the front end and don't have
to remove the furnace.
Since it works sometimes I feel like it's just -so close- that might bring it
back to the land of the living!

It's 31 deg and I'm gonna wait a few days for a little warmer weather
to pull it and try again. I'll report back later. I sure don't want to spend the
$100 on a new board unless absolutely necessary.

DAN, Sorry to hear you are still having a problem, you can try and clean the electrode rod and make sure it's in the same position after. Check all your connections again make sure that your ground is clean and well connected. Also make sure the wire from the electrode is not grounding out before it plugs into the board. If it is still doing the same thing I would have to think it's your board, lets hope not.

[garbonz]

Originally Posted by gljurczyk

DAN, Sorry to hear you are still having a problem, you can try and clean the electrode rod and make sure it's in the same position after. Check all your connections again make sure that your ground is clean and well connected. Also make sure the wire from the electrode is not grounding out before it plugs into the board. If it is still doing the same thing I would have to think it's your board, lets hope not.

ITS THE BOARD. THIS is exactly what they do as they go bad. Dino boards are dependable and work well, but make sure you get the right model #.

There is nothing worse than an undependable furnace.

[KyDan]

Now that I know how it's done, I can pull out the burner/spark gap assembly
in under 5 minutes. Really- it just takes a little power phillips screwdriver and
it's pretty easy.
So I pulled it yesterday afternoon and bent the spark gap to be a little more
over the center of the burner. I shined it with a scotchbrite pad and put it
back.
I tested it yesterday and it lit the 1st try and stayed lit.
I tested it again this morning and it lit and stayed lit.
I plan to test it every day for a few days and then later on this winter.
If it fails to work even one time, I'll order a dino board but I'm gonna give
it one last chance to work......

If I was planning a cold camping trip without electric I'd probably just
replace the board now but our next outing is TopSail Hill Preserve SP
and they've got full hookups.
I plan to test the furnace just about every day on that trip and if it works-
great. If not, I'll spend the buck$ for a new board.


Fingers crossed!

[Witch Doctor]

Originally Posted by KyDan

Now that I know how it's done, I can pull out the burner/spark gap assembly
in under 5 minutes. Really- it just takes a little power phillips screwdriver and
it's pretty easy.
So I pulled it yesterday afternoon and bent the spark gap to be a little more
over the center of the burner. I shined it with a scotchbrite pad and put it
back.
I tested it yesterday and it lit the 1st try and stayed lit.
I tested it again this morning and it lit and stayed lit.
I plan to test it every day for a few days and then later on this winter.
If it fails to work even one time, I'll order a dino board but I'm gonna give
it one last chance to work......

If I was planning a cold camping trip without electric I'd probably just
replace the board now but our next outing is TopSail Hill Preserve SP
and they've got full hookups.
I plan to test the furnace just about every day on that trip and if it works-
great. If not, I'll spend the buck$ for a new board.


Fingers crossed!

Glad you got going again, intermittent problems are almost impossible to diagnose. You can only do what you can do, you now probably understand it better then any service rv outlet, good for you..

[garbonz]

Originally Posted by KyDan

Now that I know how it's done, I can pull out the burner/spark gap assembly
in under 5 minutes. Really- it just takes a little power phillips screwdriver and
it's pretty easy.
So I pulled it yesterday afternoon and bent the spark gap to be a little more
over the center of the burner. I shined it with a scotchbrite pad and put it
back.
I tested it yesterday and it lit the 1st try and stayed lit.
I tested it again this morning and it lit and stayed lit.
I plan to test it every day for a few days and then later on this winter.
If it fails to work even one time, I'll order a dino board but I'm gonna give
it one last chance to work......

If I was planning a cold camping trip without electric I'd probably just
replace the board now but our next outing is TopSail Hill Preserve SP
and they've got full hookups.
I plan to test the furnace just about every day on that trip and if it works-
great. If not, I'll spend the buck$ for a new board.


Fingers crossed!

Make sure you cycle it through a couple of times, not just one. Some times the boards will allow one cycle and then refuse to light the 2nd or 3rd time through. I had that happen once time. If you use it overnight and it cycles a few time I think you got it licked..

PS When I get a new component like that, I immediately put into service and keep the old "good" one as a spare. I figure why leave the old part in when you have a new one and figure the old one will be fine as a spare. Just a thought.

[KyDan]

Originally Posted by garbonz

Make sure you cycle it through a couple of times, not just one. Some times the boards will allow one cycle and then refuse to light the 2nd or 3rd time through. I had that happen once time. If you use it overnight and it cycles a few time I think you got it licked..

Went out to my trailer a couple days ago and buttoned everything back up.
I have to raise up the sofa/bed remove a thin plywood panel under the sofa,
remove a corner trim piece and some other stuff.... It involves about 5-10
minutes mostly on my old knees taking apart or putting back.
Fired up the furnace one last time and the dang thing failed!
It lit immediately but kept sparking and after a few seconds shut off.
Same old thing. On the 3rd try it stayed lit but there is NO WAY I'm gonna
trust it now. When it's freeking cold and we need heat we want it to work
every time!

Dino board is on the way now. I was hoping it would get here before the
next cold wave but that's not gonna happen.
They're predicting heavy rain today, 6" snow tomorrow and an ice
storm on Sunday! Bummer!

One thing sure, I know exactly what tools and what screws to take out
to get that thing outta there!
I've already pulled the furnace out and it's laying in there on the
floor waiting for the USPS to deliver my new dino board.

[Goomph]

Darn !!! When my furnace failed (It was completely silent) I tried everything I could think of too. I was lucky, I found a take out board at an RV surplus store in Indiana for around 70 dollars, and it is back alive now. I still have the old board somewhere ... I wish I had the know how to troubleshoot and fix the board itself.

Good luck with the furnace, and be safe, that storm looks definitely very nasty !