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Old 10-29-2013, 08:55 AM   #1
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Suburban Furnace NT20S intermittent operation

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???
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Old 10-29-2013, 09:07 AM   #2
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Flame sensor can be the spark electrode itself I have had that on a home system. They can be one in the same.
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Old 10-29-2013, 09:12 AM   #3
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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????
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Old 10-29-2013, 09:15 AM   #4
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Quote:
Originally Posted by onetonford View Post
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!)
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Old 10-29-2013, 10:12 AM   #5
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Dan
Jumper the limit switch and see if it runs. It could be weak and trippng due to false overheat.
Good luck
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Old 10-29-2013, 10:44 AM   #6
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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.
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Old 10-29-2013, 11:04 AM   #7
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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.
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Old 10-29-2013, 11:29 AM   #8
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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.
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Old 10-29-2013, 02:06 PM   #9
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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!!
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Old 10-29-2013, 02:51 PM   #10
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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.
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