Furnace Trouble Suburban SF-30FQ

[LizardKing]

Hello all,

Suburban Sf-30FQ furnace, about 6 years old.

Trouble started when it wouldn’t ignite. The fan would turn on, I heard the igniter clicking and no flame. The flame would start, I would feel heat in the exhaust, then the flame would go out. Igniter would start again, flame would start, then go out. This happened 3 times, then would turn off.

Troubleshooting Steps:

Replaced the regulator.
Checked for cob webs in the regulator.
Tried two different propane tanks.

Removed the furnace and started bench testing.

Disconnected and reconnected each wire in the furnace, board, relays, etc.
Took apart the burner and confirmed 1/8’ gap between the igniter/flame sensor, and the burner.
Confirmed burner assembly was not rusted or dirty
Checked and cleaned line from the gas valve to the burner and orifice.
Used mild emory cloth to clean the igniter/flame sensor.
Connected propane tank with a single working regulator to the furnace.
Connected red wire to positive terminal of a car battery
Connected yellow wire to negative terminal of the car battery.

Connected both blue wires together (Thermostat wires)
Furnace turns on, everything works fine on my bench.
Tried this several times with no problems.

Reinstalled furnace in camper.
Turned on furnace at thermostat


Tomorrow I’m going to:

1. Reinstall the furnace in the camper but connect it with an external propane tank with single regulator (that I used on my bench) to eliminate propane pressure problem.

2. Bypass the thermostat and hook the two blue wires together to eliminate the thermostat.

Not sure what else to try, Any suggestions?

[DES-1]

Replace sail switch?
My readings indicate that's the issue. Ours has exact same problem. Last trip I could get it to work by cupping my hand over the intake and removing quickly. Ours no longer responds. Not sure how deep in the inside the sail switch is but it's a cheap fix.

[boondocking]

Originally Posted by DES-1

Replace sail switch?
My readings indicate that's the issue. Ours has exact same problem. Last trip I could get it to work by cupping my hand over the intake and removing quickly. Ours no longer responds. Not sure how deep in the inside the sail switch is but it's a cheap fix.

Quote from OP:
"Furnace turns on, everything works fine on my bench.
Tried this several times with no problems."

Cannot be the sail switch.
Sounds more like a propane issue.

[DES-1]

Originally Posted by boondocking

Quote from OP:
"Furnace turns on, everything works fine on my bench.
Tried this several times with no problems."

Cannot be the sail switch.
Sounds more like a propane issue.

Sure it can. The load on the sail switch is different when installed in the trailer as the intake path is longer.

[Larry-NC]

Originally Posted by DES-1

Replace sail switch?
My readings indicate that's the issue. Ours has exact same problem. Last trip I could get it to work by cupping my hand over the intake and removing quickly. Ours no longer responds. Not sure how deep in the inside the sail switch is but it's a cheap fix.

No, no. If the sail switch isn't closing, ignition won't take place. The OP says that he can hear the clicking sound of the spark ignition.

There are only a few things that can cause failure to ignite:

1. Bad fuel-air mix: (I fought this one for days before actually building a manometer and proving that my regulator was bad.) The furnace is essentially turbo-charged by the blower, so it is quite sensitive to fuel-air mix. This could be caused by a bad regulator, cob-webs in the intake or exhaust port, or even a gas valve that isn't opening completely. It can also be affected by the state of cabin ducts in the air circulation path, since both blowers are driven by the same motor.
2. Spark gap: The manual (attached) shows how to adjust the spark gap. This gap is the distance between two electrodes in the fuel-air path. Easy to check while the unit is still on the bench. Be careful opening the firebox so you can reuse the gasket. (The gasket is cheap and easy to find online if it does get damaged.)
3. Pinched high-voltage wire: This is pretty easy to check.

The peculiar thing about this furnace arrangement is that the double-ended blower speed changes between the bench and installation, when the second blower must force air through the trailer ducts. The furnace is surprisingly sensitive to this. When my regulator was bad. the unit would work perfectly when exposed, or even when installed, as long as the plenum cover was open. Just closing that last panel, to force air through the ducts, was enough to cause failure to ignite.

I've attached the Suburban manual and some pictures of the manometer I built. It's filled with water and a couple of drops of food coloring. At rest, the water is at the 0 line. The lines are spaced 1/2" apart, but measure inches of water column, since one side goes down 1/2" as the other goes up 1/2".

[DES-1]

Originally Posted by Larry-NC

No, no. If the sail switch isn't closing, ignition won't take place. The OP says that he can hear the clicking sound of the spark ignition.

"The flame would start, I would feel heat in the exhaust, then the flame would go out. Igniter would start again, flame would start, then go out.".

It's entirely possible the sail switch is behaving badly and causing these symptoms since ignition and initial burst of heat are achieved.
Hopefully Mr. LizardKing (Jim Morrison ) will post the cause/fix.

[Larry-NC]

Originally Posted by DES-1

"The flame would start, I would feel heat in the exhaust, then the flame would go out. Igniter would start again, flame would start, then go out.".

It's entirely possible the sail switch is behaving badly and causing these symptoms since ignition and initial burst of heat are achieved.
Hopefully Mr. LizardKing (Jim Morrison ) will post the cause/fix.

Not sure that's likely, but you've made me think of another possibility. The OP didn't say how long the furnace ran before the flame went out and then reignited without faulting. That's exactly the behavior you would get if the over-temp switch opened. The furnace fans keeps running but the gas cuts off. When the furnace cools down, the ignition sequence (gas and spark) takes place again.

If the fault occurs after a few minutes and the OP has happened to place throw rugs over a couple of the furnace vents, that would perfectly explain the behavior.

There have been some past reports on this forum of "clever" owners who replaced their floor vents with vents with adjustable dampers.

On most Suburban models, the over-temp switch is easy to get to--right behind the cold air return grille. The OP could put a voltmeter across the switch and start the furnace. It should read about 0 volts DC. At the time where the furnace starts blowing cold air, it would jump to 12 Vdc. And when it relights, it would go back to 0.

[Asrabbit]

Is it possible that the flame sensor is not functioning as it should? A bad flame sinsor will cause it to cycle three times then go into lock out. The flame will ignite, burn for seven seconds, then shut off if not flame is detected. It will do this three times and then lock out. I do read that the unit worked correctly on a bench test, so that may not be the issue.

[DES-1]

FWIW, Our new camper has the same furnace symptoms. Been reading up on similar experiences of others. I was surprised to read the logic board is sometimes the culprit (although I shouldn't have been too surprised...had to replace board in a 3 year old propane fridge...even the electronics are substandard!). I think our next step is to attempt to pull the furnace and see if the board access is easy, and if it is install a replacement Dinosaur board. I hate the idea of letting a dealer hack on it.

[Larry-NC]

Originally Posted by DES-1

FWIW, Our new camper has the same furnace symptoms. Been reading up on similar experiences of others. I was surprised to read the logic board is sometimes the culprit (although I shouldn't have been too surprised...had to replace board in a 3 year old propane fridge...even the electronics are substandard!). I think our next step is to attempt to pull the furnace and see if the board access is easy, and if it is install a replacement Dinosaur board. I hate the idea of letting a dealer hack on it.

It's not that hard. When you take off the return air grille, you will see a sheet metal box with a bunch of ducts attached. DON'T PANIC. You don't have to undo those.

That sheet metal box is the "plenum." You will leave it in place. You will open the cover and remove an assembly which includes the firebox, motor, blowers, and circuit board.

Here are the steps. (This is for an SF-25. Your geometry will be a little different.)

• Turn off propane at tanks.
• Remove 12v furnace fuse at your Power Center.
• Remove return air grille--usually four screws.
• Remove plenum front cover--usually two screws.
• Disconnect propane line at furnace--usually a flare nut. These can be hard to access. I have used a crowfoot socket from the top where I didn't have clearance to use an open-end or flare-nut wrench. Don't be afraid to buy a new tool if needed.
• Remove the anchor screw that holds the firebox in place.
• Now the whole assembly should slide out.
• There are four wires leading to the circuit board in the back. As you slide the unit out, you need to stuff these four wires in from the side. There should be enough extra wire to allow this. You will also be pulling the intake and exhaust pipes free (slip fit) from the outside grille.
• Reverse this procedure to reassemble.

Attaching the manual for your reference.

[DES-1]

Originally Posted by Larry-NC

It's not that hard.

Very helpful Larry, thank you. Will let you know how it goes

[Canadian Eh]

My furnace did the same, stopped and started. Would ignite then turn off. Now it wouldn't ignite at all. This all happened after we drove into a park on a very rough gravel road. I don't know if that contributed to the fault. I've looked at the schematic of the furnace. Ours is in a 24' C class with an outside access panel. Am I able to perform all the work from there?

[Larry-NC]

Originally Posted by Canadian Eh

My furnace did the same, stopped and started. Would ignite then turn off. Now it wouldn't ignite at all. This all happened after we drove into a park on a very rough gravel road. I don't know if that contributed to the fault. I've looked at the schematic of the furnace. Ours is in a 24' C class with an outside access panel. Am I able to perform all the work from there?

Are you certain that the outside access is not your water heater? That is much more likely.

The only outside part of the furnace is the intake/exhaust. The unit is accessed by removing the return air intake grille from inside the trailer.

[Canadian Eh]

Originally Posted by Larry-NC

Are you certain that the outside access is not your water heater? That is much more likely.

The only outside part of the furnace is the intake/exhaust. The unit is accessed by removing the return air intake grille from inside the trailer.

Yes, I know the hot water tank. I drain every winter. The outside panel has the exhaust and intake. I've since looked at Youtube videos and hopefully it is only the sail switch.

[5picker]

Originally Posted by Larry-NC

Are you certain that the outside access is not your water heater? That is much more likely.



The only outside part of the furnace is the intake/exhaust. The unit is accessed by removing the return air intake grille from inside the trailer.

My SF series Suburban has an outside removable panel to access the inner furnace parts.

All R/Vs are not the same. I'm sure I read that in someone's signature line. [emoji6]

[Canadian Eh]

Well thanks to Youtube I was able to get a better understanding. The sail switch was under the fan cover. Pulled it out and tested it. Reading was not consistent so I jumped between the terminals and the furnace fired up. A win.

[DES-1]

Originally Posted by Canadian Eh

My furnace did the same, stopped and started. Would ignite then turn off. Now it wouldn't ignite at all.

Thanks for posting, confirms symptoms we've experienced. My furnace was a *bear* to remove as there is no outside access panel. I'll test the sail switch AND will install a new igniter board to lessen the chances I have to take it out again. 100 buck in parts is getting off cheap compared to having to pay an RV tech. Doubt they would even touch the job for less than $1k.

[LizardKing]

I am going to test the propane feed hose from the front of the camper to the first junction, then the next junction, etc.

The reason I still think it’s propane pressure related, is that the furnace runs perfectly, I mean perfectly when connected directly to an external propane tank with a single regulator.

When connected to the rv propane, the flame ignites, sometimes it will stay lit for 5 seconds, other times it will run for several minutes. When it runs for several minutes, the heat exchanger never gets as hot as it does when running on an external tank.

It’s been rainy, and I haven’t been able to attack it. Hopefully this weekend. I will post an updated.

Thanks all!

[DES-1]

Originally Posted by DES-1

Very helpful Larry, thank you. Will let you know how it goes

It turns out that the igniter board was bad (on a brand new trailer ). Replaced with a Dinosaur replacement and it works nicely but I'll put it through 10 startups before I proclaim victory. Larry's list was a very valuable reference and of course there were a few twists due to manufacturing differences.

Here is the replacement board, recommended if you need one:


https://www.dinosaurelectronics.com/Fan_50_PIN.html

[LizardKing]

I realize this is an old post, but I finally got to fixing this problem.

I suspected there was a propane flow problem. The hot water tank, stove and fridge all worked, but the furnace wouldn’t. As I traced the propane hose from the front to the rear, I noticed the hose was pinched by the mounting bracket. It actually was crushing it!

I removed the bracket, squeezed the hose close to its original shape and the furnace lit immediately and stayed running. After confirming it wasn’t leaking , I made an aluminum sleeve to go over the pinched hose, and made a new aluminum bracket that mounts the hose to the bottom of the camper. Hopefully it will last. If it collapses again, I’ll have to have someone cut out the damaged part and splice back together.

Thanks for everyone who chimed in!