Suburban SF-20 furnace wont stay lit

[sf3291]

Over the weekend, I ran the furnace a couple of times. Whenever there was actual combustion, I'd get a pretty good whistling noise. Figured I had some debris in the combustion chamber.

Got it home and today, it won't light and stay lit. Here is the sequence:

1. Blower comes on.
2. I hear the gas valve pop open and the igniter start clicking.
3. I can hear combustion start with a "POOF" noise
4. Warm air begins to exit the exhaust tube, but igniter is still clicking.
5. I hear the gas valve cycle and the igniter stops clicking.
6. It will do this for 3 cycles and then go into shutdown mode.

No whistling during the brief episodes of combustion though. I pulled the unit from my TC and tore into it. Combustion chamber looks pretty darn clean as does the orifice. So, I think I'm left with two possible culprits:

1. Circuit Board
2. Bad propane regultaor

I'm leaning toward 1 since the gas fired hot water heater works just fine.

Propane bottle feels about half full, but gas fired hot water heater seems to work just fine. I was on batteries and solar all weekend, now plugged in at the house.

[sf3291]

wa8yxm wrote:
Some Suburban heaters use the same circuit board as my Attwood.

From the time the Thermostat calls for heat

1: Blower starts (Working)
2: Sail switch closes (Working)
3: Gas valve opens and ignightor starts (Working)
4: Gas ignites (you hear it light so working)
5: Thermocouple heats (Working, since it is burning)
6: Control board looks for .480 volts from thermocouple (Suspect)
5: Absent .480 volts control board shuts it down.

Possible issues.
Thermocouple, Defective or dirty enough that it does not heat up in time.
Thermocouple/control lead: Poor connection causing low voltage delivery.
Control board: Defective

On my Attwood they use a SINGLE WIRE for both Ignition (Roughly 1,000 volts Peak based on the spark gap) and flame detection (less than 1/2 volt) Now imagine what would happen if the change over from MAKE SPARKS to Is it hot? Went flakey and the detector chip got hit with one of those killovolt peaks instead of the less than 1/2 volt it loos for.. it would become a Popped tart.

I replaced my $250 dollar control board with a $100 dollar Dinosaur board. (108 including S&H) and I will tell you a few things about Dino Boards.

First: I am trained as an Electronics technician with some engineering training as well. Also over 40 years as a Licensed Ham Radio Operator (WA8YXM, original issue 1968)

The first thing that struck me when I opened the box that Dinosaur board was in was the QUALITY of the unit, All the solder joints looked first rate, The traces were heavier than on the Attwood board and it just gave me a feeling of QUALITY the Attwood OEM board did not.

Then I looked closer.. I do not know for a fact that what I described above happened. But on the DINOSAUR board there was a device (Gas discharge tube) that did not exist on the Attwood board.. From it's location on the board (next to the connector for the wire mentioned above) I suspect it is there to prevent just the type of failure I think happend.

So.. Test everything, but if you end up replacing the control board.. Go with Dinosaur.. They are 1st rate.

Good detailed post. I like it. I did order a new board, though not a dinosaur. I wasn't sure which one I should get, so for $80 I found one that looks nearly exactly the same as the OEM one. I also bought a new igniter/flame sensor. Yes, mine is both in the same lead as well.

Anyway, for anyone that may stumble across this thread while trying to figure their own heater out, here is what I did and what I learned about my particular setup.

1. The actual working parts of the furnace are encased in a shroud that has the knockouts for connection to flexible ducts as you so desire. For my application with an access door on the outside of the camper, I thought I needed to remove the entire thing...shroud and all. Not an easy task for my application since I had to disconnect 5 hoses connected to the knockouts with very little room to get my hands in there...not to mention working blind on all but one hose. Well, the shroud can stay in place. There is a single, yes just one, screw that holds the furnace in place inside the shroud. It is the screw that has a label next to it that says something like "Do not remove this screw unless you are performing service to the unit." If I had just removed that screw, disconnected the gas and chassis power, ground, and t-stat wires...out it would have come and save me all kinds of frustration and a little bit of blood fussing with those sheet metal knock out holes and the flexible tubing ducts.

2. Like the problems I had earlier this year (think my camper is just reaching that age at 6 years old now) with my dometic tundra 12V compressor fridge...I have no idea exactly what the problem was, but both work fine now. I replaced CPU boards on both. However, I suspect a more likely cause is a bad ground that may have worked its way loose and was made better again once I put it all back together. I now know which connection was the likely main culprit for the fridge, but since all of the ground leads for the furnace lead to a common spade terminal block riveted to the unit itself, not so sure it was one of those connections.

3. Getting to the igniter/flame sensor is such a huge PITA and making sure that the gap is setup properly after replacing being even more of a PITA requiring even further disassembly, it was worth it to gamble on not replacing. Even though it seemed that for whatever reason, the CPU board wasn't sensing the flame starting or the CPU board just wasn't sending the command signal to keep the gas valve open and secure the spark after combustion was sensed. The gamble paid off. As mentioned previously, furnace works flawlessly now.

4. The installation instructions and owners manual as well as service technician guides all reference observation of the flame and checking the flame pattern. Can't be done inside the camper on my particular unit. The observation port is little more than about a 3/8" diameter circular hole that is covered by the shroud assembly. Even if it wasn't covered by the shroud assembly, it couldn't be viewed while installed in my TC. Only way to check the flame front for me would be to pull unit out, hook up a temp 12V power supply, temp gas line, and a switch (more likely just a jumper wire) to replace the t-stat input and then test and observe on the bench. Within my skill level, just wasn't in my time and patience level just yet.

[wa8yxm]

Some Suburban heaters use the same circuit board as my Attwood.

From the time the Thermostat calls for heat

1: Blower starts (Working)
2: Sail switch closes (Working)
3: Gas valve opens and ignightor starts (Working)
4: Gas ignites (you hear it light so working)
5: Thermocouple heats (Working, since it is burning)
6: Control board looks for .480 volts from thermocouple (Suspect)
5: Absent .480 volts control board shuts it down.

Possible issues.
Thermocouple, Defective or dirty enough that it does not heat up in time.
Thermocouple/control lead: Poor connection causing low voltage delivery.
Control board: Defective

On my Attwood they use a SINGLE WIRE for both Ignition (Roughly 1,000 volts Peak based on the spark gap) and flame detection (less than 1/2 volt) Now imagine what would happen if the change over from MAKE SPARKS to Is it hot? Went flakey and the detector chip got hit with one of those killovolt peaks instead of the less than 1/2 volt it loos for.. it would become a Popped tart.

I replaced my $250 dollar control board with a $100 dollar Dinosaur board. (108 including S&H) and I will tell you a few things about Dino Boards.

First: I am trained as an Electronics technician with some engineering training as well. Also over 40 years as a Licensed Ham Radio Operator (WA8YXM, original issue 1968)

The first thing that struck me when I opened the box that Dinosaur board was in was the QUALITY of the unit, All the solder joints looked first rate, The traces were heavier than on the Attwood board and it just gave me a feeling of QUALITY the Attwood OEM board did not.

Then I looked closer.. I do not know for a fact that what I described above happened. But on the DINOSAUR board there was a device (Gas discharge tube) that did not exist on the Attwood board.. From it's location on the board (next to the connector for the wire mentioned above) I suspect it is there to prevent just the type of failure I think happend.

So.. Test everything, but if you end up replacing the control board.. Go with Dinosaur.. They are 1st rate.

[nevadanick]

Mine did that and it was the sail switch that was bad.

[sf3291]

Old-Biscuit wrote:
Spark electrode must be engulfed in main flame....then the main flame creates a signal thru 'flame ionization' and if spark electrode is good this signal is sent via electrode and high tension wire to control board. This milivolt signals proves main flame and power is keep on gas valve to hold it open.

Spark electrode not engulfed in main flame.......no signal
Bad electrode or high tension wire......no signal
No signal.....gas valve closes

If the furnace worked fine for nearly 6 years, isn't it unlikely that just now the electrode shifted out of the flame path?

[Old-Biscuit]

Spark electrode must be engulfed in main flame....then the main flame creates a signal thru 'flame ionization' and if spark electrode is good this signal is sent via electrode and high tension wire to control board. This milivolt signals proves main flame and power is keep on gas valve to hold it open.

Spark electrode not engulfed in main flame.......no signal
Bad electrode or high tension wire......no signal
No signal.....gas valve closes

[spadoctor]

from the outside...look in the upper right corner...you will see 2 1/4" hex screws...loosen and retighten each a few times and retry lighting. This is the point inside that the ground is connected...if this becomes high resistance it will do as you describe....I know from personal experience.

[enblethen]

Look at your water heater circuit board. It could be the same as the furnace. Switch them out to see what happens if they are the same.

[Mandalay_Parr]

I suspect the thermo sensor/igniter assembly. It is not sensing the flame and tells the board to restart.. Any tech familiar with it can fix it.