Suburban SF-35Q Intermittent Issues

**runningfromordinary** · 08-26-2021, 06:47 AM

We have a 2020 Grand Design Solitude 3740BH equipped with a Suburban SF-35Q furnace. We have lived in the RV full time since May of 2020 and have used the furnace without issue off and on throughout the year, including in sub 30 temps without issue.

Here’s our problem experienced the last theee nights — befofe we go to bed, we set the thermostat to 68 deg. Thermostat clicks, furnace fan kicks in, you can faintly hear the burner kick in, heat comes out. We go to bed. I wake up cold at 3am, go out to check thermostat and it reads 55deg. Temps outside are in the 40s. I turn thermostat to “off”, then back to “heat”. Click, fan, burner, heat - good for the rest of the night. This pattern has repeated the last 3 nights in a row.

I checked propane - solid blue flame coming out of stove top and hot water heater seems fine as well. To be rule out a partially empty tank issue, I switched over to our other 100% full tank last night, but had the same issue.

I also tried letting it get to temp, cycle off, then cool down and get to a temp where it should kick back on - and it did so without any issue 🤔.

Because our furnace has bug covers, the fact that it always turns on without issue when manually cycling it off/on, and given that it’s only a year old I’m doubtful it’s dirt/debris/airflow obstruction.

Based on my googling/research, it seems likely that it could be: (1) a faulty circuit board, (2) a faulty thermostat, or (3) something else?

One other fact of note - we are, for the first time using the furnace at a higher elevation - we are in Crested Butte, CO (8850ft where we are currently parked). I offer this information in the off chance that the issue we are experiencing is related that, although this seems to be more of an issue for older furnaces from what I have read.

Any tips or suggestions anyone has would be greatly appreciated, as we plan on staying in Breckenridge this winter for 2 months and do not want to be dealing with this issue then! My fallback is to verify the issue still occurs when at lower elevations, and if it does to replace the circuit board (perhaps with a Dinosaur Electronics version) and the thermostat. And if neither of those solve it, bring in the “big guns” and hire a mobile RV tech.

**runningfromordinary** · 08-26-2021, 09:22 AM

I should also note that I have confirmed that the furnace cycles on/off correctly in the morning. So maybe it's overheating because of how long it's running at night?

**DECelt** · 08-26-2021, 10:38 AM

Originally Posted by runningfromordinary

I should also note that I have confirmed that the furnace cycles on/off correctly in the morning. So maybe it's overheating because of how long it's running at night?

Not sure what you have for a thermostat, but did you happen to set a timer or program by mistake?

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**runningfromordinary** · 08-26-2021, 11:00 AM

Originally Posted by DECelt

Not sure what you have for a thermostat, but did you happen to set a timer or program by mistake?

Sent from my moto g power using Tapatalk

It looks like we have the AirXcel AT9430A3382 - DIGITAL HOT/COOL T-STAT BLK (Digital Replacement for 7330F3852), which doesn't appear to have a timer function (unless it's hidden somehow

)

**runningfromordinary** · 08-26-2021, 03:25 PM

I think I know what's happening, as I can get it to happen from time to time -- when the furnace is off, and then is triggered to kick on, it goes through three attempts to ignite. When it fails after the third time, it triggers the "lockout" mode which means I have to cycle it off on the thermostat, and then back on to try the process over again. So what's happening here is that is sporadically is choosing to not ignite, which makes me think it's either:

1) A bad igniter
2) Bad/incorrect propane flow
3) An incorrect gap for the igniter

I called Suburban and they told me their furnaces are rated for 4500 ft and below, and I'd need to install a different orifice if I go above that (and then swap it back out when I go back below that elevation). Which obviously is not only impractical, but shouldn't be necessary considering we and other RVers have used this model of furnace at much higher elevations without issue.

**runningfromordinary** · 08-27-2021, 12:17 PM

Took everything out to inspect. Control board looked good, all connections clean and solid. Sail switch freely moving without any issues. All airpaths clear and clean.

But lo and behold - the ignitor gap was not correct with what the factory service manual specifies, which is 1/8" - this one was set at 1/16" (too close). I gently bent it up to be as recommended, buttoned everything back up and triple checked connections, and furnace fired right up and lit on the first go. True test will be tonight, so crossing my fingers that this solves the problem.

Also - Time and time again I find that I'm really impressed with Grand Design's build quality. The wiring was setup perfectly for servicing a unit like this, and they even used Wago connectors!

**runningfromordinary** · 08-27-2021, 12:18 PM

I should also mention that before I even touched the furnace, I soapy-water-tested all of the propane connections to ensure it wasn't a low-flow related issue.

**runningfromordinary** · 08-28-2021, 07:00 AM

My adjustment seems to have solved the issue, furnace worked like a champ all night

**Sparkybeau1** · 08-29-2021, 09:40 AM

Thanks for posting the issue as well as following up with the processes involving the solution! Too many posts start out strong then just suddenly end leaving everyone to wonder........

**runningfromordinary** · 08-29-2021, 11:51 AM

Originally Posted by Sparkybeau1

Thanks for posting the issue as well as following up with the processes involving the solution! Too many posts start out strong then just suddenly end leaving everyone to wonder........

No problem! I can confirm that the furnace has worked the last two nights at elevation with the above-mentioned adjustment, so I’m reasonably confident that solved it permanently.