Suburban sf 20 furnace

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DJohn

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I am lost. Please help. I have a 2021 trailer. Suburban SF 20 furnace. The furnace works great on shore power, batteries still online. But when on battery power only, furnace will initially come on. Heat to set temp, but once there. It shuts off. Temp drops, evacuation fan comes on, but furnace will not fire, and everything shuts down. I go to thermostat, shut off, wait a till it clicks off. Then reset to heat, it comes on again and works fine, until temp reached, then it will not start until I cycle thermostat. Plug in shore power. All is well again. I started with 12 rv battery. Added a second thinking power problem. Still did it. Replaced them with 2 glass batteries. No change. Added solar panels. No change. Replaced converter. Replaced circuit board. Still doing it. I am at wits end. Batteries are always showing 12.6 or better. HELP.
 

Lou Schneider

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Sounds like the furnace is hitting the overtemperature lockout when it shuts down. Once the overtemp trips you have to turn the thermostat off and then back on before it will relight. Are all your ducts and outlets free and open? Restricted airflow will make the furnace run hotter and the slightly slower fan speed on batteries vs the higher converter voltage may be enough to push it over the edge.
 

John From Detroit

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Sounds to me like either 1 your batteries are old or 2: Connections need attention (Cleaning) or 3: Both (Most likely)
Short version of furnacec cycle
T-Stat says "I'm feeling chilly" (Calls for heat)
Fan starts
SAIL SWITCH CLOSES
Burner starts

What is happening
T-Stat calls for heat
Fan starts but never gets up to full speed so sail switch never closes.
End of what happens.
If the batteries are getting old then they contain enough OOMPH for the first cycle but when it comes time to blow the switch down again they are too tired. (Kind of lime me now days). Dirty connections make it worse.
Also.. If the heat ducts are in need of cleaning or heat registers blocked this adds to how hard teh blower needs to blow.
So Clean the ducts AND battery terminals, clear the outlets and ... TEST THE BATTERY.
 

Kirk

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I started with 12 rv battery. Added a second thinking power problem. Still did it. Replaced them with 2 glass batteries. No change. Added solar panels. No change. Replaced converter. Replaced circuit board. Still doing it.
As I suggested in your thread last night, you can download a copy of the Suburban Service Manual here. That might be a good place to start if you don't have a copy. Looking at the schematic for your furnace, the high limit thermostat opening will do the exact same thing as the sail switch opening (it is at the top and just to the right of the sail switch). Once it cools down it would close again so you may be cycling on it, but I'm baffled why it would matter if you have shore power or not.
1674312505156.png
Since the furnace does not use shore power in any way, the only thing that I can think of which will change would be the voltage of your 12V supply. Have you monitored the voltage at the furnace when the blower comes on to see if it might be dropping when on the battery but not with shore power? Since shore power does supply the converter to in turn supply 12V power it may somehow be contributing that way. Other than the voltage of the supply to the furnace, I can't think of anything that would be different when you are connected to shore power.
 

DJohn

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That's my quandary trying to chase down and eliminate things. The furnace is dc. Shouldn't matter. But it does. In this I would say battery problem, but I have eliminated that. I'll take your schematic and see what I can find. Thanks
 

DJohn

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Sounds to me like either 1 your batteries are old or 2: Connections need attention (Cleaning) or 3: Both (Most likely)
Short version of furnacec cycle
T-Stat says "I'm feeling chilly" (Calls for heat)
Fan starts
SAIL SWITCH CLOSES
Burner starts

What is happening
T-Stat calls for heat
Fan starts but never gets up to full speed so sail switch never closes.
End of what happens.
If the batteries are getting old then they contain enough OOMPH for the first cycle but when it comes time to blow the switch down again they are too tired. (Kind of lime me now days). Dirty connections make it worse.
Also.. If the heat ducts are in need of cleaning or heat registers blocked this adds to how hard teh blower needs to blow.
So Clean the ducts AND battery terminals, clear the outlets and ... TEST THE BATTERY.
All batteries have been brand new. Connections clean. I would and have looked at the batteries as problem. But from 1 new rv battery, to two new rv batteries, to two new glass, AGM, batteries. Then solar charging abilities. I'm confused and overlooking something or I do not know what to look at!
 

DJohn

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Jan 20, 2023
Posts
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Location
81212
Sounds like the furnace is hitting the overtemperature lockout when it shuts down. Once the overtemp trips you have to turn the thermostat off and then back on before it will relight. Are all your ducts and outlets free and open? Restricted airflow will make the furnace run hotter and the slightly slower fan speed on batteries vs the higher converter voltage may be enough to push it over the edge.
Best I can tell all are free, I'll go through again. But I did not know the overheat proceedure and switch. Sounds like what it is doing. Thanks, I have a direction to go with and a schematic to use
 

DJohn

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Best I can tell all are free, I'll go through again. But I did not know the overheat proceedure and switch. Sounds like what it is doing. Thanks, I have a direction to go with and a schematic to use
Is there a switch or sensor easy to locate for this lockout
 

Kirk

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Former fulltimer, Mesquite, TX
Is there a switch or sensor easy to locate for this lockout
It is a metal disc thermostat that will be attached to the heat exchanger.
61cQsZ9OAnL._AC_UL232_SR232,232_.jpg
 

Gary RV_Wizard

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It's easy to validate whether the overtemp safety switch is causing the shutdown. It is a normally closed switch, so you can jumper around it and prevent it from ever triggering a shutdown. The problem, of course, is that furnace is still overheating, so you can't just leave the jumper in place. The switch itself is rarely the issue, i.e. falsely signalling an overheat.
 

John From Detroit

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Davison Michigan
Argument for and against high limit
You said it works find on Shore power (Argues against high limit)
Slightly slower blower speed on battery (Argues FOR High limit).
If this were my Furnace I',d be hooking up a couple of indicator lights.
One would indicate the closing of the Sail Switch
And one for the high limit.
 

DJohn

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Argument for and against high limit
You said it works find on Shore power (Argues against high limit)
Slightly slower blower speed on battery (Argues FOR High limit).
If this were my Furnace I',d be hooking up a couple of indicator lights.
One would indicate the closing of the Sail Switch
And one for the high limit.
I'm not voting anything out. It's actually a warranty item, but cannot get trailer in for almost 6 months. Took 3 months for recall here in colorado with camping world. I'm lost so I'm trying anything.
 

DJohn

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It's easy to validate whether the overtemp safety switch is causing the shutdown. It is a normally closed switch, so you can jumper around it and prevent it from ever triggering a shutdown. The problem, of course, is that furnace is still overheating, so you can't just leave the jumper in place. The switch itself is rarely the issue, i.e. falsely signalling an overheat.
I rechecked all vent and air inlets and exhaust. No blockage.
 

John From Detroit

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If you look at the schematic that I posted in #4, you will see that the sail switch and the high limit thermostat are in series so that won't work. :giggle:
View attachment 161113
Actually it will one is connected to the cold side of the sail switch and the othe rto the cold side of the limit switch.
IF (Depending on which is the hot side) one lights.. but the other does not, than the swich on the "hot" side.. is good and the problem is the other switch.

OF neither light lights it's the switch on teh hot side that's bad.. The other is "Not tested".
 

DJohn

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After reading the replies, and looking at the testing I could do, I think, 99% sure, I have located the problem. I was confused about how it was doing what it was doing. But after returning with new ideas and testing,
It is Forest Rivers design! A flaw in the way the trailer and furnace were designed. With one tube venting towards back of trailer and one tube venting towards front and the slight difference in shore power and battery power fan speed, the furnace is unable to push enough air through unit to properly cool. Add that when build, the vents were connected to front of furnace box instead of at back of it, air was overheating at sensor.
I removed the back cover and the furnace runs great. No flaws or issues. Put cover on, and it starts locking out again. So I am figuring where to run another vent tube to move more air in trailer without cutting air flow to current vents or heating area of thermostat to quickly.
Thank You for the help. It may have come to me later, but your help made it sooner. And if anyone else has similar problems, have them check the "flow".
 

Kirk

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So I am figuring where to run another vent tube to move more air in trailer without cutting air flow to current vents or heating area of thermostat to quickly.
That does seem to confirm that it is the high limit problem. While I have never seen one that reacts to shore power in that way, I would certainly think that you have one and I have seen an RV furnace that needed to have another vent added. You need to be careful not to get the additional vent too close to the thermostat, as you mentioned. If possible you might want to place it as close as possible to one of the other vents, or possibly add 2 new ones, each near to the existing vents.
 

Lou Schneider

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After reading the replies, and looking at the testing I could do, I think, 99% sure, I have located the problem. I was confused about how it was doing what it was doing. But after returning with new ideas and testing,
It is Forest Rivers design! A flaw in the way the trailer and furnace were designed. With one tube venting towards back of trailer and one tube venting towards front and the slight difference in shore power and battery power fan speed, the furnace is unable to push enough air through unit to properly cool. Add that when build, the vents were connected to front of furnace box instead of at back of it, air was overheating at sensor.
I removed the back cover and the furnace runs great. No flaws or issues. Put cover on, and it starts locking out again. So I am figuring where to run another vent tube to move more air in trailer without cutting air flow to current vents or heating area of thermostat to quickly.
Thank You for the help. It may have come to me later, but your help made it sooner. And if anyone else has similar problems, have them check the "flow".
Congratulations! I had a similar problem several years ago on a Sierra 5th wheel, with the same symptoms on shore vs. battery power. In this case it was a restriction on the supply air side. They mounted the furnace under the sofa and were relying on the gaps around the edge of the sofa for the intake air. I solved the problem by cutting a hole in the sofa's kick panel to supply more air and covered it with a grate.
 

DJohn

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You beat yourself up trying to find a problem with the mechanics of furnace and it's working properly. Well doing what it's supposed to do with situation.
 
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