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Old 11-29-2019, 05:48 PM   #1
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Help with propane furnace.

I have an issue with my furnace. It starts okay, but sometimes it does not turn off when it reaches its set temperature. Even if I turn the thermostat to the off position it will not turn off.
If I pull the fuse and reinsert it, it turns off.
It sounds like a suck relay.
I think it is a Suburban brand.
Does anybody know what might be causing my problem?
Thanks
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Old 11-29-2019, 06:36 PM   #2
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The last few I remember with that type problem had a bad control board.

But, before you run off buying a new board, remove the thermostat wires going to the furnace once it gets up to temp and see if the furnace shuts down. If it does, it's the thermostat.

Can you tell us which thermostat you have?
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Old 11-29-2019, 07:03 PM   #3
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Sounds like you have the cheap analog thermostat. Get a good digital thermostat.
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Old 11-29-2019, 07:43 PM   #4
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This is my thermostat. Click image for larger version

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Old 11-29-2019, 09:31 PM   #5
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That's a Dometic Single Zone thermostat. Try to initialize it and see if that does any good.

Quote:
"A system initialization will need to be performed by the installer after
the system is installed.
Make sure the Single Zone LCD thermostat is in the Off condition.
Press the “+” button and, while holding it, also press and hold the On/Off Mode button for three seconds. LCD will show ― ― .
Press the On/Off Mode button again to turn system off.
This completes the initialization."
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Old 11-29-2019, 10:32 PM   #6
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Originally Posted by Bama Rambler View Post
That's a Dometic Single Zone thermostat. Try to initialize it and see if that does any good.
I'll try that.
Thanks
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Old 11-30-2019, 01:28 PM   #7
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The furnace will run for a while after the burner shuts off to remove the residual heat. Even if you turn the thermostat off the furnace will run until the limit switch allows the blower to stop. Pulling the fuse to shut off the blower before it removes this heat could damage your furnace.
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Old 11-30-2019, 01:54 PM   #8
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Quote:
Originally Posted by Oafdawg View Post
The furnace will run for a while after the burner shuts off to remove the residual heat. Even if you turn the thermostat off the furnace will run until the limit switch allows the blower to stop. Pulling the fuse to shut off the blower before it removes this heat could damage your furnace.
That is the case with my Suburban furnace. It might blow up to 5 minutes.
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Old 11-30-2019, 02:04 PM   #9
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That is the case with my Suburban furnace. It might blow up to 5 minutes.
If the fan continues to run for what seems like an excessive amount of time check the ducting for restrictions.

I had a similar issue with a restricted duct, having a bunch of unnecessary 90 degree bends (one was actuually 180) and crushed duct at the furnace outlet.

My furnace would cycle against the upper limit, starting and stopping the flame, and then run for at least 5 or more minutes after the T-Stat said "enough".

I removed the lousy duct and replaced with better material that had fewer bends. No 90's, just sweeping bends that allow more air flow. Furnace shuts down completely after T-Stat is "happy" in 2-3 minutes.

TT also heats quicker with more air flow through the ductwork.
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Old 11-30-2019, 02:38 PM   #10
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I had the same issue with my furnace a day after I picked it up from the dealer this past May 2019. set the thermostat to 69, we left for a couple of hours and when we returned it was 84 degrees and the furnace was still running. I called the dealer and was told to unplug from the pedestal and disconnect from the battery, wait a couple of minutes and then power up again, that should reset it. It worked and it has worked perfectly since.
Give it a try.
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Old 11-30-2019, 05:23 PM   #11
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Quote:
Originally Posted by bikendan View Post
Sounds like you have the cheap analog thermostat. Get a good digital thermostat.
I'm still very much a newbie to analyzing my RV problems after two seasons and trying to learn how to approach problems that arise. Can you tell me what there was in the OP's post that led you to this possibility?
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Old 11-30-2019, 05:27 PM   #12
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Quote:
Originally Posted by Adrian Gordon View Post
I'm still very much a newbie to analyzing my RV problems after two seasons and trying to learn how to approach problems that arise. Can you tell me what there was in the OP's post that led you to this possibility?
This statement:

"It starts okay, but sometimes it does not turn off when it reaches its set temperature".
Analog thermostats often have inaccurate temperature swings. Digital thermostats have more accurate temperature settings. But the OP has a digital thermostat so it's likely not the issue.
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Old 11-30-2019, 07:48 PM   #13
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Anticipator

Quote:
Originally Posted by bikendan View Post
This statement:

"It starts okay, but sometimes it does not turn off when it reaches its set temperature".
Analog thermostats often have inaccurate temperature swings. Digital thermostats have more accurate temperature settings. But the OP has a digital thermostat so it's likely not the issue.
The hysteresis in an analog thermostat is adjustable via the anticipator on the back side of the thermostat base plate. Suburban recommends starting with a setting of 0.8. Given all the bad things we've heard about FR assembly, it's a good bet that this is never checked when the thermostats are installed.

I'd rather have a simple, mechanical thermostat than one where I have to remember to hold my nose, hold temp down and temp up while holding heat and cool at the same time, squinting my left eye, and hoping there's no full moon. I've never heard of anyone rebooting an analog thermostat.

I'm no Luddite. I was a computer designer for IBM for 45 years. But I do believe simpler is better--especially on a cold Sunday night 50 miles from the nearest RV supply.
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Old 11-30-2019, 08:57 PM   #14
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My issue is like what jamesr7332 was having. It only happens now and then. Never predictable.
I will try jamesr7332 approach.
I have a front and rear furnace, but only the front one is having this issue. They are both 6 1/2 years old and the front one just started this issue.
No sharp or 90* bends.
Turning off the thermostat or lowering the thermostat does not stop the furnace from blowing hot air, even after 2 hours.
My thinking is the control model may be going out. Getting to it won't be easy. It is under the oven.
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Old 11-30-2019, 09:15 PM   #15
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Getting the furnace out

Quote:
Originally Posted by wrapperman View Post
My issue is like what jamesr7332 was having. It only happens now and then. Never predictable.
I will try jamesr7332 approach.
I have a front and rear furnace, but only the front one is having this issue. They are both 6 1/2 years old and the front one just started this issue.
No sharp or 90* bends.
Turning off the thermostat or lowering the thermostat does not stop the furnace from blowing hot air, even after 2 hours.
My thinking is the control model may be going out. Getting to it won't be easy. It is under the oven.
I'm assuming it's a Suburban SF series. I don't think there were any other options 6-1/2 years ago.

It's easy to find the troubleshooting guide for the SF-series furnaces. Click the "Library" link at the top of the page. Go through their steps before taking the furnace apart.

Getting the furnace out is really pretty easy.
1) Turn the propane off at the tanks
2) Take off the grille
3) Turn off the propane at the tanks
4) Disconnect the propane line at the upper right of the plenum. (Mine had an elbow which had a flare fitting on one side. I had to use a crowfoot socket to disconnect it there. I was able to leave the elbow on the furnace.)
5) Remove the plenum front cover. It's one or two sheet-metal screws at the top.
6) Remove the firebox anchor screw. It's a single sheet metal screw at the bottom center.
7) There should be enough slack in the four wires going to the furnace that you can pull the firebox straight out, while stuffing the wires into the plenum at the back--or there may be a connector on the four wires so you can unplug it.*
8) Pull the firebox out. The control board is on the back side.

*If the wires are too short and there's no connector, you can add one.Just buy this cord. Connect it to itself, pull the loop taut and cut the four wires so half remains on each connector. Now, one at a time, cut one of the four furnace wires and connect the two pieces to opposing pins on the connector. (Make sure the +12 supply is on a female contact, not male.)

Reassembly is the reverse. When sliding the firebox back into the plenum, you have to engage the intake and exhaust tubes into the mating tubes on the outside vent. The Suburban manuals advise you to take the vent off, and install it last. Some folks have been able to engage the firebox tubes to the vent tubes but I never have.

Be sure to get the propane flare nut really tight. You can check for leaks with a little soapsuds.
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Old 12-01-2019, 12:41 AM   #16
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Originally Posted by Bama Rambler View Post
That's a Dometic Single Zone thermostat. Try to initialize it and see if that does any good.
Where did you find that? I need to do the same for our 3rd A/C in the garage but itís the new capacitive touch thermostat by Dometic.
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Old 12-01-2019, 07:57 AM   #17
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Where did you find that? I need to do the same for our 3rd A/C in the garage but itís the new capacitive touch thermostat by Dometic.
It's in the "manual" for the single zone thermostat.

Here's one version of the Operating Manual for the Capacitive Touch Thermostat that lists an initialization procedure.
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Old 12-01-2019, 07:33 PM   #18
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I'm assuming it's a Suburban SF series. I don't think there were any other options 6-1/2 years ago.

It's easy to find the troubleshooting guide for the SF-series furnaces. Click the "Library" link at the top of the page. Go through their steps before taking the furnace apart.

Getting the furnace out is really pretty easy.
1) Turn the propane off at the tanks
2) Take off the grille
3) Turn off the propane at the tanks
4) Disconnect the propane line at the upper right of the plenum. (Mine had an elbow which had a flare fitting on one side. I had to use a crowfoot socket to disconnect it there. I was able to leave the elbow on the furnace.)
5) Remove the plenum front cover. It's one or two sheet-metal screws at the top.
6) Remove the firebox anchor screw. It's a single sheet metal screw at the bottom center.
7) There should be enough slack in the four wires going to the furnace that you can pull the firebox straight out, while stuffing the wires into the plenum at the back--or there may be a connector on the four wires so you can unplug it.*
8) Pull the firebox out. The control board is on the back side.

*If the wires are too short and there's no connector, you can add one.Just buy this cord. Connect it to itself, pull the loop taut and cut the four wires so half remains on each connector. Now, one at a time, cut one of the four furnace wires and connect the two pieces to opposing pins on the connector. (Make sure the +12 supply is on a female contact, not male.)

Reassembly is the reverse. When sliding the firebox back into the plenum, you have to engage the intake and exhaust tubes into the mating tubes on the outside vent. The Suburban manuals advise you to take the vent off, and install it last. Some folks have been able to engage the firebox tubes to the vent tubes but I never have.

Be sure to get the propane flare nut really tight. You can check for leaks with a little soapsuds.
Thanks Larry.
I'll try the troubleshooting section first. Sounds easier than it looked.
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Old 12-12-2019, 07:19 PM   #19
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Same kind of stuff was happening to mine. The mobile RV guy come over changed out the thermostat and control board. Cost me $150.0
For everything.
Good luck
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Old 12-12-2019, 08:58 PM   #20
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The furnace will run for a while after the burner shuts off to remove the residual heat. Even if you turn the thermostat off the furnace will run until the limit switch allows the blower to stop. Pulling the fuse to shut off the blower before it removes this heat could damage your furnace.
I agree with this comment. What the OP reported may actually be absolutely normal furnace behavior.

Only if the fan runs for a long time after reaching temperature or being turned off would there be a concern. If warm air is still coming out of the vents, it is still cooling down. If the fan continues to blow for some time after the air from the furnace is cool, then there could be a problem.
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