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Old 11-25-2022, 10:47 AM   #1
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Dometic Thermostat Issue?

I just replaced the Suburban propane furnace in my 2018 Sandpiper Fifth Wheel and the furnace comes on, heats the camper, shuts off, but then seldom comes back on without turning the thermostat off and back on. We're having to get up multiple times a night to turn off the thermostat and turn it back on. Any ideas what is causing this?
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Old 11-25-2022, 11:36 AM   #2
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Is the unit cycling off on the thermostat being satisfied the coach has reached temperature, or could it be that once the furnace comes on to heat, the flame is not being consistent (flame sensor issue) and the control goes into lock-out (reset when the stat powered is cycled)?

One way to know the above - assuming stat doesn’t have an “Auto/On” setting for the fan and it’s set to “On” - is if the fan is cycling with temperature.

If fan hasn’t cycled (assuming no “On” switch on stat) when you have to get up to cycle the stat then very likely a control or flame sensor issue.
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Old 11-25-2022, 12:00 PM   #3
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Just replaced the Suburban Furnace?????


Typically when furnace cycles once then locks out it is the circuit board
Turning T-stat off/on resets lockout

Why did you replace the furnace?
What was issue with old furnace?
Was replacement a NEW or used furnace?
Check connections on furnace/circuit board.......
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Old 11-25-2022, 12:01 PM   #4
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Cycling

Yes, it does cycle off once the RV reaches the temperature on the thermostat, but then doesn't cycle back on when the temperature drops, even by 10 degrees. It happens both when set to Auto and when set to Furnace (might say Heat, I'm not at the RV at this moment). But, not all the time. Sometimes it cycles back on, usually during the day, when it's not as annoying.

Quote:
Originally Posted by MrMark52 View Post
Is the unit cycling off on the thermostat being satisfied the coach has reached temperature, or could it be that once the furnace comes on to heat, the flame is not being consistent (flame sensor issue) and the control goes into lock-out (reset when the stat powered is cycled)?

One way to know the above - assuming stat doesn’t have an “Auto/On” setting for the fan and it’s set to “On” - is if the fan is cycling with temperature.

If fan hasn’t cycled (assuming no “On” switch on stat) when you have to get up to cycle the stat then very likely a control or flame sensor issue.
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Old 11-25-2022, 12:27 PM   #5
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New Furnace

Old furnace failed to come on one day about a month ago, so I took it out for the dealer to "bench test". The dealer told me it would take $550 to replace the motor, so I bought a new furnace from RV Suburban (online) and installed it (camper is long term parked, we live in it, and the RV dealer no longer does service calls). Long before the motor failed the old one had the same issue intermittently, but it might go months working fine, then act up.

Quote:
Originally Posted by Old-Biscuit View Post
Just replaced the Suburban Furnace?????


Typically when furnace cycles once then locks out it is the circuit board
Turning T-stat off/on resets lockout

Why did you replace the furnace?
What was issue with old furnace?
Was replacement a NEW or used furnace?
Check connections on furnace/circuit board.......
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Old 11-25-2022, 12:29 PM   #6
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Could be a problem with airflow out of the furnace. Might be tripping the high temp safety circuit..
Why was the Suburban replaced? Did it have the same problem?
Did you change the thermostat at the same time.
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Old 11-25-2022, 12:41 PM   #7
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Replaced Furnace Only

It did have the same problem, but very intermittently. It might go months working fine then behave this way for a couple days. Ironically it had been working fine until it didn't work at all. I pulled the unit out and took it to the dealer to "bench test". They said the motor needed to be replace, so I preferred to install a new unit. All the ducts were clear when I installed the new unit. I did not change the thermostat.

Quote:
Originally Posted by glwinger View Post
Could be a problem with airflow out of the furnace. Might be tripping the high temp safety circuit..
Why was the Suburban replaced? Did it have the same problem?
Did you change the thermostat at the same time.
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Old 11-25-2022, 12:42 PM   #8
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Quote:
Originally Posted by glwinger View Post
Could be a problem with airflow out of the furnace. Might be tripping the high temp safety circuit..
Why was the Suburban replaced? Did it have the same problem?
Did you change the thermostat at the same time.
When Hi temp switch opens Fan continues to run until heat exchanger cools down, HI temp switch closes and flame is reestablished until Hi temp switch opens again.....repeat
Hi temp switch doesn't completely shutdown/lock out furnace

Flame Proving (spark electrode sensing) can....3x attempts to ignite/prove flame then lockout

Circuit board failing....lockout

*intermittent issues...circuit board and/or wire connections to board
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Old 11-30-2022, 08:45 AM   #9
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Quote:
Originally Posted by Old-Biscuit View Post
When Hi temp switch opens Fan continues to run until heat exchanger cools down, HI temp switch closes and flame is reestablished until Hi temp switch opens again.....repeat
Hi temp switch doesn't completely shutdown/lock out furnace
The above quote is true. However, and isn't there always a 'however' , there is an exception to the statement quoted!

The Suburban Service and Training Manual makes this statement,

"If during the heating cycle, the limit switch circuit opens and remains open for 5 minutes, the module board will go into lock out and shut down the blower motor. If this occurs, the thermostat will need to be reset for the furnace to operate."

My motorhome exhibited the same problem you are experiencing. I determined that the burner was cycling off and on with the High Limit Switch. The limit switch is an electro-mechanical device that is not designed for repeated operation. The switch is supposed to stay closed for the life of the furnace. It should only open if something is wrong.

In my experience the limit switch would cycle the flame off and on simulating normal furnace operation. Eventually the limit switch would wear out and intermittently start hanging open for >5 minutes and the furnace would shut down.

The root cause of my furnace problems was not the furnace. It was the installation of the furnace. The coach designer did not put in enough outlet registers. This caused the furnace to over heat and the limit switch shut the furnace down. The flame would then cycled off and on based on the limit switch.

My solution was to add 3 4" round wall registers to supplement the floor registers the coach manufacturer installed.

Attached is the Suburban Service and Training Manual. Go to page 20, there you will find minimum ducting specifications for most Suburban furnaces.

You could have plugged registers, blocked cold air return, mountains of stuff plugging the fan, etc. Anything that inhibits air flow can cause High Limit Cycling.
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