Gas furnace issues. 2002 Windsor. Help appreciated

[Wayne Thomas]

My gas furnace doesn’t turn off but the roof top heat pumps turn off when they reach set temperature, my furnace is located under my refrigerator on my 02 Monaco Windsor. I believe it could limit switch?

[Tom Cherry]

First test.  When you turn off the thermostat, by using either the zone and mode functions…. Does the furnace shut down?  Next….if NOT, then try the ON/OFF button.

The final “try this”…. With the Thermostat off (switch), then hold in the MODE and ZONE buttons….keep holding and turn ON the thermostat…. Should see FF on the Thermostat.  
 

If that doesn’t work, one final thing to do before tearing into the furnace.  If you have a manual on the furnace and can get to the control wiring, there is a “control” or thermostat set of terminals.  The funace is designed so that the control module in the HVAC zone (most likely Zone 1) closes a set of contacts.  There are two “blue wires” that make this circuit.  It is like an ON/OFF or light switch.  When the thermostat says “heat”, the two wires in the controller are closed….so that the furnace comes on.  There is NOT any DC or voltage on that line.  It is called a “dry contact” line.  You can download the install manual for the original Dometic Penguin.  There is a schematic of the control module.  If, when the HVAC System is OFF, the circuit to the two blue wires (they change color when they leave the control module) is OFF….then time to start into the furnace.

My logic would be to verify the thermostat and control module first.  Simple and a quick continuity test on top (assuming you have a 15K unit….13.5 K are supposed to be INSIDE in the plenum)….pull the cover…

THEN the chase begins….

[cbr046]

In addition . . . . Make sure you're looking at the same zone the furnace is on (probably zone 1).  Also, it'll run for 2 minutes after you shut it down or reaches temperature.

- bob

[Wayne Thomas]

On 1/9/2024 at 10:05 AM, Tom Cherry said:

First test.  When you turn off the thermostat, by using either the zone and mode functions…. Does the furnace shut down?  Next….if NOT, then try the ON/OFF button.

The final “try this”…. With the Thermostat off (switch), then hold in the MODE and ZONE buttons….keep holding and turn ON the thermostat…. Should see FF on the Thermostat.  
 

If that doesn’t work, one final thing to do before tearing into the furnace.  If you have a manual on the furnace and can get to the control wiring, there is a “control” or thermostat set of terminals.  The funace is designed so that the control module in the HVAC zone (most likely Zone 1) closes a set of contacts.  There are two “blue wires” that make this circuit.  It is like an ON/OFF or light switch.  When the thermostat says “heat”, the two wires in the controller are closed….so that the furnace comes on.  There is NOT any DC or voltage on that line.  It is called a “dry contact” line.  You can download the install manual for the original Dometic Penguin.  There is a schematic of the control module.  If, when the HVAC System is OFF, the circuit to the two blue wires (they change color when they leave the control module) is OFF….then time to start into the furnace.

My logic would be to verify the thermostat and control module first.  Simple and a quick continuity test on top (assuming you have a 15K unit….13.5 K are supposed to be INSIDE in the plenum)….pull the cover…

THEN the chase begins….

Thanks Tom The system will turn off with the mode switch after a couple of minutes but not when it reaches temp or when its turn down real low and the heat and air on the roof units work fine had the sensor over the passenger seat replaced a few years ago

[Wayne Thomas]

I turned down the thermostat to 50 degrees and the furnace turned off then moved it up to 55 and it stayed off then up to 60 and it came on it shut off when it got to 70 according to the mercury thermometer 

I hung the mercury thermometer at the same level as the sensor under the overhead cabinet. I guess my thermostat is 10 degrees off