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Heater and AC run even though thermostat is off

My heater and AC will run even though the thermostat is off. I have to pull the fuse on the heater and breaker to off on the AC to get them to shut off. The AC only runs when plugged into 110 power of course and heater will run off batteries or house power. I swapped out the thermostat and it's still doing the same thing. I have a Conquest travel trailer with dual control thermostat. Dermatic is the brand. Please help.

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It sounds like you have a short in the control unit at the ceiling pack. Remove the inner shroud from the AC and pull down the control unit. Remove the cover from the control unit to access the relays. With the thermostat off, check for power at the white (or orange depending on the unit) wire coming from the thermostat to the relay board. Also check power from the yellow and gray wires from the thermostat to the control board. None of these should be hot with the thermostat off. If there is power then you have a short in the wiring between the tstat and the upper unit. If there is no power, then you have a bad control board.

Hope this helps,

Jeff

Rate please!

Posted on Jul 05, 2009

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http://waterheatertimer.org/images/Gas-valve-ECO2-358.jpg
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1 Answer

Difference between thermostat and thermal fuse


Hello Mary
Thermal fuse is a heat-sensitive safety feature that shuts down system when overheating is detected.
In electrical wiring, the thermal fuse reacts to high temperature caused by too many amps on the electric circuit. When temperature exceeds the fuse amp rating, then the fuse trips and cuts power to electric circuit.
Example of thermal fuse is a household circuit breaker.
Gas water heaters have a different type of thermal fuse located in the combustion chamber or on the combustion chamber door, and also inside a copper probe inside water tank. Water heater thermal fuses will trip when water temperature exceeds fuse rating, or when heat inside the combustion chamber exceeds temperature rating of fuse. This action will cut off gas supply to water heater.
So a thermal fuse is a safety feature.

Thermostat is not a safety feature.
Thermostat is a measuring tool.
Thermostats measure temperature.
Example of thermostat is outdoor thermometer, or cooking thermometer, or Heat-AC control located on wall inside house.
Thermostats can give a simple temperature reading, or they can cause a separate control to turn on-off depending on application.
For example thermostat on Heat-AC will read room temperature, and when temperature exceeds setting, this triggers a switch that turns Heat-AC on-off.

Many appliances have both thermal safety fuse and thermostat.
Take for example a gas water heater. A copper tube extends into the water tank. Inside the copper tube are 2 things: A thermostat probe to measure water temperature, and a thermal fuse called an ECO for energy cut off. The ECO is a thermal fuse, and is a safety feature.
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The thermostat reads temperature of water, and turns water heater on-off to keep water hot. A person can adjust water temperature by adjusting the thermostat dial on front of water heater.
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