What’s wrong with the thermostat (fan / limit control)


I previously asked about wiring the White-Rodgers fan/limit control on my wood stove.

It's now wired and the wood stove is fired up. I ran the fan wires to the fan (hot) and I capped the limit wires. Since it's a wood stove I think I don't need the limit circuit.

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But the fan control is not working the way I think it should. The stove is hot but the fan is not turning on.

When I open up the thermostat box, the temp dial is not oriented the way I think it should be. I snipped this from the installation manual for the thing:

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I drew the N to show where North is for the dial in its current state. This seems wrong to me. The stove is pretty hot. I'd guess near 200F on the outside surface. Surely it's higher inside the box. At this level of heat, I'd expect the dial to rotate counterclockwise, to the limit of the temp scale. That would obviously turn on the fan. But that's not happening.

If I manually rotate the dial, it actually does close the circuit and turn on the fan: the microswitch is operating properly. when I let it go, though, the dial returns to its position as indicated in the drawing.

Right now I have the lever on the control set to "Manual" to override the thermostat and keep the blower running.

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What's the deal? Is this something I can fix or adjust?

Best Answer

Looking at your thermostat specs etc, I assume it is a basic bimetallic rotary spring operated mechanism with manual SPST switching. since you can operate it to "on" and you know the contacts close in manual mode, and when physically turned/twisted, I suggest a couple of tests to verify operation of the bi metal spring. Disconnect the electrical power. Use an air heat gun to blow on the sensor unit and watch to see if actuator moves in the switch housing as you would expect. Connect an Ohm meter across the load/fan terminals. As the temp rises, the ohms should change from infinite/open to 0 ohms/closed when contacts close at the set limit. Although this is not a calibrated test, it will verify that the spring is responding to temp changes. If this test works properly and you are satisfied the unit is working, I may suggest you look at where you are trying to sense the heat from the wood stove. It may be that the area you have selected to mount the thermostat is not seeing enough heat from the stove. you may have to monitor the temp closer to the flue or at an area that radiates heat better. You may also have to fabricate some type of small plenum directly on the surface of the stove for the sensor to mount into. In that I mean just the spring/temp sensor assembly, not the actual electrical switch assembly. Good luck.