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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?

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Did the temperature indicator move at all when the stove started to get hot? –  Mike Powell Dec 15 '10 at 15:02

1 Answer 1

up vote 4 down vote accepted

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.

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From the other question linked to in this post, I think this unit is mounted directly to the side of the stove with the bimetal probe sticking into the stove, so it should be getting hot. –  Mike Powell Dec 15 '10 at 15:01
@Mike Powell, that's correct. I'll see if I can post a photo. @shirlock - yes you are correct about the basic mechanism of the termostat. I'll try some of your tests; I'd be surprised if it's not getting enough heat as it is inside the exterior stove box, the surface of which is too hot to touch. –  Cheeso Dec 15 '10 at 20:31
after fiddling around with it tonight, the problem seemed to be that it was sticking. Not sure how or why, but the wheel was sticking inside the housing. I discovered this by just tapping on the face of the temp dial. When I tapped on it, it jumped in rotattion. Manipulating the dial gently back and forth allowed the mechanism to rotate more freely under normal operation. By opening up the flue I was able to see the stat rotate slowly, and the fan kicked on. It ran for a while before the stat rotated back, turning off the fan. All's well. –  Cheeso Dec 16 '10 at 7:10
glad to hear you got it working. –  shirlock homes Dec 16 '10 at 17:09

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