Earthstove MP40 fan issue

[WildBill57]

I have a MP40 Earthstove. I only burn pellets in it. The room blower fans run constantly. I replaced the fan blower disc switch and the fan/limit control disc switch but the blowers still always run. If I turn the rheostat knob to the left until it clicks, then the blowers will turn off, but of course this does me no good. Any suggestions would be greatly appreciated.

 

[Lake Girl]

Hi WildBill - welcome to the forum Can't find a manual on-line to guide on this. Fan/limit control disc - do you mean the low limit switch? The only other switch left is the manual reset (high limit). The room blower will stay on to cool down the stove as it still thinks its over-temp. Usually that means that the auger won't turn though...

Good luck and let us know how you make out...

 

[Stovensen]

Can't find a manual on-line to guide on this. ...

The manual for MP35/50 can be downloaded here: (broken link removed to http://www.whitfield.com/resources/manual/Lennox_Hearth_Earthstove_MP3550_ES_Pellet_Stove_Installation.pdf)

MP35/50 employs many of the same parts as the MP40.... snap disks, control board, auger motors, blowers and so on, so it is very likely that the wiring diagram and troubleshooting guide could be useful for you WildBill. I hope so.
Welcome to the forum.

 

[Lake Girl]

Thanks Stovensen! Any suggestions on where WildBill should start? Curious as to what the symptoms of rheostat failure are...

 

[Stovensen]

Any suggestions on where WildBill should start? Curious as to what the symptoms of rheostat failure are...

A rheostat for controlling the blower speed is a very direct and simple motorcontrol. Basically it's a variable power resistor. In other words a potentiometer capable of handling much larger currents than potentiometers normally used in electronics.
A great advantage of rheostat motor control is simplicity and easy repair... if it stops working... replace the rheostat.
A disadvantage is the inevitable heat loss dissipated inside the rheostat. This heat loss is worst when it is turned down, since the voltage over the resistor winding will increase and the same current will flow through both the blower and the rheostat ( P = U x I ). From this we can conclude that there's no heat loss in the rheostat at full throttle.

A typical rheostat will have to be designed in this way in order to handle the excessive heat: A resistive element wound on a ceramical base, and with a wiper sliding on the side of the windings to change the resistance.

Picture of an opened rotary rheostat:


For a rheostat to fail like the one in WildBill's MP40 stove all the individual turns of the winding would have to be shorted... and that's not at all unlikely. Some conductive material from the wiper must have been ground off and settled between the windings making an electrical connection. One way of fixing this would be to open the rheostat and carefully clean the contacting surface with a wire brush (a soft brass brush will be optimal ).
Disassembly of the rheostat in a MP40 for cleaning may not be possible, though, so it will be necessary to replace it with a new one. Here's link to an aftermarket version for only $26: (broken link removed to http://woodheatstoves.com/rheostat-for-whitfield-pellet-stoves-p-325.html)

Other stoveowners here at Hearth.com have had exactly the same issue with their rheostat control. Here's an example: https://www.hearth.com/talk/threads/convection-blower-only-works-on-high.101290/

Good luck and take care. Keep us posted.
Bo

 

[Lake Girl]

Always learn from you Bo ... appreciate the time and explanation. Thanks.