Harman XXV Pellet Feed Switch for Shutdown Fail

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Feeling the Heat
Nov 14, 2011
Central MA
Had been thinking about adding an inline switch to the pellet feed motor power for the times that a normal shutdown doesn't seem to be working and a cleaning of the ESP probe doesn't help. For me a removal and cleaning of the exhaust vent piping usually is the fix to get the shutdown to work properly, but not something I have the desire to do during a period when I need/want to run the stove, thus this is the 'Band-Aid' until I can remove and clean the vent. I had been using the pellet hopper lid with its feed motor switch to kill the stove previously, but I think having it open for a period of time alters the draft of the stove and I had concerns about the possibility of the remaining fire working back to the hopper.

The wiring for the feed motor is easily accessible and connected with quick connect type spade connectors, so it's easy to break the connection and add a switch in series with the motor. (I did change out the quick connect spades on the motor power leads as the spades I had didn't fit what Harman had used). The cable from the switch is routed through vent slots at the bottom of the stove and avoided drilling any holes. The small utility box with the switch is fastened to the stove with heavy duty Velcro, is out of sight from the front of the stove, and removal is easy if needed. Labeled on top to show 'ON' and 'OFF'. If I get ambitious I'll paint the utility box black.

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So your stove keeps feeding pellets after you turn it off?
Did you try cleaning/replacing the potentiometer switch?
Yes, it feeds pellets very slowly when the pot is set to the off position. This is an occasional complaint here and usual suggestion is to clean the ESP or replace. Have not cleaned/replaced the pot, although I occasionally sweep it end to end which if there is any crud on it may dislodge it. If I ever should clean it, my magic elixir is DeOxit, but the pot has never proven to be a problem.
As I mentioned in the first post this non-shut down problem seems to be remedied by cleaning out the exhaust vent which does collect a fair amount of ash and I think the ash buildup tends to retain heat in the section of vent near the ESP which skews what the control board sees. I think cleaning the ESP may be similar as cleaning it would allow it to sense the exhaust temperature more accurately. I've had the stove since 2012 and at least once a season it fails shut down and the vent cleaning has been the fix. If the vent cleaning is not be the fix at some point in time in the future I'll dig a bit further.
The feed switch has worked well. I'll set the pot for a normal shutdown 1/2 hour before bed time and after 15 minutes if it doesn't look a clean shut down is going happen I'll kill the feed motor with the switch and that takes down stove quickly. After the shut down I reset the switch to 'On' so the stove is ready for the morning.
I guess I need to see a schematic to see how the ESP can override the off switch.
I would like to see it to, but I've yet to find any schematics/logic diagrams for the control board and how sensors and functions are programmed.