Harman XXV Distribution Fan Only Runs at High Speed

[QuadraBurner]

I have an older model Harman XXV, not sure of the mfg. year as it came with the house when I bought it 10 years ago. It has run fine, I did replace the distro fan last year but that is really all I have had to do to it outside of regular maintenance. Over the last couple of days I have noticed that rather than ramping down to low speed, the distro fan stops running altogether and then it will suddenly run at high speed for a few minutes and stop again. Outdoor temps here have been mild for this time of year so I expect that it should ramp up and down more often, more like it would during a shoulder season. I have played with the settings, tried it in auto and manual, room temp and stove temp, and the results have been the same. The rocker switch is always set to HI, and when I switch it to LOW the fan does run in LOW mode, so I don't think it is the fan.

Any suggestions for diagnosing this problem? I don't mind replacing parts but I'd rather not do it blindly! I am due for a thorough cleaning so I will be doing that today. That seems to fix a lot of problems so maybe it will help, but this feels like something different.

Thanks in advance for any advice!

 

[jackman]

I would try rotating the knob in both the Stove Temp and Room Temp. The speed should vary with knob rotation.

 

[QuadraBurner]

It seems like cleaning it may have done the trick, I'll keep monitoring and update the thread if it acts up again for some reason. One thing that I did while cleaning that I don't normally do is clean the ESP probe. I'm not sure if that would be related, but I don't normally clean it so I thought it was worth mentioning. It was dirty enough that it was difficult to remove, so it needed to be done. It slid back in no problem after cleaning.