If it has been running well for years and you haven't changed any of the settings, my first thought is that the boiler/smokepipe/chimney may need to be cleaned. Another possibility is that the characteristics of this coal are different than previous years...even if you get it from the same source the quality can change from vein to vein, etc. I recently switched to a different kind of coal and had my first outfire in two years. In my case the new coal produces more heat, and the boiler was hitting the high limit setting (200) and skipping timer cycles. I reduced the timer setting, and it is running fine once again.
Follow the wires connect to the aquastat (the device you found with the 160/200/10 settings)...one of them should lead to a repeat-cycle timer. Make sure that is working, and let us know what it is set at. 2 minutes every half hour is pretty standard for rice coal, but you may need to cut that back a little if the boiler runs too warm.
Also, what are you using for feed and air settings?
Richard S. wrote:I don't have an EFM but similar Van Wert. Winter settings are at 180/160. Summer setting at 160/140.
EFM recommends a 40-50 degree spread between the low and high limit settings to reduce the chance that a timer cycle get skipped. The larger Van Werts have a deep firepot and are much less sensitive to timer settings than a 520 EFM. The Van Wert manual I have doesn't even list a timer setting for anything larger than a VA600.