Most if not all rice and pea stokers are of older designs. And the bigger stokers have one thing coal likes...size, size of the fire-pot.
But for smaller units it gets touchy at times.
Back in the day the majority of the anthracite coal was the high quality deep mined type. Even w/ the then quality of the deep mined coal the coal companies would still push the lesser quality coal off to the side to maintain the best quality product in the then, highly competitive coal mining industry. Reputation was everything.
Fast forward to today and now residential coal sales is somewhat of a novelty, and the deep mines are a thing of the past.
Open pit strip-mining is where it's at now. Now it is less trouble in many aspects, regulations are far easier on the operators than what it is in the deep mines.
The strip mines also require far fewer people and then there is now nearly no limitations on what size machinery you can have to get the coal out of the ground and through to the breakers. Blah, blah, blah, sorry...to the point...The dang coal ain't what it used to be
...sometimes a decent batch will come though, but it is a whole new ballgame.
These old stoker designs were worked out on that deep mined stuff, and it liked to keep burning all by itself.
Seems that today's strip mined coal now either fades fast when there is a lack of air or even sometimes chokes in it own excessive ashes.
All this get us to the main point here...you just may need a little more air to keep the fire happy. When the fan is off longer on milder days the fire can start to shrink.
When a call for heat does come, and as the call is usually a short call during these milder days, the call does get satisfied but perhaps the shrunken fire has barely recovered only to sit again again shrink once more.
It's getting all the air that it needs during the heat call...it's the idle fire that needs a little air boost to maintain that optimum idle fire to be ready for the next coming heat call.
The heat lost up the chimney is of little consequences if the idle fire cannot be effectively maintained w/o the added air or you are needlessly dropping quantities of partially and unburnt coal in the ash pan.$$$
Y'all hand fired nut coal burners may find YMMV from what's described here...IDK