That's the beauty of a good stoker design. Your EFM underfeed stoker will keep a small 'baseball sized' fire going deep down in the burn pot. The fire will look like it has burn out. No flames, no red spot. But call for heat, and the auger starts feeding fresh coal, the fan runs and revs up the fire, and soon you have a 6-8" diameter fire and 16" flames. Pretty neat.
I'm probably going to fire up my big boiler with the antique 'Iron Fireman' underfeed stoker, It will idle all day like your's on only a few pounds of coal, then keep the house and hot water warm at night.
During 55* days and 35* nights I can't run a hand load 24/7, but the stoker does fine. With propane now at $2.05/gallon. Coal is much cheaper.
I can't wait for the AA to be done and installed.. I'm slowly getting it rebuilt. I have some of it repainted, a few more weeks and it will be ready to plumb in.