I seem to be in the minority on this topic, but I am not a "fan" of secondary combustion air blowers as advocated in some of the previous posts. IMO secondary blowers are generally unnecessary and counterproductive for several reasons. Running a combustion blower when there is not a call for heat takes the btu's from the coal on the grate and pushes them up the chimney as waste heat. With little or no burnable coal on the grate, the fan is blowing cooling air through the dead ash into the combustion chamber, ultimately increasing the amount of coal that has to be burned to maintain boiler temperature. When there is a call for heat, the burn grate is basically empty, so there is a delay while the unit runs to fill it up again. Not to mention that the blower running 24/7 uses electricity, draws make-up air into the house and creates maintenance requirements, and that having the fire high on the grate moves it closer to the hopper and contributes to some small risk of a hopper fire.
From this and other threads on the forum, my understanding is that the main reason people want secondary combustion air fans is because they reduce the amount of black coal in the ash. I agree they do that, but for the reasons outlined above I believe it's false economy. Personally, I'd rather burn less coal and have the unit be more responsive when needed, even if it means there is a little more black in the ash.
Am I missing something here? I freely admit that I like the older equipment, but I also try to keep an open mind about ways to make things work better. So far, I don't see how secondary blowers are one of those.