Hi John, the stoker boilers have a 'keep-fire' timer that signals the stoker mechanism and combustion fans to run on a set schedule. I have my timer set to run the stoker for 2minutes every hour. But I use this timer only during the warm months when the boiler will sit idling all day long, only run for a heating demand maybe once or twice an evening. The fire needs some 'freshening up' during the long day of idling. Not fan run or augers run while the boiler idles. only an aquastat is monitoring the water temperature, if it drops below a setting, then the stoker will run to bring the temp back up. With an AA or AHS, there is probably 20-30# of coal in the firepot, idling, burning slowly, the firepot is surrounded by the water jacket of the boiler, so it it kept warm from the large fire. this large fire will stay burning for hours if there is no call for heat from the aquastat to maintain water temperature.
Each type of stoker has different requirements. the underfed stokers like the EFM have a smaller idling fire than an AA or AHS boiler, about a large softball sized fire, deep in the firepot after several hours of idling, the timer will run to keep the fire alive, on EFM's the idle timer is typically a minute or two every 30 minutes, or just slightly more frequently than with an AA, it's just a characteristic of the design, the smaller fire needs a little fuel and freshen up more often that the big fire in the AA and AHS. The EFM requires a better chimney to pull air through the fire when idling to maintain the fire.
The flat bed stokers have a much smaller idling fire, a strip of coal maybe an inch deep and an inch or two long, going acrosss the width of the flat bed grate. Depending on the BTU output of the flat bed stoker, the grate may be 7" wide or 12" wide, all depending on output. The fire is more fragile to maintain, because of the size and amount of coal burnng is much less.. so the timers run usually every 15 minutes for 1 minute to keep this idliing fire alive.. And the steady draft of a good chimney is important.
The idling fire in any stoker mechanism is like the pilot light on a gas appliance.. since a coal fire is not simple to start like a gas flame or oil flame, the necessary idling fire is the 'inefficiency ' achillies heel of coal appliances.. you have to burn fuel all the time to be able to have a big fire to respond to a thermostat's call for heat. The smaller the idling fire, the less coal burned when the boiler idles.. but the longer it takes to go from an idling fire, that has run only on the timer to a full output fire. In the EFM and flat bed stokers, it can take 10 minutes to stoke up from idle to full output.. with the AA and AHS, this only takes about 2 minutes.. but there is a much larger amount of 'idling' coal waiting to burn at full intensity.
If during the winter, your home needs heat every hour or so, the boiler's fire will keep alive just from the hourly call for heat. otherwise if it is more than say 2-3 hours for a heat load, then the idling fire must be 'freshened up' periodicly to keep the fire fed and healthy...
I think that covers it.. once you read some more, and think about what these stokers are trying to do, each in their own methodology and design.. you will see the ingenuity of the designers of these stoker mechanisms.
Ok, back to bed, and to sleep I hope. Insomnia is a