Great thread! I think it is agreed upon now that for steam applications sizing by radiation is the best approach. Whereas I can agree with Mike that a coal appliance has different characteristics than an oil or gas burner I still feel it should be somewhat close to the ideal BTU for the radiators in the building, but like Mike says, preferably on the plus side.
I would like to know more about this poster's steam system. Is it one pipe or two? How was the 300,000 BTU figure attained?
I will also say what is just as critical in a steam set up as boiler sizing, if not more so, is the near boiler piping and the venting. And that includes the supply pipe size on the boiler. Here is a PDF of exit velocity of steam from the boiler and how pressure and temp are major components. The lower pressure the better the efficiency of the system. That's one of the reasons I love my 260, that four inch supply pipe. http://www.gwgillplumbingandheating.com ... ge?pid=224
I also have to say that this Axeman is incredible in the steam set up I have. I was initially worried that since my timer that came with my boiler is kaput I would have trouble with maintaining a fire. Nothing could be further from that worry. Even during some of the mild weather we have had here in Connecticut I have not lost a fire. With a long twelve hour plus stretch for a call from heat, once that fan kicks on it is not long before there is a serious ball of fire in that boiler! Yanche, Coaledsweat, Greg, Steam-up and Rob each articulate well how this system operates. I was at Coaledsweat's house once sitting around his boiler. His 260 was just idling along on a mild day. We were actually talking about the subject of boiler fire temps and I was wondering aloud how hot his fire was since there was not much of a call for the motor. He took a piece of solid steel bar about three feet long, an inch wide and a quarter inch thick and opened his firebox plate and shoved the steel bar into the fire. I don't think it was over 3 to 4 minutes and he took the bar out and the end in the fire was solid cherry red and glowing! Yet, by looking into the firebox you would not have known the coal was even burning.
You have to see it to believe it, this 260 makes steam like there is no tomorrow. Even since I added the hot water coil in my boiler, it still cranks out steam at an amazing rate.
Burning coal, be it in an Axeman (AHS), an EFM, a Keystoker or any other unit set up properly, is THE best fuel for steam. Size your header, your equalizer, your loop properly, and VENT THOSE MAINS and you'll pump out some serious steam. If you're in CT please stop on by and we'll crank the thermostat and time the boiler's response. You'll be amazed.
I would love to see an EFM 700 or 900 set up for steam, or a big Keystoker with twin 4 inch supply pipes and a massive header like mine. I bet those units would rock as well.