1. You get steam when your boiler water gets to 212 deg and then absorbs the "latent heat of vaporization". If you add a DHW load when the boiler is trying to make steam, those btu's come pretty much entirely from steam production. Put another way, with a steam system, if the DHW load pushes your boiler water to 211 deg, you get 0 steam into your radiators. With a hot water system, even if the DHW load pushes down your boiler water temp, you'll still get some heat from your radiators.
I really don't know where you are getting all this from, hope not from experience. Who is to say my DHW is going to push my temps to 211? If it does and the thermostat is not happy, guess what, the boiler turns on. What if my steam is 220? I drop to 219 all is well. You are trying to argue some point which i have no idea what it is. If you are running hot water and your DHW drops your water temp to below the low setting, your pumps don;''t run either, Ha ha. I can get steam at 211, or even less. Can you guess how?
2. My point is that any boiler, including an AA, if run without its original jacket basically can't make steam per its original specs. If they have any future scenario that would involve running a coal boiler without a jacket, they need to factor that in when they compare it to the performance they get from their current oil-fired boiler.
Like i said, show me a AA with a jacket, you won't find one. How do you know the original spec had a jacket on? Please post the spec. Did you work there or something 50 years ago? I can show you a spec from AA that says btu output on steam is only 97,500. Therefore, with your thinking, my house could not possibly be heated last winter because the oil boiler i took out was rated at 165,000. I also have no jacket so that brings it down to 80,000 btu output, so no way could it even begin to heat 9 radiators. I told you it hardly runs, i have a good draft, fire burns hot, makes some steam at idle with timer running 2 minutes each half hour.
3. As far as I know it's not a big trick to add hydronic heating zones to a steam boiler (without converting the existing steam radiation to hot water) and several forum members have done so. My last house had a hydronic loop and my current house has an indirect water heater; both use water from steam boilers.
If the original poster had stated a little more about his steam system, we would have more to go on. He is scared away never to return. It could be a 1 pipe or if the original owners of his house had money, they might have installed a 2 pipe steam system.
4. If you burn 20 lb/hr in a boiler with a given heat exchange area, and I burn 20 lb/hr in a boiler with, say, a 25% larger heat exchange area, all else equal I'm pretty sure more of my btu's are going to end up in the water. A given boiler may work most efficiently near its maximum output, but that doesn't tell you anything about how it compares to the efficiency of other boilers with larger hear exchange areas.
Where are the the other btus going then? We are talking steam ,unless you are talking something else again to try to get some point across i still haven't figured out. They are being used as heat, yes in the basement or heating the floor. 20/lb/hr only gives off so many btus. If you can get more btus out of a lb i will ask one of the moderators to lock this thread. I can put a heat reclaimer on the top of my chimney also but......(don't steal my idea)......then i can claim more btu's per pound of coal. A AA260 with hardly any load is a waste of coal. I don't know the difference between a 520,700,900 but i'm sure it's SIZE. No matter what i am still sticking with my guess that a AA130 would heat his house. Measuring the 13 radiators as suggested would be a great start.
Of course, if you find you have an underpowered steam system you can always shut off radiators, manage the timing of your DHW consumption, insulate the boiler and live with cold floors, etc. To me, that misses half the point of the cheap btu's you get from coal - you can choose to have a quality of life that involves greater comfort in - and even expansion of - the habitable area of your home, as well as prodigious amounts of DHW, all while saving a large percentage of the cost. Especially with steam, I just don't see the point of cutting it too close on capacity.
In that respect, economically speaking, he installs, better yet, he pays to have a AA260 installed. Maybe $6000. He got a good deal. All is good. Steam works all 13 rads, all the way across if needed. He doesn't remodel the basement because it is scary down there, the kids don't live in the attic because of the bats. So now, few years go by, say 2 -3 years. He adds some insulation on top of the bat guano,installs a few cheap windows from Lowes, etc.etc. This may be a true story, it has happened before. People insulate and upgrade the windows all the time. All of a sudden the AA260 is spitting out unburned coal. Such a waste. Solution, remove the oversized boiler, install smaller coal boiler, AA130, spend more money on new install. He currently has 2 coal stoves. They can supplement the small boiler, while he insulates,if needed. Maybe on the other hand he should just forget the coal until he gets the house all snugged up and r-96 insulated.
P.S. Mozz, how many sf are you heating to what temperature, and what is your annual coal consumption? Thanks.[/quote]