Rob R. wrote: In this case, the homes are existing, insulation values are probably unknown, and it was my impression that they already own the boiler....so there is no sense going nuts with heat loss calculations, the 520 will either handle it or it won't.
I agree with Rob on the calculations. If this project proceeds, the best way to make sure the 520 can handle the load is to be extra vigilant regarding factors that may prevent it from producing its max output. Things like monitoring your ash ring to ensure you have a good fuel/air mix, frequent cleaning of heat exchange surfaces and avoidance of junky and/or undersized coal can all help to make sure you don't come up short in the dead of winter.
That does sound like a substantial load for a 520. AFAIK two 2000 sf houses would normally have considerably more exterior surface area than would a single 4000 sf house, and any heat loss from the conduit would add further to the load. (Hmmm, I seem to be agreeing with Sting more than I thought here,
though I still wouldn't want anyone to be foolish enough to rely on such heat loss calculations to limit the sizing of a coal boiler for an old house.) One option you might want to consider is swapping out the 520 for a bigger boiler up front. Another possibility would be keeping the 520 head and base but swapping out the S-20 stoker for one with a bigger feed tube that would reliably burn buck at 20-25 lb/hr.
Random thoughts on things I don't really know anything about
- good to make sure the pipes moving boiler water to/from the second house are sized to account for extreme demand conditions as well as heat loss into the ground - otherwise the second house may not always keep up; and,
- x2 on the idea an indirect water heater might be needed in the second house. I'm not generally a proponent of them when a DHW coil is available, but I think getting satisfactory DHW performance in the second house off the coil might require a recirc line, and entail extra expense and heat loss. The one possibility to consider would be "bundling" the DHW line with the trunk line for boiler water going to the second house. If the trunk line is kept hot, the DHW wouldn't cool as it sits in the pipe. If anything, it might need a tempering valve in the second house.