lsayre wrote:Is there something different that needs to be considered between the output BTU's for say an oil fired boiler vs. a coal fired boiler?
I agree with steamup that there are good reasons for oversizing a coal boiler somewhat relative to whatever the heat loss computations may show. The rated capacity of the boiler is based on assumptions that may not be fully valid at the time you are looking for the boiler to deliver its maximum output. For example:
- sizing of coal
- In my old house, a refurbed EFM 700 stoker had trouble carrying the load when it was really cold. Eventually I figured out that much or perhaps all of the issue was the fact that I was burning rice rather than buck (not available locally), which EFM originally specified for this unit. If the coal being used is the wrong size, or has too large a percentage of undersized pieces, the boiler may not achieve its rated output.
- For a while I used red ash coal in an Alaska 140 (w/paddle stokers). When I tried to crank it up in cold weather I had problems with clinkers and "plowing". To the extent you have to limit air to the fire to control clinkers, at least in a flat stoker, you may be holding it back from its maximum output.
- boiler maintenance
- Burning coal generates fly ash, and boilers need to be cleaned periodically to ensure that accumulated fly ash does not interfere with efficient heat transfer, draft, etc. If that cold spell hits before you do your mid-season cleaning, your boiler may not be able to perform up to its ratings.
- As covered in detail in other threads, it takes something like 40k btu/hr of boiler output to maintain a 1 gpm stream of DHW output. If you make DHW from your boiler, you need to be able to cover the load associated with all of the showers, dishes, laundries, etc. that need to be done while you're counting on the boiler to also cover the max heating load. I agree with steamup that the heating load can "coast" to an extent, but if you use a lot of DHW the numbers can get big fairly quickly.
Now older - not sure about wiser - I take comfort in relying on a stoker that I only have to run at about 70 percent of its capacity to make all of the heat and DHW that we normally need. It held the set temp at -21 deg, and had some in reserve for contingencies. So I guess my suggestion would be to do all of the load and heat loss computations you wish based on the things that you know, but don't be afraid to throw in a contingency for the things you don't yet know about how your system may actually perform at any given time under a heavy load.