NoSmoke wrote:I have an architect friend that once told me that it was really easy and simple to design a structure that was way over-built, but quite another to build a building that was economical, just met the load requirements and was safe.
I think the same principal applies to sizing heating appliances. Anyone can throw a huge wood stove in a home and heat the building, but it is quite another to design an efficient boiler that just meats the heating requirements, but no more, is safe and is economical with a decent return on investment.
Naturally when you have an engineered system, you must have all aspects of the system run in harmony, but get a few things wrong and the unit will not run properly. I have seen everything from undersized boilers, to check valves put in wrong, to unqualified people install heating appliances. We are human and mistakes happen, but overall I have a lot of respect for the technicians who set up a heating system that just meats the requirements, but no more, and I think as society with a diminishing energy supply, it is best if we use logic to heat our homes instead of sheer size. If it is an AA 260 or an EFM 520, so be it, but calculations should be made first.
The problem with applying an engineered solution to coal boiler sizing is that there are too many random variables over which the operator has limited, if any, control. As discussed in other threads, these include variations in coal quality, coal sizing, coal iron content/clinkering, ash buildup, DHW load, fuel/air mixture, severity of actual weather conditions relative to engineering assumptions, etc. If you really sized a boiler to protect against the reasonably foreseeable variations in these factors, it wouldn't bear much resemblance to the one you would pick based on traditional heat loss computations.
Then there is the issue, discussed at length in other threads and mentioned by steamup, that under some circumstances a larger boiler may be more efficient than a smaller one that just meets the load because the larger boiler (generally) has a greater heat exchange area. Yes, there are engineering factors that also work against big boilers, but actual efficiency and performance go beyond following some spreadsheet and installing the size it shows.
I've lived in a few houses with heating systems that originally were powered by coal, and all of the boilers were sized "generously". In the era when coal heat was commonplace, I believe that the people who did boiler sizing had a greater recognition of the significance of the above factors. Coal heat is able to provide enhanced comfort as well as economy, but you won't get that comfort if you "engineer" out the capacity that enables it to be provided. Everyone is certainly free to make their own choices, but I'd suggest studying a little more closely what the Dead Men did before discarding what history has shown.