Oversized boiler - qualitative effects on fuel usage

michael_15

It's well known that an oversized boiler causes fuel wastage (as does an undersized boiler), but I'm curious about what level of wastage there is. I'm thinking about this with respect to steam systems, because, well, that's what I have.

Let's say you have attached radiation demanding a 100,000 BTU gross output boiler. Your actual boiler, however, is:

a) A 100,000 BTU boiler,

b) An 80,000 BTU boiler,

c) A 150,000 BTU boiler, or

d) A 150,000 BTU boiler downfired to 100,000 BTU (is this even plausible? CO problems?)

If the boiler is not so oversized such that it causes a premature shutoff on pressure before all the radiators are fully heated, what would you rank the above in terms of actual fuel used? Would this differ between oil and gas due to the differential "wastage" upon starting and stopping with oil versus gas?

Shortly after moving into my current home, I replaced some air vents and added a bunch of main vents, which improved my fuel efficiency (BTU/degree day) by around 20%. I then go around to insulating all of my piping (it was entirely uninsulated) -- once I did that, my fuel efficiency <i>dropped</i> by 20%. My boiler is sized just right if you count the effective EDR of my pipes when uninsulated (though it does make for quite a warm basement), that is, adding around 0.8 sq ft for a linear foot of 2-inch pipe, and so forth. Thus, after insulating, it become oversized. . .

-Michael