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Is oversizing a big problem with a modern boiler with a modern aquastat and controller?
I live in New York and have a 5600 sf. (plus 700 sf. cellar) house currently heated with a Burnham V8 8 section boiler. The block is cracked and I'm leaning toward replacing it with a Buderus G215 series. The Burnham has 239,000 net BTUs. The 6 section Buderis is 223,000 net BTUs and the 7 section is 256,000. The house is 100 years old, stucco, 3 floors plus cellar with a ton of window glass area. Main heat piping is 4" cast iron. We have only owned the house for only 3 years.
The Burnham, depending on the outdoor temperature, might cycle on for 5-15 minutes, off for 5-15, on for 5-15 etc... which I assume means that it isn't working excessively hard. However, I'm nervous about replacing it with the slightly smaller 6 section Buderus because there is about 1000 sf. of the house that is under renovation and has never been heated during the time we've been here. I have no easy way of knowing how this will impact things. I don't yet know what the space will be like in terms of doors, windows, type of heat (it might be radiant floor). If I get the 7 section Buderus at 256,000 BTUs, it will likely be a bit big but is that a such a bad thing in a modern system? I've read a lot about the problems of oversizing a boiler but is this advice outdated? My understanding is that intelligent aquastats and controllers can adapt based on water temperature, outdoor temperature, cycle times, etc. Does this mean that it's now okay to err on the side of big with such a system like the G215-7? I'd hate to find out, after completing the unfinished sections of the house, that I went too small so if there is no harm in bigger, I'd like to play it safe.