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Better to Undersize or Oversize
Hi. While I'm waiting for quotes to come in from the contractors, I'm wondering whether it's better to undersize or oversize a boiler when your EDR falls between two sizes.
Let's say I have 420 square feet of radiation. That puts me smack in the middle of the Peerless 63-04L and 63-04. For the sake of argument, assume those are my only two boiler options and that tricks like downfiring aren't possible. Also assume that the pipes are insulated, the house is mostly unchanged since the radiators were installed, and that there are no plans to add or remove radiators.
In this case, is it better to go with the 63-04L with 383 square feet or the 63-03 with 458 square feet?
I've read the article about boiler sizing (http://www.heatinghelp.com/article/323/Boilers/1551/Taking-Another-Look-at-Steam-Boiler-Sizing-Methods-by-Dave-Boilerpro-Bunnell), but I'm not sure how to apply it.
My naive assumption would be that the larger boiler would be better on the coldest days while the smaller boiler would be better the rest of the time. Therefore, if you expect few "design days," slightly smaller is better than slightly bigger. Conversely, if you expect many design days, slightly bigger is better.
Is there a rule of thumb about how far off you can be from the rated capacity of the boiler? For example, if I had 384 square feet, the 63-04L with 383 would work, right? At what point does it flip and I'd be better off with the bigger 63-04?
I'd appreciate any input from the experts who know what happens in the real world. Thanks.