This is a key assumption that I don't think should be accepted without question. "Most of the time" - i.e., most of the spring and fall, parts of the winter and essentially all of the summer (if used for DHW) a hydronic system will be idling, so factors like standby losses favor a smaller system. However, "the rest of the time" is when the vast majority of the coal actually gets burned, and that's when factors like the potential efficiency advantage of a large unit with a big heat exchange area become relevant. IMO conditions that prevail "most of the time" don't necessarily tell you anything meaningful about the efficiency and/or effectiveness a system will achieve when it's actually doing the thing it's there for.Sting wrote: Plus if you undersize your wet system slightly - it will perform better most of the time
Mike