[QUOTE=AWill;14067881]Since you're interested, I got the information from the Natural Resources Canada at http://oee.nrcan.gc.ca/equipment/heating/12244
[B][I]"[COLOR=#000000][FONT=Verdana]A condensing boiler can have an AFUE rating of 90 percent or higher. But in practice, condensing boilers in hydronic (hot water) heating systems can have difficulty achieving this efficiency."
If you'll read the quote a little closer, you'll notice that it it doesn't say that a high efficiency boiler is a bad choice for high temp hydronics it says it "can have difficulty achieving" its efficiency rating. The reason for this is simple: the lower the return water temp to a mod/con, the higher its operating efficiency will be. If it's hooked to a high temp system where the return water temp is constantly above 140*, it won't be in its condensing mode and the efficiency will drop to about 88-90%. If the return water temp is below 110*, then its efficiency will be 97%+.
A mod/con comes with outdoor reset. This feature adjusts water temp to match the load: i.e. the warmer the outdoor temp, the lower the water temp. You only need 180* water when it's 0* outside. The Nation Resources statement doesn't account for this and is in that sense inaccurate about a mod/con. Outdoor reset allows the mod/con to run in condensing mode the majority of the heating season resulting in a boiler that's operating well above 90% most of the time.
Regarding your current boiler: it's doubtful that its efficiency was at 45% when it came out of the crate. And, that's a 1920's boiler. Someone may have brought it in from a previous house when yours was built. It's a gravity flow that may have been converted to forced flow. So is your system which is actually good news: they operate very efficiently with lower temps that mod/cons like vs. the higher temps required by baseboard.
Do yourself and your wallet a favor and install a good mod/con. I would not put a dime in what you've got.
Also, make sure you choose a good hydronics pro: that's far more important than the brand of boiler.
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.