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Posted by Tony Conner on March 24, 2005 at 08:51:13:
In Reply to: New Steel Boiler that saves Energy posted by Raman Rayes on March 23, 2005 at 15:15:15:
I always take published efficiency values for any equipment with a grain of salt. There is HUGE confusion regarding efficiency, and how it is calculated. A proper boiler eff is net heat out, divided by heat input, multiplied by 100. This is the "big picture" and will take into account all losses. It doesn't identify where those losses are, though.
Combustion eff is NOT the same as boiler eff. It's very possible to have excellent combustion eff and still have poor boiler eff. A scaled-up boiler is one example.
AFUE is yet another measure of eff, and is calculated differently again. I dislike this one because it requires a complex calculation based on a large number of measured values. The chance of an error occuring either in some aspect of the metering, and/or the calculations is pretty high. I also suspect that some (not all) manufacturers play games with this one. I think that AFUE eff calculations leave too much room for the old "figures don't lie, but liars can figure" effect to come into play.
The next time somebody gives an "efficiency" number for a steam/hot water boiler, furnace, unit heater, etc., ask exactly how that number was calculated.