Posted by Harold Kestenholz - Hydronic Network on June 14, 2001 at 09:45:02:
In Reply to: Most efficent direct vent hot water boiler posted by J. Taylor on June 13, 2001 at 14:15:24:
The highest efficiency boilers will be direct-vent because they were made to provide controlled combustion air for efficiency. The old chimney units rely upon wasted heat used to heat the chimney to make a draft to send the products of combustion up the stack; the direct vent fans permit increased heat exchanger size in relation to the burner input as there is no need to waste heat, or even spend money on a chimney, to move the lower-temperature spent gases out.
http://www.aceee.org/consumerguide/topgboil.htm