A good source of information about boilers that are more than 100% efficient.....
http://www.boilerservices.net/technical/high-efficiency-boilers/
and specifically about efficiency.
http://www.boilerservices.net/technical/
and generally
http://www.lovekin.net/boiler-efficiency-gross-and-net.html
http://www.boilerservices.net/technical/high-efficiency-boilers/
and specifically about efficiency.
http://www.boilerservices.net/technical/
and generally
http://www.lovekin.net/boiler-efficiency-gross-and-net.html