Vic
Here is some info on condending gas heat exchangers..
Good Luck...
http://www.chxheat.com/chx.html
http://www.chxheat.com/overview.html
Any thoughts of a fan assist exhaust ..just remember to SUCK the flue gasses through the heat exchanger ..THat will keep an negative pressure on the hx ...Imperative to keep neg press on hx..A positive press would force CO and CO2 out of HX into occupied heated space...Not good...
Tits up...
Keep us informed..
Alex