Quote Originally Posted by FRANKHE View Post
I am new to this forum. I have a question regarding tankless efficiency. Dana posted a chart on 12-09-10 regarding condensing efficiency. It showed that if the return water temp was too high heater efficiency was greatly reduced. I realize the actual transfer of heat through the heat exchanger is decreased because of the smaller delta t. If concentric venting were used wouldn't the exhaust temp approach the air intake temp? Would the overall efficiency be higher? Thanks
The short answer is no.

A concentric vent raises the temp of the incoming air, raising the temperature of that 2000F+ flametip exhaust ever so slightly, which results in a very microsopic DROP in raw combustion efficiency, since to get the heat into the water the stack temp needs to be low enough at the water/exhaust heat exchanger to condense, transfering that heat to the water, not the incoming combustion air. Condensation in the flue doesn't buy you any further efficiency, since that heat isn't transferred to the water.

But if the vent is long, pre-heating the cold combustion air in winter with the outgoing flue exhuast may make a very small difference in the space-heating load.