High pressure steam boiler with 65 degree F stack temperature?!?

Now this is a possibility through Innovative Boiler's Heat Recovery Systems

NEW: Condex CONDENSING economizers can recover up to 4X the energy that conventional economizers are capable of. Utilizing the built in by-pass technology, these economizers can recover heat to multiple heat sinks with built in temperature control of each section. Any waste stream of moist air is a possible application – boiler flues, generator flues, oil fired equipment, process washers, etc.

Cain economizers
are specifically designed to recover the heat from any gas fired appliance from the flue stream. Typically utilized on steam boilers to recover the heat for the boiler feedwater.