Got my LP backup boiler today , have three choices for install. 1 Garn side of the heat exchanger unpressurized, 2 Supply line pressurized. 3 Return line pressurized. The Lp boiler is a Burnham cast iron 105k btu holds 3gal going to install in the same building as the garn. I'm installing this so I can get away and not freeze up. I ran scenario #2 by Heaterman with the water continuously circulating through the backup, he said it would work. But before I start cutting back iron is it the most efficient ?