When i first installed my 140 I tracked my coal consumption daily over a few weeks alternating the front and rear burners.
I noted the daytime and nighttime temps and wind speeds and cloud cover and found the rear burner slightly more efficient. My unscientific guess was 5% less coal used on comparable days
The other advantage of running the rear is when needed it makes the front burner easier to light .
I just tweeted my domestic hot water system and added a tempering tank . During this cold spell running both burners my dhw ended up 180 degrees. I just added a thermostat and another Aquastat and zone valves to maintain 140 degrees in my water heater and divert the excess into the tempering tank. Last night my tempering tank got up to 120. I'll start another thread on that mod and post some pics.
Did you head up north for bulk coal yet ?