I am looking at the WB-051 or WH-055 knight boilers. The biggest differences I see are in the head loss, gallon capacity, and heat exchange surface area. The WB has a head loss of 11.0 at a delta T of 20 deg. while the WH has a loss of only .27. Why such a large difference? In what situation does one work better thank the other? The one with the lower loss has a 2.5 gallon capacity while the high head loss model has a .6 gallon capacity. Again, why such a difference? This seems huge to me. I was going to go with the WH-051 but found the other model on their web site and thought I would look into it.