It's coming soon - hot water recirculating system.

The Mr. Heater portable buddy cranks some heat...quickly and quietly. We used a one pound bottle on this trip, but I do normally carry a one gallon refillable tank and hose that can be used with either my Weber Q or the Portable Buddy. Total burn time on low (which is hot) is about 5-6 hours per pound.

Wife was a bit more skeptical and a bit uncomfortable with the heater than me. It has a small open flame pilot and glows bright red and is very hot directly in front and on top. It does have a built in oxygen sensor and auto shutoff if tilted more than 45 degrees. I don't think I would leave it running all night and you really do need to crack a window or two just in case. It's really great for sitting in front of under the dome or standing in front of after a shower!

Doug, I tried to find my 45 year old Thermodynamics textbook to work out the energy equations...then laughed at the thought of me trying to understand that again!

Ya know, we could probably find an old auto heater core, a small fan, a pump, a sink full of 140 degree water and do some testing in your kitchen! I won't tell my wife if you don't tell yours! 

Seriously though, given the amount of heat the furnace throws outside and reading that the furnace is, at best,  70% efficient *and* I still got 6 gallons of 140 degree water; I gotto believe the hot water heating system would be more efficient. But...I was also positive we would have flying cars and a robot maid named Rosie!

fred