This is a thermal image video of the 15 gallon water pre-heater that Jeff Bosch and Rodney Morgan and I installed on my rocket heater earlier this year. In the first month of operation it saved us about $100 on our electric bill by reducing the amount of work our on-demand electric water heater had to do to heat the shower and kitchen water. We designed it to NOT collect the heat too efficiently from the house heater--we do want it to heat the house--but we also don't want it to get any warmer than about 165 degrees Fahrenheit. Steam is a bad thing in a water heater design as it makes things explode. It worked so well that the guys were firing it up for a hotter shower than the electric heater would give them, even when it was nice outside, so the house would get really hot. Watch for video soon of the outdoor rocket water heater we built for when the weather is nice.