[gonzy]

It's basically a tank in a sealed box tilted at about 45 degrees and it is piped in series before the standard water heater. It is very simple to make if you can find a 30 gal. tank paint it black and put it in a sealed box. The water will automatically circulate to your existing water heater as it heats up and being on the input side it would draw the already heated water into existing water heater. This is best done if you have a location where the batch heater can be located within 10 ft. or so of the existing water heater. I've seen a lot of designs on the internet for them.