MAILBOX: Heating Water with Wood, Off-Grid

Input and output temperatures on a wood heater’s hot water loop.

I enjoyed the article in HP177 on off-grid water heating. We heat our off-grid, renewable energy-powered home in the winter using an indoor wood heater. Several years ago, I installed a stainless steel loop inside it and placed it on a parallel propylene glycol loop to the second coil inside our hot water storage tank. The first loop is attached to our evacuated-tube solar water-heating system. Both loops use El Sid low-wattage DC pumps operated by an Art Tec controller.

I purchased the stainless-steel loop and a kit for cutting through the cast-iron side of our wood heater from Hilkoil, which specializes in making these loops. We have continuous hot water even in the solar shoulder season, because the pumps come on whenever they sense a heat source that exceeds tank temperature. I had so much hot water that I had to install a second hot water storage tank to contain it all!

James Li • Coastal Maine

Connecting a heat exchanger to a wood heater is an excellent low-tech solution that I use in my off-grid homestead as well. My solar water heating system is set up as a pumped glycol system, and my wood heater is passive thermosyphon with domestic water. Both systems deliver heat to the same tank. I never run out of hot water—summer or winter—and the backup propane heater has been off for a few years now.

But caution is in order: If something happens in your system to cause the pump to stop working, the water in the coil could quickly flash into superheated steam, which can be quite dangerous—possibly even explosive. Safety is one reason why thermosyphon systems are better, and why pumped wood heater systems have fallen out of favor.

Ian Woofenden • Home Power senior editor

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