Hack a Wood Stove into an Efficient Water Heater

Editor's Note: The following hack carries a risk of explosion if not done properly. It will also likely void any warranties on your wood stove. Proceed with caution.

Shade of Green: Burnt forest

Items Needed: 1 wood stove (well, duh), 1 pressure relief valve, 1 hot water jacket or heat-resistant loop, miscellaneous copper pipe, miscellaneous copper fittings, 1 circulator (optional), 1 aquastat thermostat (optional), 1 pre-heat holding tank (optional), 1 plumber, stove gasket cement

A view of the back of the stove/water heater mod, showing in and out water pipes.

Why this Hack: Because you're extremely adventurous. This is an advanced hack, requiring specialized knowledge and equipment and a clear understanding of the danger involved. Wait... come back... where are you going?

Listen: It's true that running a water loop through a wood stove is serious business; without a pressure relief valve, this sort of hack could cause, well, an explosion. This either scares or excites you. There is no in between. But the rewards are at least equal to the risk (and really, there's not much risk if you do it right): Piping hot water with the same quantity of wood you already burn, saving you literally hundreds (if not thousands) of dollars in the long haul.

This hack requires real plumbing skills, so you might want to hire it out. But since few plumbers will be familiar with this sort of tomfoolery, be sure to do the research first. There's a great book available from Lehmans.com. Rather fittingly, it's titled "Hot Water From Your Woodstove." Pure genius, eh? Anyway, it's got all the nitty-gritty, as well as sources for the hot water jacket you'll need. What's a hot water jacket? It's a rectangular slab of metal that installs inside your firebox, and it is designed to mate with copper pipe that runs cold water in and hot water out.

Heart of the Hack: The tricky part is fitting the jacket (and if a standard jacket doesn't fit, try hilkoil.com for custom loops); most stoves are not pre-drilled for water, so you'll have to break out the hole saw and git to hackin' in a most serious way. If yer stove's still under warranty, and you'd like to maintain that warranty, stop reading. That said, if you measure correctly and exercise due caution, you won't do any damage drilling two half or three-quarter-inch holes in your stove to fit the cold water in and hot water out pipes. Actually, you'll want to drill 'em 1/16th-inch oversize for ease of installation and to accommodate expanding/contracting. Once everything's installed, seal everything up with stove gasket cement.

Once the jacket/loop is fitted, it's time to run your copper; cold comes in the bottom and hot exits through the top hole. There are two basic ways to run this system: Bonus hack points for going with a thermo-siphon system, which uses the expanding properties of heat to send hot water to an elevated holding tank. Of course, that means plumbing a holding tank into your second floor, which definitely increases the hassle factor. The other option is to utilize a circulating pump that's thermostatically switched to pump water whenever the fire's a'blazin.'

You can plumb this to dump heated water directly into your existing hot water tank, or you can plumb it to dump it into a pre-heat holding tank (whether you do a thermo-siphon or circulator system). Either way, you'll be tapping free BTUs and saving fossil fuels. Greenhacking at its best.

Handy Tips: Get the book from Lehman's. Though this writer has been as clear as possible with his directions and warnings, he is limited by space and his own somewhat suspect intellect. This is an award-winning hack, but it requires a real understanding of the thermal dynamics involved.