By Patricia Greene – A good hot shower or bath is essential to everyone’s well-being. A shower or bath on a cold day with free hot water from your wood-burning cook stove that doesn’t waste fossil fuels, now there’s a luxury that can make your day.

A wood-burning cook stove with a large enough firebox to heat your home is an amazingly useful piece of equipment. It keeps you warm, cooks your dinner, bakes your bread, and dries your clothes. Add a heat exchanger coil, hot water tank, copper tubing, valves and fittings, and your wood-burning cook stove can heat all your domestic water, too.

A basic thermosiphoning hot water system has a stainless steel heat exchanger coil bolted to the inside of the firebox and passing through the rear of the wood-burning cook stove to connect with pipes that run up to a regular 30- to 120-gallon hot water storage tank above the stove by at least 18 inches, and ideally placed on the second floor above the stove. The system is plumbed at about a 45- to 90-degree angle so that rising hot water and falling cold water circulate continuously as long as the stove is hot, and is connected to the house’s hot water system.

Variations on this basic theme use a circulating pump and are thus able to connect to the regular gas or electric water heater in the basement for off-season use. Some people have tried homemade coils installed in the stovepipe or on the exterior of the stove wall. The system also complements solar hot water perfectly by heating water in the less sunny part of the year. If installed with a flip switch, it can also work in tandem with your current water heater.

To install this system yourself, you will need basic plumbing and mechanical skills, seasoned with a sense of adventure, plus the ability to use a soldering torch, and some plumbing tools. Each system will be a little different and require some creative thinking.

There are many advantages. It really can supply enough hot water for a family. If you are burning hot, the system can supply about 20 gallons of 120-degree water an hour, but it can get much hotter. It will retain that heat for 48 hours in a properly insulated tank, even after the fire is out. So when you’re not running your wood-burning cook stove continuously, you’ll still get that early morning shower.

Best of all, cost and payback are good. If you install it yourself and can scrounge a hot water heater, it will cost you approximately $250-$700 for the coil, $400 for copper pipes and fittings, valves, and gauges, and $50 for pipe and tank insulation. Let’s say your electric water heater is costing you a painful $40 a month, and you live in a northern area where you can run your wood-burning cook stove hot six months of the year. The bottom line is $40 a month x 6 equals $240 that you’ll be saving annually. So in less than three years you’ll have paid off the cost and be enjoying free hot water using this low-cost construction technique. (Ed. Note: Prices from 2010)

System Details

Although this hot water system may be installed in any wood-burning cook stove, many newer cookstoves are designed to heat water and have a coil that you can buy directly from the manufacturer. The safest, most widely used, and efficient heat exchanger coils are made from pressure-tested stainless steel and designed in a simple U or W shape to be installed inside your firebox. They come in different sizes with mounting hardware, gaskets, and instructions. You may also order a pressure relief valve (a necessity!), and a hole saw with a bit for drilling your stove. Custom coils are also available. Cost is from $170 to $270. (See the end of the article). Lehman’s Non-Electric Catalog also has a hot water jacket that installs in the firebox for $395, and by the way, don’t forget to order their useful booklet Hot Water From Your Wood Stove, for $9.95. (Ed. Note: Prices from 2010)

Once you’ve measured your firebox, decided what size and shape coil is best, and ordered it, you’ll need to find or buy an electric or gas water heater that is the right size for your needs. If you’re scrounging a second-hand tank, make sure it’s rust free and water-tight. The ease with which the fittings and connectors can be removed from an old water heater is often a good indication of what shape it’s in. Sometimes plumbers will have used water heaters they’ll be happy to part with that have nothing more wrong than a broken thermostat. You could also use a galvanized steel tank to save money, but you’ll have to insulate it with fiberglass, as you will any water heater tank. In placing your tank remember this: you can move the tank up to two feet away from the wood-burning cook stove for every foot it is above the coil exit from the stove back.

Remove the cover of the water heater, and unscrew and remove the electric element and thermostat on the tank. Using a hole saw, you’ll drill two holes from inside the wood-burning cook stove where the threaded ends of the coil will come through and be sealed with nuts, a flat washer, and a gasket.

Basically, hot water from the coil comes out of the stove and rises through 1″ copper pipes to enter the tank through the upper element. (See diagram). Cold water returns out of the bottom drain valve down through 1″ pipes to re-enter the coil and be heated again. The hot water pipes are installed at 45 to 90-degree angles and connected to the regular hot water plumbing pipes to the kitchen and bathroom. In order to facilitate flow, the hot water pipe must slope only up for at least several feet after exiting the stove. After that, you may have 90-degree bends that will slow the flow but two 45 degree fittings are better than one 90.

You will need a drain valve, plus a temperature gauge in a place you can easily see, and two pressure/temperature relief valves on the hot water output near, but not too close to the wood-burning cook stove and plumbed to a safe place, such as a five-gallon bucket or into your sewer system. At the tank, you will install a temperature regulating valve set to 120 degrees, and at the highest point another temperature/pressure relief valve, a vacuum relief valve, and an air bleeding valve. Make sure you follow the plumbing codes.

Problem Solving

Generally, this system is easy to maintain, but here are a few tips.

In the beginning, the system will get really hot and may blow off the pressure relief valve, but as creosote builds up on the coil this problem will subside. Burn your wood-burning cook stove a little cooler.

If you have hard water, lime scale will buildup on the inside of the pipes after a number of months. Using the drain valves, you can flush the pipes with vinegar at least once a season.

Creosote will build up on the outside of the coil and can be scraped off to keep the heat exchange at maximum efficiency. And speaking of creosote, check your pipe or chimney more often as the heat exchanger will draw BTUs from the firebox and make your fire burn somewhat cooler.

For insurance purposes, you may need to use a coil that is certified for use with your wood-burning cook stove.

This system may affect EPA emissions certification because it takes heat from the combustion process. If you are concerned about this, you may possibly be able to use flue mounted collectors to solve the problem.

Keep an eye on your temperature gauge for a while to assess how hot your system is burning. Draw off more water if it’s burning too hot. Hey, an unexpected bath is a wonderful thing!

If you don’t feel you have the skills, but still want a wood-burning cook stove hot water system, consult solar hot water installers in your area. Many of them are starting to install these systems.

Resources

Therma-coil.com and hilkoil.com both manufacture and fabricate stainless steel heat exchanger coils for wood-burning cook stoves. Lehmans.com sells wood cook stoves and jacket heat exchanger systems, and a booklet entitled Hot Water From Your Wood Stove. 

If you love wood-burning stoves, here are some great tutorials from Countryside Network for masonry stove plans and a wood-burning outside oven using local stone.

Published in Countryside January / February 2010 and regularly vetted for accuracy.