Dual-Mode Hot-Water System Heated with Solar and Wood

Reader Contribution by Christopher James Marshall

How the Dual-Mode Solar-and-Wood Water Heater Works

Having hot water for showers and dish washing is certainly a joy that makes off-grid feel as luxurious as on-grid, yet without utility bills or fossil fuels. Where I live, summer sunshine is abundant, but winter cold requires the heat of a woodstove.

In summer, I connect to the solar hot water panel in the greenhouse, and in winter to the hot water coil on the wood stove pipe. I have cold and hot water tanks, located upstairs, to provide gravity-fed hot and cold water to a kitchen sink and a shower on the lower floors. The hot water tank is filled manually, by a pipe and valve plumbed from the cold tank.

I let the hot water tank heat all the water in the tank to a usable temperature and then use it. Hot water is circulated to the hot water tank by heat convection — no pumps.


The solar collector heats my ten gallon hot water tank to 120 degrees Fahrenheit in about three hours on a summer day, even if partly cloudy — the stove pipe coil heats the tank above 130 degrees F in about two hours, or three hours if the starting temperature of the tank is near ice-cold.

A surprising discovery is that the hot water coil will lower the stove pipe temperature by 100 degrees F when water is circulating. Depending on how hot the water in the tank is, and if I mix cold at the faucet, showers use 2-1/2 gallons and washing dishes use 2 to 4 gallons per day. The tank will drop about 20 degrees F overnight or after the woodstove has stopped burning.

Solar Hot Water Collector

The solar hot water collector panel is made with an array of CPVC pipe on a flat plate collector (3 feet by 5 feet). It has ten 1/2-inch collector tubes that “T” into a top and bottom manifold that is 1 inch CPVC and routes to the hot water tank. CPVC pipe can withstand 200 degrees F, while the hottest temperature on my flat-plate collector is 130 degrees Fahrenheit.

Stove Pipe Hot Water Coil

The hot water coil is 3/8-inch copper tube that is coiled around the stove pipe about a dozen times and joins 1-inch copper pipe that is routed to the hot water tank. Copper pipe can withstand boiling water without damage. Do not use plastic fittings with copper pipe because plastic will melt. The stove pipe can get very hot, over 500 degrees F, and it could easily turn the water in the hot water coil to steam in a closed system, unless you have an open tank with a loose-fitting lid so that no steam pressure will build up.

Summer and Winter Modes

When the season changes, I simply swap the connection to the hot water tank between the pair of CPVC pipes from solar for summer, to the pair of copper pipes from the woodstove for winter.

Heat Convection Loop and Hot Water Tank

Two key points to remember about using heat convection to circulate hot water from the heat source to the hot water tank:

1) Heat convection flow will stall if there is too much resistance. That means the pipes must be large diameter, 1-inch minimum, and routing must have gentle angles — never use a 90 degree turn, instead use two 45-degree fittings or a flexible hose (the metal type used on typical hot water tanks). Heat convection flow will stop if the tank water level drops below the hot water inlet.

2) Heat convection requires that the heat source (where the heat is collected) be lower than the hot water tank (where the heat is stored). The tank should have four ports, one is the fill inlet and one is the hot outlet, and two for heat convection. The hot water inlet port that brings hot water from the heat source must be ABOVE the hot water return port.

Similarly, hot water output is from the TOP of the solar collector and/or the hot water coil, while the cold water return is supplied to the BOTTOM of the solar collector and/or the hot water coil.

More ideas for your homestead and small house are in Christopher James Marshall’s holistic guide, Hut-Topia: How to Create Sustainable Small Homes and Homesteads. Read all of his MOTHER EARTH NEWS posts here.

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