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


| 11/10/2016 10:07:00 AM


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.

Performance

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.



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