Check valves for entirely passive solar water heaters


A check valve, or one-way valve, permits flow in one direction only. Certain solar water heating systems require a check valve to prevent a reverse (cooling) flow through the solar water heater when the heater is cold.

A check valve for an entirely passive solar water heating system must be very sensitive. In an entirely passive solar water heating system, water can move through the solar water heater only by natural convection – a movement produced only by the small difference in gravitational force due to the density difference between warmer and cooler parts of the water.

floating-buckley-conducting.GIF Shawn Buckley invented a particularly sensitive fluidic check valve while inventing a modular solar heat collector/heat store, the thermic diode. A fluidic check valve for water uses an auxiliary fluid, for example, oil, that differs slightly from water in density.  The two fluids are the only moving parts of a fluidic check valve. Buckley claimed the fluidic check valve  in U.S. patent 4,245,617 (1981).  The thermic diode and its fluidic check valve are described less formally in Chapter 23 of Buckley's book, Sun Up to Sun Down, 1979, McGraw-Hill, ISBN 0-07-008790-3, pp. 133-137.

See also an old Mechanix Illustrated article about the thermic diode: Solar Diodes, Mechanix Illustrated, #583, December 1976, (v.72) pg. 31.

William A. Shurcliff described Buckley's thermic diode and its fluidic check valve in his book New Inventions in Low Cost Solar Heating, 100 Daring Schemes Tried and Untried, 1979, Brick House Publishing Company, Andover, Mass., ISBN: 0-931790- 02-6, pp. 62-65.

In the same book, pp. 66-69, Shurcliff described his proposal for a novel floating check valve that requires no auxiliary fluid, but is less sensitive than Buckley’s fluidic check valve.  Shurcliff's description of the floating check valve is reproduced here immediately after his description of Buckley's thermic diode.

We can use the idea of floating the valve to create a floating fluidic check valve that may allow easy one-of fabrication. See Fig. 1.