I want a thermal expansion valve. They are common in HVAC systems but I want one that operates around 200 degrees F. The ones designed for HVAC operate at much higher temperatures I believe. I did find one for sale at this link http://www.mcmaster.com/#temperature-regulating-valves/=o1znnq but I don't want to spend the $1000+ on it. So the easy solution is to make one. Now how... I have some ideas but I am just thinking out loud to see if there is any interest. Essentially, a bulb of gas expands in the presence of heat forcing a piston to extend from a cylinder. The piston causes a valve to open. As the temperature cools, the gas contracts, the piston recedes, and the valve closes. No electricity. Direct(ish) control.
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