A couple of questions regarding using the solar water heating system on Shabbos night. Even if you stop the cold water entering the tank do we have to be concerned with the water in the pipe going from the tank to the faucet that has now cooled down. Even if it gets pushed some could heat up. Or is this similar to the water droplets in a cup that we are not concerned about when pouring hot water from a rishon. Another question, do you know a person who rigged his dood shemesh like this?
The comparison between the hot blending with cold in the pipe and the drops in the cup is not so strong since the basis for the lenient approach to drops is that the drops are an insignificant amount of water and therefore an insignificant change (they also don’t add significantly to the resulting cup of hot). The issue with the blending is much more problematic since the result is much “hot water” (though perhaps slightly cooled by the amount of cold blending in). There is not consensus that the blending in the pipe is a direct result of my “action”. This is how many poskim understand but there is a minority opinion that can be relied upon for a sick person is that the blending is a result of “grama”. I personally don’t know anyone who rigged their dud this way. (Mainly because most people haven’t the foggiest idea of how and what to do with a dud). Additionally there are “vacuum pressure issues that exist depending on the exact situation which may present flow of water after the valve is shut. There may be a possibility under certain circumstances to set the “thermostat” so that it never allows heating above yad soledes in the tank. This would allow for full usage (but might produce only lukewarm water)