A. Joseph Stoddard responds: In the situation you describe, you can get acceptable results without a pump. The hot water "passive recirculation" loop is a technique I learned from an old plumber years ago. I’ve successfully used this technique a number of times when the bathroom is located above the hot-water heater. By providing a return piping run to the hot-water tank, a natural circulation (called a thermosiphon) is created. The cooler water in the upstairs bath sinks back towards the hot-water heater, and the hot water from the heater rises and replaces it.
I’ve had acceptable results using 1/2-inch-diameter copper return lines, but would caution against using a smaller-diameter return: It might not allow enough water to circulate through the loop.
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