Yea, I thought it was pretty cool too!
I asked the same question of the folks demonstrating the system and
they couldn't tell me. My assumption is that the controller for the
pump motor was just a timer, so the system couldn't tell when the hot
water had reached that location. You don't want to pump hot water back
into the cold water line, so that must have been their way of
"shutting off" the pump until the controller shut down the pump.
I'd go with a smarter controller and, like you say, use a solenoid
valve and monitor the temperature of the water.
Another thing I was thinking about on this system, is that you could
install one pump at the water heater and push button/valve/temp sensor
at each sink/tub, with wires running back to a common controller. That
way you would have the expense of only one pump for the whole house.
You would probably have to add check valves around the pump to pass
the higher flow rate of hot water, so it might be cheaper to just add
a pump at each location.
>>I've also seen a system that used a pump and a thermostatic valve,
>>plus a control circuit. Before you turn on the hot water you press a
>>switch that pumps water from the (now cold) hot water line into the
>>cold water line via the thermostatic valve.
> That's a nifty idea. But why would the valve need to be thermostatic ? Why
> just a solenoid valve that only opens when the pump is running ?
> Seems like this would be easy to build yourself, and removes the concern over
> the hot water line being heated all the time.
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