One of the features of the much-debated Margarido House is the D'MAND hot water system. It's basically a water pump that circulates or flushes your hot water line when you need hot water. The water that was in the hot water line is then pushed back into the hot water furnace to be reheated. This saves the water that would have gone down the sewer if you just let the water run until it is hot. In my house the second bathroom can have a wait of 3 minutes in the morning for the water to be hot, so that's quite a bit of water. I'm interested to know if anyone has firsthand experience with this device.
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