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Instant hot water systems that are installed after a house is built use the cold water pipes to return hot water back to the water heater, or discharge that water through normal cold water use. (I imagine that systems that are installed during construction often have a dedicated hot-water return pipe.) This means that sediments from the hot-water pipes, generally contributed by the anti-corrosion rod, finds their way into the cold water system. This suggests that if you've installed one of these, you should be extra diligent about occasionally draining the water heater until the water runs clear. OTOH, it seems that normal cold-water usage ought to keep it fairly free of sediments, anyway. So which is true? Should you just drain the water heater occasionally as always, or more often?

  • Hqve you worked with on-demand heaters before? Could you be talking about circulating hot water systems? Your sense of how they work is rather quite different from mine. – Harper Nov 13 '17 at 17:11
  • @Harper, I'm referring to circulating hot water systems. I'm going to install one, if only for the master bath, as it takes 65 seconds for the hot water to get there, and I wondered if I need to be more diligent about draining the water heater more often than I do. I'm fairly sure our water system is remote manifold to the bathrooms, and trunk and branch to the kitchen. – BillDOe Nov 13 '17 at 19:48
  • Ok that makes more sense, I would still revisit your knowledge of how circulating hot water systems return water. I'm pretty sure it's not by the cold water pipe, you'll need to add a new pipe. I say since you're buying copper either way, I'd rather run wire than pipe, and install electric PoU heaters, and not need a hot water pipe at all. Way more efficient to boot. Last I priced 1/0Al is cheaper than 3/4 copper pipe + insulation. – Harper Nov 13 '17 at 20:24
  • @Harper, this is the recirculating pump I purchased amazon.com/gp/product/B000E78XHG/…. It does use the cold water to recirculate the hot water. – BillDOe Nov 13 '17 at 22:31

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