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I would like to be able to feed water from the cold water pipe into the hot pipe directly bypassing the waterheater (WH) upon demand, such as the WH being off for whatever reason. The switch would be a shutoff valve on a tee after the shutoff for the WH. If the WH shutoff is off and the bypass/bridge valve is on, the water gets fed from the cold pipe directly into the hot without going through the heater.

I understand that most water terminals (faucets etc) works fine with just cold being fed, except that the dishwasher doesn't. I want to be able to run the dishwasher (even though I am using cold water) when the WH is off for whatever reason (every couple of years the thermocouple goes out and takes several days to replace).

My question is, is this design something commonly done, am I calling it right, and to see if there are any (code) issues with it?

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  • The big question is when does the dishwasher heat water? If it heats water only for certain parts of the cycle, to "sanitize", but the main wash relies on "incoming hot water is hot enough" then you will likely have problems. If, on the other hand, it always heats water if less than specific temperatures, then running the dishwasher that way will work just fine. Oct 14 at 15:33
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    Shouldn't the bypass/bridge valve be before (upstream) of the WH shutoff?
    – HoneyDo
    Oct 14 at 15:44
  • If needing to bypass the tank, then two tees, two pieces of pipe and a shutoff valve should do. Most tanks have the cold inlet and hot outlet close together. Add @HoneyDo's suggestion.
    – crip659
    Oct 14 at 15:44
  • Should add a shutoff valve/s between bypass and tank if not wanting water to go to tank(removal of tank). Usually tank will act as a bypass itself if not working to heat water. If part is needed every couple of years, why not have a spare/s handy?
    – crip659
    Oct 14 at 16:10
  • Not just the dishwasher, also pressure balanced fixtures (shower) won't work without the bypass. This, in case you intend to use the shower or tub faucet for other purposes.
    – P2000
    Oct 14 at 16:34
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If the water heater is simply off water can still flow through -- no need for a bypass.

The bypass can come in handy for a situation where the heater has a leaky failure: pressure relief valve or drain valve or tank liner leaks, or the entire heater is to be disconnected for an extended period. In those situations you need not only a bypass but also a shutoff for both the cold supply and the hot discharge pipes.

If you're into elegant-looking plumbing, consider using a pair of 3-port ball valves. They're harder to find but doesn't cost tremendously more than ordinary valves, and the cost will somewhat be equalized by needing only two of the 3-port versus three of the conventional valve. (image: supplyhouse.com)

3-port ball valve

Whether you use three 2-port valves or two 3-port valves, arrange them in an H so that water going in or out of the heater tank is stopped by valves in the lower vertical legs of the H and the bypass goes across the horizontal part of the H.

This is not commonly done for water heaters though it is routine for water softeners. It's anybody's guess why, but I'll submit that a plumber doesn't spend time and money (ie, his profit) installing valves nobody required him to install. It's a good idea, though, for the "continuity of operations" and serviceability reasons you identified.

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