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Is crossover hot water recirculation a good choice for branch-type piping configuration?

I am considering replacing my aging hot water heater with a tankless. Several products offer a recirculation feature that sounds appealing, but I am uncertain whether recirculation would work for the unique way that piping has been run through my house.

The diagrams that manufacturers use to describe crossover hot water recirculation show a pair of long main lines that serve each of the fixtures from pairs of short branches. A thermal bypass valve is installed in the fixture that is farthest from the hot water heater, and the recirculation occurs through that valve. My understanding is that recirculation occurs only through the loop between the hot water heater and the thermal bypass valve; and that there is no recirculation to the fixtures that do not have a thermal bypass valve (but who cares? they are so close to the main that is carrying the recirculated water that it won’t take long for hot to get there).

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My house is 110 years-old, and has undergone several different remuddles that each added new fixtures to an existing system. My hot and cold water mains (if you can call them mains) are only 18” long, with 10’, 20’ and 40’ branches that serve single fixtures or small groups of fixtures.

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My assumption is that if I opt for a hot water heater with crossover recirculation and install a thermal bypass valve in one of the second-floor bathrooms, then only the second floor bathrooms will get the recirculation; and the fixtures on the other branches will have to wait for hot water.

My three questions are:

  • Is my assumption correct; will recirculation occur only on the loop/branch that has the thermal bypass valve?
  • Can I get whole-house recirculation if I put a thermal bypass valve
    at the end of each branch?
  • Is crossover hot water recirculation a good choice for branch-type
    piping configuration?

2 Answers 2

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You can decide for yourself if it’s a good choice, but I can tell you that it works well in my house with a storage-type water heater. I would answer your three questions “yes”.

I don’t think this will work at all with a tankless water heater. Those need a certain minimum flow to start up, and the flow from the crossover is minuscule. Opening a faucet will start up the water heater, so you’ll end up waiting for hot water, crossover or not. If you want prompt hot water from a crossover valve, use a tank-style water heater.

I have three separate branches for my 1st and 2nd floors and my basement. I put one crossover at the farthest sink on the first floor, which pre-warms the water going to 2 baths, a kitchen and laundry room. I put a second crossover in a second floor bath (on its separate branch) which also reduces the wait time for another second floor bath. My basement has a short run and doesn’t need a crossover.

As you suspected, a crossover on the farthest sink on a branch greatly reduces the wait time for hot water for any sink / shower / etc. on that branch.

I’ve got to warn you that the cold water will no longer be really cold, since the cold water pipes will contain warm-ish water that has passed through the crossover on the way back to a water heater. Mrs. MTA disliked the warm-ish water so much at the kitchen sink – it was wilting the salad greens – that I relented and ran a new cold water line to the kitchen sink from the cold water main before the water heater.

I have a circulating pump at the water heater in the basement to provide a slight pressure differential between hot and cold, but the second floor crossover works even without the pump running due to the different densities of hot and cold water. First floor, not so much without the pump.

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When I posted my question, I sent the same question to Rinnai, and this is their response:

Thank you for contacting Rinnai. The answer to your question is, in theory, you can. However, in practice, each branch is going to be different lengths, with differing numbers of fixtures and different numbers of elbows, so the the possible recirc flow through each branch and the pressures capable are different. Water will always follow the path of least resistance, and all the cold water lines from each branch eventually join into one main line that returns to the heater. And because they all return to the main line at different rates and forces, whichever branch is strongest will actually recirc its water through the heater and the others get pressured out. The first attachment illustrates. The recommended thing to do is add a snaking dedicated line, in order to get recirculation.

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See second attachment. This is why we never recommend multiple crossover valves. Does this answer your question?

See second attachment. This is why we never recommend multiple crossover valves. Does this answer your question?

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  • FWIW, I've decided to place a thermal bypass valve in the farthest 2nd floor bath and another in the first floor bath. This should theoretically create a recirculation loop between the water heater, the 2nd baths and the 1st bath. I'm not physically able to snake a cold water line from the second baths to all the other fixtures, but I'm not sure I'd want it if I could. The laundry doesn't need instant hot water, and not putting the kitchen directly onto the loop should mitigate some of the cold water issues that @MTA raised.
    – jcropp
    Feb 14, 2023 at 20:49

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