I have two water heaters (different brands) running in parallel. When I run hot water in the house only one tank will drain while the other sits there full. I have manual shut off valves so if I close the hot water on the first tank, then the second will start running.

Both tanks are 50 gallon.

Any help would be appreciated.

  • 2
    Did this arrangement ever work or is it a new problem? Why do you need 2 water heaters? What do you mean by "drain". The water heater will fill up with cold water as the hot water is used.
    – Barry
    Commented Mar 21, 2018 at 22:37
  • Apparently water moves more easily through one than the other, so that one supplies all the water, and then runs out. What were you hoping to happen? Commented Mar 21, 2018 at 22:47
  • Are these tanks draining by gravity from a height above the delivery points or are they under pressure? What country is this? Commented Mar 22, 2018 at 1:08
  • I've only seen parallel tankless water heaters, and they use special control boards to synchronize them...
    – Hari
    Commented Mar 22, 2018 at 22:24
  • I bought the house and it came with two tanks in parallel. One of the tanks began leaking so I replaced it with a different brand (same capacity and similar footprint). I didn't move any of the original piping. I was. I know my father in law has a parallel system as well in the mountains and it works fine even though the piping is different distances (ie one tank is closer to the cold intake than the other). Commented Mar 23, 2018 at 16:35

1 Answer 1


Piping 2 water heaters in parallel is much more than just piping, it is an art. If either tank has longer sections of pipe for the in and out connections, before the common piping, then the one with the shortest piping will get most of the flow.

The preferred method would be to start with 2 tanks from the same manufacturer, the same size and the same model number so both are exactly alike. You may need to contact a "good" plumber and ask him to do this for you. Myself, I prefer to pipe 2 tanks in series, it is easier to do, yields the same results, and is fool proof.

One last idea, if you need this much hot water to fill a large Jetted tub, you could use a single 50 gallon water heater and for the tub add a dedicated electric "point of use" water heater that would only be used when the water heater begins to run out of hot water. This is what I did.

  • In a series arrangement would you set the thermostats to stepped temperatures to balance the workload?
    – isherwood
    Commented Mar 22, 2018 at 16:50
  • The temperature of the 1st tank, that which receives the cold water, could be adjusted as necessary from a low to normal setting based on the needed capacity of hot water. I never liked parallel hook ups, unless there were more than 2 tanks in the set since it was very hard to get the piping right for a perfectly balanced flow.
    – d.george
    Commented Mar 23, 2018 at 9:31
  • I think I'm just going to change it to in series. Thanks for the feedback. Commented Mar 23, 2018 at 16:36

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