Hot water in an aptartment complex where I am the service electrician is running out because they only put 30 gallon heaters in.

Their plumber told the owners to hook up the elements so they run at the same time for quicker recovery.

I know the wire and the 30 amp breaker are not enough to handle that load but the plumber that gave the original advice did not. This leads me to not trust anything he is saying.

Will running both elements simultaneously cause too much pressure and cause the pop off valve to blow sending water all over the apt?


The unit is absolutely NOT designed to run with both elements on. This will void any warranty or safety standards of the unit. For the plumber, or anyone else, to suggest this is ok is EXTREMELY irresponsible.

Besides, while running both elements will heat the water up faster, it will not likely increase volume by much. To increase the volume, you need to......increase the volume. IE: bigger tank.

  • Perhaps it depends on the heater model/design. On page 16 of this manual I see at least two "non-interlocking - simultaneous" configurations. americanwaterheater.com/support/manuals/res-elect.pdf which suggest that a blanket denial of it being acceptable is incorrect.
    – Ecnerwal
    Apr 11 '15 at 1:02
  • 3
    Thanks for the input. I knew that if they were meant to run together, than they would be wired that way from the factory. The owner only is worrying about his bottom line. That's why everything in the complex was obviously done by the lowest bidder.But now he is trying to find ways around his previous mistakes. I can't seem to make him realize that doing something against code knowingly can cost him everything in a law suit.
    – rob
    Apr 11 '15 at 1:06
  • @Ecnerwal, Go back and look at those wiring diagrams again and tell me what you notice about the single phase "simultaneous" one. (The 3-phase one does NOT apply so that does not concern me). Hint: Look at the feed Apr 11 '15 at 1:06
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    Dual input, separate breaker, yes. Also both elements have a high limit, rather than only the top one. And then there are the cute 120A (3x40) "electric tankless" things - which are amusing as heck given how low standby losses are on a modern tank. I'll nuke my answer now.
    – Ecnerwal
    Apr 11 '15 at 1:08

There should be an expansion tank plumbed somewhere on the hot water line to handle the pressure fluctuations. Without this there will be potential pressure problems regardless of how the heating elements are run. With one (appropriately sized and plumbed) there will be no problems.

Hot water expansion pressure is created in a system with a backflow check (which one should expect to be present) by the heater volume multiplied by the change in temperature from the coldest (incoming) to the hottest temperature the water is heated. It doesn't matter how quickly that set temperature is reached, so running both elements doesn't cause an increase in peak pressure.

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