I have 2 series-connected water heaters. I have sediment showing up in my shower head clogging it. So I am thinking I need to flush my water heaters. I understand if I have 1 water heater to shut off the inlet valve and drain the 1 unit and flush. However, I have 2 and they are in series. There is only 1 inlet shutoff valve that goes to the first water heater. Do I drain and flush the first water heater then try and drain and flush the 2nd (downstream) tank? I can see how to drain both but not sure how to flush the 2nd tank (downstream).

Thanks, Marvin

  • If they are electric, you need to turn off the power to them first. The heating elements will burn if not covered with water.
    – crip659
    Apr 3, 2021 at 0:36

1 Answer 1


Since you didn't say it, and it matters - first, shut off the power or fuel to both heaters, so they are not dry-fired, which will quickly destroy them. This is very important any time a water heater is not full of water.

To flush the second tank, if they are connected in series, consider the first tank as if it was a pipe.

So, shut off supply, open hot taps, drain both, turn on the supply to flush the first, close the drain on the first, let the first fill until it runs into the second, flush the second, close the drain on the second, fill until no more bubbles come out the hot taps, and only then turn the power or fuel back on.

Might want to check the anode rods while you are there.

Be alert to the possibility that malfunctioning thermostatic mixing valves (as on a shower) could route cold water back to the hot side.

  • Thank you for your direction. Yes, I will turn the heat knob on the units to pilot mode, I understand this to keep the gas pilot light on but will not fire up and run the heat on the tanks. Apr 3, 2021 at 1:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.