We recently moved into a property with 2 water heaters installed. These were installed less than 3 years ago, so they are relatively new. Now, in an effort to conserve electricity (we are just 3 people in the house), I switched off one completely. We then noticed that the other (functional) water heater's pilot was going off every other day, causing no hot water. We then switched both heaters on 2 days ago, and the problem hasn't reoccurred...

We had a technician come over today, and he said it could be a problem with the thermocouple. But when I mentioned that one of them was switched off, when the problem occurred, he evaluated the plumbing, said that both feed into the same water line, and that that could be the problem causing the pilot on the functioning heater to switch off.

My question is - Does the operation of one water heater affect the other in a setting described above? Of course, I will evaluate whether the problem reoccurs with both heaters being on over the next few days.

enter image description here

  • Can you provide more info about how they're connected to the rest of the plumbing (series, parallel, separate hot water distribution pipes?). Also, when you turned off one heater, did you close any valves? – Hank Feb 15 '14 at 17:48
  • Not sure about the plumbing..all I was told by the technician was that they are reliant on each other - so whatever that entails...I'll try taking a picture from just above the heaters. I did not close any valves - just turned the dial to OFF. Thanks. – rs79 Feb 15 '14 at 17:54

It sounds like you are describing two different problems.

FIRST PROBLEM: Turning off a hot water heater.

First, there are two ways to hook up two water heaters: series and parallel.

enter image description here


  • In series, the water comes in to the first water heater, then passes through the second hot water heater, and finally goes to your fixtures.
  • If you turn off the second water heater, then the water in the second one cools down between uses. This results in very long warmup times.
  • If you want to turn one of these off, turn off the first one. (This is actually a legit way to save on energy costs if you don't need the extra hot water volume because in many places the water supply is colder than room temperature and the first tank allows the water to sit and reach room temperature before it's heated.)


  • In parallel, the incoming cold water is split between the two water heaters. Hot water coming out of the two heaters is combined and sent on to the fixtures.
  • If you want to turn off one of the water heaters, you also have to close the valve to the inlet or the outlet. Otherwise you will just be mixing the cold water in one water heater with the hot water from the other heater.
  • Leaving one heater closed off for a long time is a bad idea because the water can become stagnant and foul.

SECOND PROBLEM: Pilot light going out

Just turning off one heater should not cause the pilot light on the other to go out.

  • I agree, the pilot flame should not go out regardless of the setup. – Louis Somers Feb 15 '14 at 20:59
  • Would you be able to assess, looking at the picture, whether they are in series or parallel? Thanks. – rs79 Feb 24 '14 at 15:36
  • 1
    No, not enough of the plumbing is visible – longneck Feb 24 '14 at 16:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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