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We just had two 50 gallon electric water heaters installed inline. Rheem professional classics. And a new check valve.

The check valve was for the circulating pump. They had to leave the old one on. They said it was stuck in an open position. There's little tank connected in case something goes wrong--the tank takes some of the overflow. It is connected to the water heater pipes.

When we take a shower the water fluctuates between mildly cool and a little warmer about every six to eight seconds. This is for the entire time we shower. It is not like when you flush a toliet. It did not do this before the new heaters were put in.

They did put a little tank on top to the side of the new heaters. We have had them out three times and they can not fix this. Do you know what is causing this? Help.

  • Is the valve on the shower a newer anti-scald valve? I believe your description possibly describes a malfunctioning anti-scald valve. – Tyson Mar 22 at 17:41
  • If you turn on the tub in that bathroom or a faucet (one that has a hot and cold handle) do you have a temp fluctuation problem? A single-handle shower valve has a pressure balancing system that could be acting up. If a two-handle faucet doesn't have that problem, I'd focus on the hot water pressure or junk in the valve from the heater replacement. – JPhi1618 Mar 22 at 17:41
  • @Tyson, that's what I'm thinking too, but how does a plumber not consider that? – JPhi1618 Mar 22 at 17:44
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The small tank is a thermal expansion tank, there should be two of them. You must have a check valve if you have expansion tanks. If you have two matching water heaters, they should have been plumbed in parallel and not in series. Also, the piping associated with each heater needs to be the same length or it will cause one heater to do more work than the other. This could possibly cause a fluctuation in the water temperature. Also, make sure that both heaters are set to the same output temperature. Everything must be "balanced" in a parallel setup. Here is a diagram of what two water heaters in parallel should look like:

enter image description here

Here is a diagram of two heaters connected in series:

enter image description here

Images are from a Rheem/Ruud technical service department pdf I have on file.

  • Thank you guys. I will talk to my plumber. You are much appreciated. – Katy Mar 22 at 19:54
  • Do Rheem/Ruud specifically discourage series connection, or is the parallel graphic given only as an example? I've known people to install tanks in series such that the first tank warms water to some intermediate temperature, then the second tank heats to the final temperature. "They say" less thermal shock due to smaller temperature difference between tank and incoming temperature, less heat lost to the room due to one tank operating cooler, and so on. The parallel arrangement has benefits too of course. – Greg Hill Mar 22 at 22:47
  • Rheem/Ruud suggest using parallel when both heaters are identical. This way, for every gallon used an additional gallon of cold water is introduced. They will both wear the same. They suggest the series method when you have different sizes of tanks (I/E: a new bathroom added to the home and you want to add another smaller water tank). The largest tank is first in the series and the smaller tank next. I added a series diagram to the answer as well. – Jerry_Contrary Mar 24 at 12:56

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