A Reddit question about a water heater pipe behaving strangely turned into a discussion filled with important warnings and concerns, as plumbing enthusiasts said that a setup that seems merely unusual can also pose health risks.

Commenters said the home had two electric water heaters installed in series, a configuration they argued creates problems that go beyond simple inefficiency.

What happened?

In a post on Reddit, the homeowner said the connector between the two tanks stayed cold during a shower. In the system they described, cold water enters the first tank, passes through a jumper to the second, and the hot-water line exits the second tank.

The thread drew 167 comments and fostered discussion and critique of the series layout, with presumed experienced users saying it was a bad way to arrange the heaters. The top response said: "Series WHs is a dumb setup IMO. You'll overwork the first one and there's no actual gain."

Efficiency was not the only issue people raised. Several commenters warned that in a series arrangement, the first tank can remain at a lukewarm temperature that may favor Legionella growth. The Centers for Disease Control and Prevention says that this type of bacteria grows best between 77 and 113 degrees Fahrenheit.

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Why does it matter?

Legionella bacteria can cause Legionnaires' disease, which is a serious form of pneumonia. Multiple commenters said some jurisdictions prohibit this type of series water-heater setup because one tank can remain in that temperature "danger zone."

An odd plumbing arrangement is not always just a harmless coincidence. If one heater does most of the work, it can pose safety risks, weaken hot-water performance, and increase monthly costs. That's why it's important to have vetted installers.

Users also said the design may not provide the benefit homeowners assume it will. One person commented: "I prefer parallel over series installs. You have a series install."

If the goal is greater capacity, commenters said there are better ways to get it than leaving one tank underused and the other overworked.

What's being done?

As a more typical fix, commenters suggested repiping the heaters in parallel rather than in series. The discussion also touched on thermostatic mixing valves, or TMVs, which could keep the tanks at 180 degrees and then temper the water to 120 degrees where it is used.

Some of the reactions in the thread came down to short conclusions expressing preference toward a parallel system.

One commenter stated: "I've only ever done this once for an apartment duplex, and I was told specifically by the plumbers to make sure it was parallel and both lines the same distance off the top of the tank."

Another Reddit user put the alternative just as plainly, saying: "Parallel is the way this is done, full stop."

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