I have an interesting setup that I'm trying to understand (without tearing apart sheetrock). I have two electric tank-based water heaters, and a tankless gas water heater that is used for the boiler system.

This is the tankless heater:

tankless water heater

And here are the two electric tank-based heaters:

electric heaters

There is also a water recirculation pump hooked up to the water heater on the left. Here's a close-up shot:

water recirculator

I believe that the two tank-based water heaters are hooked up in series, with the right one receiving cold water and feeding the left one, which then provides hot water to the house. This explains why both the input and output of the left one are hot. However, on the right one, the output is always hot (expected) but the input is sometimes hot, sometimes cold. Why? Is it possible that the on-demand boiler is feeding into the input of the right one? Are there any other explanations?

  • Is this a house, apartment complex?
    – cutrightjm
    Dec 5, 2015 at 8:06
  • This is a small church Dec 5, 2015 at 8:33
  • 1
    I could see having 2 hot water heaters in series, the upstream one being on a timer so it only runs when the church is occupied. The downstream one provides water for anytime use, the upstream one refills it with preheated water at peak times, and off peak, does not. Doubling hot water supply in peak times, May 26, 2016 at 3:02

2 Answers 2


If the right hand heater were supplying hot water to the left hand heater, it would be strange to hide the pipework in the wall.

I guess it is more likely that they are feeding separate heating circuits.

It is not unusual for the return pipe to be nearly as hot as the output line, if the system has been running for a while.

There ought to be records of the system somewhere showing how it is connected up. If the church doesn't have them, the original installers should - or at least could advise.


Despite being obscured behind the wall, it turns out that the two water heaters were indeed in series, the right side feeding into the left side. I accepted the other answer tho since it is probably more helpful in the general sense.

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