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:
And here are the two electric tank-based heaters:
There is also a water recirculation pump hooked up to the water heater on the left. Here's a close-up shot:
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?