Weird water heater problem

Came across a weird water heater issue that I could not figure out yesterday on a warranty inspection.

The homeowner had tried to use only one of his two water heaters for some reason and had discovered the problem. I confirmed the problem but was stumped to determine what exactly was going on.

Here is the setup:

Two water heaters are installed in the garage but are separated by finished wall so the plumbing between them is hidden.

When the inlet valve to unit 1 is turned off all fixtures in the home lose supply. AND Unit two loses supply. Suggesting to me initially that the units are plumbed in series.

When Unit 1 is turned on and heating all fixtures in the home have hot water. When unit 1 is not heating and unit 2 is heating no fixtures in the home have hot water.

The homeowner explained this when I arrived and turned off Unit 1’s burner and turned on Unit 2’s burner. Then we filled all of the bathtubs to exhaust the hot water supply and allowed unit 2 to heat up during the inspection. It heated properly and supplied hot water out of it’s TPR. But did not supply any to the fixtures in the home.

I suspect that Unit 2 is improperly tied into the plumbing system. But I cannot figure out how it could be on the hot water side (confirmed by losing pressure when the inlet valve was closed on unit 1) but not supply at least some hot water. Even if the inlet side dip tube was missing it should have provided some lukewarm water to a fixture after it had heated to 140 degrees and it provided NOTHING. The water at the fixtures was absolutely cold.

I referred it over to have a plumber look at it and will be following up to see what they find.

Could be either of these issues:

  1. The two units are in series.

The hot water pipe going to the fixtures is tied (T joint) to the line that connects unit 1 hot output to unit 2 cold input.

It should be tied to unit 2 hot output.

Unit 2 hot output must be capped off somewhere.

  1. The units are in parallel. (least likely in this area)
    The output of unit 2 is capped off.

Probably plumbed in series with unit 2 being the upstream unit. When unit 1 is not being used and a hot water fixture is opened, the hot water from U2 flows into U1 and cold water from the U1 tank is supplied to the fixtures. If you run enough water you will probably eventually get warm water to the fixture after you have displaced enough cold water from the downstream tank.

If he only wants to run one tank, it needs to be the downstream tank.

Turing off the inlet valve at unit 2 does nothing at the fixtures. So I was thinking along the lines of what Bruce described. The house was PEX plumbing, but there was not an accessible manifold that could find. So I think the inlet for Unit 2 is supplied from unit 1, but just as another fixture. I have no idea where the outlet for unit 2 is going, I find it hard to believe it is capped somewhere, maybe it is hooked up wrong to an inlet line also. Whatever is going on, unit 2 acts like a dead end.

The bad part, I don’t think I would have found it. I would have noted the second unit was turned off, but he knew that. The house was inspected when he closed, and I don’t think this was found then. He made it sound like he found it himself. I’m guessing that he chose unit 2 to be the only heater initially and discovered nothing happened. I cannot think of anyway to reasonably check for this normally, but then usually you can see that they are obviously plumbed in series or parallel, or they serve different sections of the house.

He said he would let me know what the plumbers find.