Has anyone ever seen someone pipe two gas water heaters in series? Is that really a “thing” in your part of the woods? Seems rife with unintended consequences.
Piped in series to increase amount of hot water available. I hope you noticed missing vent.
BTW, it’s best to pipe multiple heaters parallel instead of in series. In series the first one is first to fail. The second tank acts more like a buffer than a heater.
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Yes it is common to pipe two water heaters in series. I prefer to parallel pipe the water heaters. Not a defect. Simon has already mentioned the flue pipe. I will add that the small expansion tank will not be large enough for both water heaters. It also cannot be supported with piping material.
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Thanks. Yeah, they’re already operational anyway. I tried the master garden tub and it spewed hot air and water in less than a minute. All the way in the hot position, though, the water pressure drops as usual. It seems to me the parallel piping would even help the water pressure marginally.
In series is nice if you want to be able to regulate how much water you are heating (only turn on the first one when guests are in town, etc.). And yeah, WTF happened to the vents??? That’s crazy!
Actually that’s what you do when your parallel pipe. You are able to shut one unit down while the other one is operating. But yes you can shut the first one down and it’s just considered a large 50 gallon piece of pipe 
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Wouldn’t the water from the “off” tank dilute the water from the hot tank in parallel?
No the nice thing with parallel piping is you shut down the water supply and the fuel supply. The thing I don’t like about a series piping is the first water heater does all of the heating and the second water heater is just storage. You will likely get inconsistent water temperature as well.
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LOL… I guess I’m realizing why I’m not a plumber 
Usually, when I see two tanks it’s on a larger house and they just go to different area completely.
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So you trap water in the unused tank in parallel? How long before the legionnaires start? Or should you keep the temp on that one high to prevent growth? Wouldn’t that reduce some of the energy saving with bypassing it?
Also, in series the first tank in the series is only “just for storage” when you don’t need the additional capacity of hot water. When you have guests over, you turn on the power and increase your hot water capacity the same as you do parallel.
In series, you don’t have the option of eliminating one unit or the other. They are inescapably co-dependent. And, because the first unit is receiving cold water at its inlet, and the second unit is receiving much warmer water at its inlet, the first unit will see a lot shorter lifetime than its Siamese sister.
You are shutting down the wrong water heater in the series. Only the last water heater in the series should be active all the time and provide hot water to the distribution in the home. The first tank in a 2 tank series can be powered off and becomes a large 40-55 gallon cold water tank. Yes, the tank that is more active is likely to fail first but that doesn’t mean it will have a shorter life than a single water heater home right? It isn’t being overworked, just more worked than the extra tank.
Yes, but usually homes with two 50-gallon water heaters are occupied by large families with a demand for a LOT of hot water. Just letting the burner go idle means the first water heater in the array serves as nothing more than a storage tank for cold water, and is not serving the intended purpose of extra BTU’s.
Agreed. If there is a constant demand for high quantities of hot water as you described, a parallel setup may provide better hot water to meet the demand. Both setups exist because neither satisfies all possible expectations and demand scenarios.
I would just verbally talk with your client about the proper way to manage them. Not really a deficiency for the report in my opinion.
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I found some good arguments from both sides of the issue on a plumber’s forum:
It’s been done that way along with parallel piping for decades and decades. Never heard of your concern. Do you understand that the water heater that is turned off still has water circulating through it when piped in series? I’m parallel that heater can be isolated and drained.
If you are draining the bypassed tank, then legionnaires would no longer be a concern. Seems like a lot of work for the typical homeowner. I thought you were simply bypassing your parallel tank and leaving it full which was my concern.
In series, the inactive tank acts as a cooling device. Gas furnaces have a hole in the center, up which hot air rises. Hot water makes hot air, which rises.
Parallel is better, as others have noted, you can disable one heater during low demand periods.
Can you provide me with something that states that Martin, as i see that installation all the time in my area.