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My (used) hot water heater was used as a thermal battery/buffer tank by the previous owner, one with cold in, hot out, and a center connector for over-pressure discharge. The previous owner used the cold in for the over-pressure discharge instead of the middle connector. Why would they do that? Did they use the middle connector as HW panel in?
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Without looking at the application, and because it's most often the most thermally efficient way for most domestic solar domestic water heating systems to be plumbed, the common way to plumb a tank is so that the return line from the collector(s) goes to the top of the tank. on standard (non-solar) tanks, that's the hot outlet to the house plumbing.
I can't think of a common reason why what's usually intended as a relieving device connection would be used as cold water inlet. Some code inspector might get porky about doing that due to reduced discharge capacity of a relieving device in a relieving event caused by decreased discharge pressure from inlet pressure line pressure drop caused by a dip tube or possible complete collapse (and so blockage) of a (relatively) flimsy dip tube.
Usually, the cold inlet on a water tank, either standard or solar, has what's called a dip tube attached to it (on the inside of the tank) that takes the cold water that's inlet to the tank inlet to the bottom of the tank. The hot outlet on the tank has no dip tube with the idea that thermal stratification of the water in the tank will mean that the hottest water in the tank will be drawn off first.
Solar tanks with extra connections are more fit for purpose, but in standard (non-solar) tanks I've plumbed for DHW service, I've used the tank drain as the tank return (tank outlet) to the collector with a "T" attached with a valve for drainage is/as necessary, and the hot line back from the collector teed to the tank outlet to the house and a thermal loop in the hot water return from the collector.
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