yes, this is correct. And definitely cut the power to the one valved off otherwise it'll build pressure and pop the T/P valve when it goes into a heating cycle.
I'm really not sure whether it's better to drain or leave full, from a corrosion standpoint. I'd be worried about anaerobic bacteria possibly growing in a stagnant full WH as well.
nate
Add 2nd valve.... I have the same dual Elec. WH in our 3000 Sq ft. home with only 2 people.They are side by side in the garage, & space is not an issue, and I like the standby issue for service or guest issues. Both heaters feed into the same hot water line in the attic, so to completely isolate one, I will install a ball valve on the hot water outlet, so that hot water does not flow back into it. I would still like a definitive answer about the best practice with regard to draining or leaving water in the heater being taken out of service for an extended time.
Two people don't need two water heaters (unless they are very modest).
I think you should turn one off. Is there a turn off valve already there for the input to the extra WH? If so, I don't think you even need the new valve you are talking about. A tiny bit of heat will be conducted to the adjacent water where the two pipes join up. The hot water in a half-inch pipe at a T-connection isn't going to conduct more than a teeny bit of heat. In my case, much of the time the water in the supply pipe from the WH isn't even hot, because it's been an hour or more since I used any hot water and the copper pipes let the water in them cool off quickly. When the water in the pipe is not hot, it can't heat the water in the pipe coming from the other WH.
I've gone out of town for 3 months, 2 months, 3 months, and 4 months, and I turn off the electricity but I don't drain the tank. Of course I don't drink the hot water when I get home, but I've never noticed a smell or anything.
FWIW my water heaters have been listed as glass-lined, but when I tore one aparet it was flexible vinyl with, I guess, some sort of glass in that. I thought it was "glass' and if I droped the WH it might shatter! No chance of that.
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