Backup water heater and code

I'd like to implement a backup water heater without creating a "hydrostatic pipe bomb" or violating any meaningful plumbing code requirements.
My thought is to connect both water heaters to the cold water supply with a single ball valve in line with each inlet connection. The outlets would be connected through 3-way valves configured so that each water heater is either connected to the house hot water line or open to a drain.
I've though of using ball valves on the inlet and outlet ports and installing redundant pressure relief valves so that if the TPE valve failed there'd be backup but think the 3-way valves are safer.
One of these water heaters is a 450,000 btu/hr oil fired unit and the backup unit will be a 9 kw electric unit.
I'm in an area with no plumbing permitting or code enforcement but that doesn't mean I have a death wish. Getting a replacement unit in can take several weeks when the weather is bad so I'd like to be able to just switch over in the event of a failure.
Comments on code violations?
Thanks, RB
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that a variant of your plan might work. Instead of valving the inlets, why not T off the incoming cold water line so that both heaters are always "fed" by the incoming water. Put your selection valve on the outgoing line from both heaters, with the backup usually being off. The backup heater that way won't ever have any pressure build-up beyond the normal water pressure. In the event that the main heater dies, turn on the power to the backup heater and switch the valve over to it.
Marc
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The primary reason for the backup is in case one develops a tank leak. This always happens when I'm out of town so my wife is left with no hot water. The valving on the inlet is necessary for the obvious reason...preventing the flood.
RB
MAG wrote:

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Put globe valves, with levers, on both in and outlet. Then use the flex connectors to connect to the tank itself.
Easy to use, and see, well worth the few bucks extra.
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Unless you are joking, I suggest that you just replace the old WH with a new one and make it a habit to replace it before the manufacturer's warrentee runs out.
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The only dual water heater arrangement I have ever seen had the two connected in series. Of course when one broke, they didn't know it until the second one also died...
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