After thinking about it, you have to create a vacuum by opening the faucet. If you just shut off the water, you will invent all sorts of new bad language when you get drenched.

Also, you must have a shut off valve on the supply side just before the heater. Closing the main water supply won't work.

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"I was a dirty bird, Carol's not grungey - she's *****in" John Milner....American Graffiti