I think PennBF received exactly the right answer about what happened. Without a check valve (one-way valve), the water pressure in the house "floats" with the supply pressure and gets no higher. With the check valve installed, and no expansion tank, the water in the water heater expands as it heats, can't back up into the supply line, and over pressurizes the plumbing in the house. Or trips the T & P valve on the heater.
When I go on vacation and shut the water supply valve, I set the water heater to "Pilot Light" setting and depressurize a hot water faucet. The water shutoff valve is acting just like a check valve when it's closed. Maybe now I'll install an expansion tank on my water heater. Thanks for the heads up.
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