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I turned off my oil hot water heater during a four week vacation. When it was turned back on, the T&P valve leaked and flooded the basement.

Why did this happen? should I just leave the burner on while away?

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Do you have a back-flow prevent valve (aka a check-valve) in the water supply? If the answer is yes and you do not have a hot water expansion tank, then as the tank full of cold water was heated it would have expanded significantly and forced open the PT relief valve. The valve then must have malfunctioned and stayed open so you ended up with a flood rather than a puddle.

You must get a new PT relief valve. I suppose you could try to remove deposits on the PT relief valve, but I doubt this is worth the risk of a repeat flood at an inconvenient time.

In the future, when you come back to a cold tank, as the tank is heating up you should let one faucet drip or run on low until the tank reaches the temperature set-point. This would prevent a pressure build up during heating. The dripping faucet can be a cold side one if the check-valve is before the branching of the supply to the hot water heater. You could let a hot water faucet drip if you would want to know when the water is hot, but this of course would waste hot water and lengthen the time to a full tank of hot water.

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