My water heater pressure relief valve is pouring water out of the valve. Do I have a problem with the tank or do I need to turn the temperature down on the heater.

More background information: We went on vacation and turned off the water heater and the water to the house. When we returned, we turned on the water and after approximately 1 hr we turned on the water heater. About 3 hours later, we found water pouring out of the relief valve (there is a pipe going from the valve to the floor). We have turned off the water heater.

As long as water is free to flow back out the inlet and out the meter or into the pressure tank, the relief valve should not open. If you have one of the, now often required back flow preventers, you must also have an expansion tank before the water heater. It took us old dogs here a while to figure that out.

The relief valve having been disturbed, even by a cooling/heating cycle or looking at it cross eyed, could have some junk in it. Try opening it wide open for a good blast and hope to clear it. If that doesn't work, try a new one.

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Originally Posted by labman

As long as water is free to flow back out the inlet and out the meter or into the pressure tank, the relief valve should not open. If you have one of the, now often required back flow preventers, you must also have an expansion tank before the water heater. It took us old dogs here a while to figure that out.

The relief valve having been disturbed, even by a cooling/heating cycle or looking at it cross eyed, could have some junk in it. Try opening it wide open for a good blast and hope to clear it. If that doesn't work, try a new one.

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Labman put his finger directly on the problem. Let me enlarge on the explanation so it's less of a mystery. Water expands when heated in a water heater. Normally this would expand out into the water service line and out into the city main. By installing a backflow preventer you have blocked expansion past the backflow preventer. In short, you have set up a closed system. The expanding hot water hits the backflow preventer and can expand no further so it begins to build up pressure. When the pressure reaches a certain point the Temperature & Pressure valve on the heater senses this and opens up to relieve the pressure. There's nothing wrong with your water heater and the T & P valve is just doing its job.
The answer is to install a expansion tank on the cold water supply to the heater so the water will have some place to expand to. When we began to receive complaints such as yours it took us some time to figure out just what was happening. And now you have the rest of the story. Regards, Tom

Tom's and my answers may appear different if you do not read them carefully. The crucial difference is the backflow preventer. If you have one, but not the expansion tank, the relief valve is doing just what they are required for. Add the expansion tank, just like Tom said.

If the water that is forced out of the tank, when it is heated, has someplace to go, either an expansion tank, or back the feed line with no backflow preventer, and water is coming out the relief valve, then the problem is the relief valve.

2 different cases, and 2 different solutions.

I am glad Tom and I can work together instead of feuding as is going on in computers.