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Put a new t&p valve on my water heater and as soon as it fills up the valve opens why would this happen?

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    Hello, and welcome to Home Improvement. What's your water pressure, and at what pressure is the valve designed to let go? – Daniel Griscom Mar 2 at 2:07
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Either

  1. The water temperature &/or pressure exceeds the valve's rating (typically 150 psig & 210 F), so it opens

Or

  1. The valve is defective

If your valve is opening the second the water tank fills up, I'm assuming it hasn't had a chance to even start heating the water yet, so it must be from high pressure or a defective valve.

Possibly there's something blocking the valve from sealing, and holding it open. In that case if it were a bit of metal or rock or something it could have damaged the valve's seal / seating surfaces, so I wouldn't trust it even after removing the blockage.

I'd attach a water pressure gauge somewhere and see what your water pressure is, and just exchange the valve for a new one.

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It could be pressure is too high or possibly the wrong type of valve. Most t/p valves have a long rod sticking out. The rod is the temperature sensitive component. Check the rating of the valve. Make sure it's not 30psi. A picture would be helpful.

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