Put a new t&p valve on my water heater and as soon as it fills up the valve opens why would this happen?
- The water temperature &/or pressure exceeds the valve's rating (typically 150 psig & 210 F), so it opens
- 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.