I had a new 75 gal water heater installed, and have had problems with it leaking from the pressure relief valve, called the installer back, he installed a pressure relief tank, still it leaked, he came back and adjusted the pressure valve installed on the water line by where the water comes into the home, still leaks... I bought a pressure gage, the pressure coming into the house is about 50 psi, when the tank began leaking, I put the gage on the drain on the water heater, it went up to 150 psi. help...

A temperature and pressure valve responds to high pressure and high temps. So, the house pressure checked with your gauge is 50 psi. good. I don't recommend testing your pressure at the hot water tank boiler drain valve. Test at a hose connection or laundry tub. Have you checked the temperature? Use a meat thermometer and hold under hot water for about 1 minute to get a complete reading. Thermal expansion tank is in, but is set to match the house pressure? IF 50 psi is the house pressure then the thermal expansion tank needs to have 50 psi to match. (I should also mention that when testing your house pressure be sure that no water is being used anywhere in the house). Is the leak from the T&P valve constant or happens off and on throughout the day/night? Here's how to do a thermal expansion test. Attach gauge to hose bib valve or laundry tub. Open valve (cold water only for this test). Do not allow any water use during the test. Turn hot water tank thermostat up just until the tank fires to heat. Watch gauge as the tank fires for approx. 5 minutes. If the pressure in the gauge starts to rise, you have thermal expansion issues. Also be sure to look for a backflow device on the hot water tank cold inlet. If there is one there, the thermal expansion tank needs to be located between it and the tank. Let's recap.
1. Double check house pressure 2. Check temperature. 3. Check pressure in thermal expansion tank with tire pressure gauge to see if it matches house pressure. 4. Look for check valve/backflow device on hot water tank. 5. Be sure thermal expansion tank is in proper location if you do have a check valve on cold inlet. If everything is in order, do thermal expansion test to be sure before you change out the T&P valve with a new one. I suggest the testing as opposed to just changing the T&P valve first because if you spend the money and time it takes to drain down the tank and the new T&P valve also leaks, you will have to do the testing anyway to get to the root of the problem. Hope this info was thorough and useful.

Note: Having the proper size thermal expansion tank is also important. The size is listed on the label of the tank.

Excellent advice pghplumber. Ill just add one thing about checking the pressure in the expansion tank. Make sure the house water is off and pressure in the system drained off completely when checking the pressure in the tank.

I figured that I might miss something with all of that typing. Thanks mygirlsdad for the back-up. We don't want the house pressure on the inside of the bladder to give a false reading on the tire gauge.