I installed a Titan tankless water heater. But when I turn on the show, all the water seems to go out the pressure relief valve.

Can I just plug it up?:mad:

No, don't plug it up it may be doing its job. Make sure the small handle on the valve is perpendicular to the valve head. Shut off your heater and try tapping some water. If the valve dumps more water replace it. It should only open under high pressure or heat and with the unit off there should be no heat. Have you tested your water pressure inside? It should only open if it sense 150 psi or 210 degrees.