Pressure and temperature relief valves are designed to go off whenever there is an increase in pressure in the water system. This would include every time the water heater begins to heat the water, since the water is expanding and increasing tank pressure. If there is no check valve on the house, the water expands into the city water supply pipe and the pressure is relieved. But if there is a check valve serving the entire home, then you will need to install an expansion tank to absorb the pressure. Roto-Rooter has licensed plumbers who can handle this for you if you are not equipped to do it yourself.