The valve you are talking about (assuming I'm correct) could be failing. It is designed to open if either the temperature gets too high, or th epressure exceeds the design parameters. The spring in the thing sometimes wears out and releases pressure before it was designed, but you could actually have a problem with high pressure, too. Some water systems have a one-way valve - water comes into the house, but cannot go back out. The cold water in teh heater expands when it gets heated, and could be causing the pressure to exceed the safety setting on the valve. These one-way valves are sometimes in or near the water meter or if you have a pressure reduction valve, it typically has one in it. If your system has an expansion tank (the only way to reliably keep the system from venting like yours), then it could have failed or the pressure has leaked out of it. If you have an expansion tank, knock on it and see if it sounds hollow on one end. If it doesn, then it is probably okay. If it doesn't, then turn off the main water valve, open a faucet until the water stops (your're relieving the ressure in the system), then take a tire pressure gauge and check the pressure in the tank. If it is zero, try to pump it up to (maybe 40# or so?). If it holds, shut the faucet, and turn the main valve on, and you may be okay. If not, then you may need a new tank. Note, I'm not a pro.