First of all, it's not an overflow valve. It's a safty valve called a temperature and presure relief valve which is designed to open when the water temperature exceeds 212 degrees OR the presure exceeds 150 psi. It is usually the presure that sets it off. You see when water is heated it expands, This is called thermal expansion. First you need to check your incoming water presure. (a test guage has hose threads that allow it to be connected to a laundry faucet or an outside faucet). If your incoming presure exceeds 80 psi, you should have a presure regulator installed on your cold waterline just after your water meter. (if you already have one, it should be replaced). along with the presure regulator, you should install a thermal expansion tank on the cold water line just before your water heater (in some municipalities, this is a code requirement). On rare occasions, the relief valve may just be defective. in either case, now that the relief valve has gone into full discharge. It should also be replaced along with any other needed repair. --- Good Luck
