Boiler blow out valve question?

6 months ago i noticed my water bill increased dramaticly I followed all my pipes and found out my blowout valve was leaking (a slow and steady drip) the heat was off at the time and the valve was rusted so I umed it just went bad and replaced it. I installed and new one with no problems untill a month ago (my heat is now on) I noticed that my place was not getting warm so i checked the boiler and sure enough the valve is releasing water and pressure. This time (because the heat is on I ume) the valve is shooting out water and steam at great pressure. The valve is set at 30lbs. same as the previous one. My boiler is only about 5 to 7 years old and this is the 1st problems I have had with it. any suggestions??