I have a Weil Maclane Gas boiler in my house. I have noticed the the pressure relief valve has been blowing when I'm not home This is the first time I've had a problem with it. I installed a new digital thermostat upstairs, I have not drained the system for two years.I have priced a new valve is $28.00. Could the problem be the new thermostat, rust that has caused the valve not to seat, or just age. Also the new valve Btu rating is 550,000 btu the old one is 650,00 btu, does this make a difference? Thanks in advance.