Harold Kestenholz
Air in the system is not the cause of blowing the safety relief valve. Not enough air in the expansion tank is the cause. Either the tank has lost its original air charge (void of water when disconnected from the sytem) or the tank was not large enough in the first instance to accept all the extra water produced by heating the system volume. You can read an explanation of this design problem in the free lessons section of : http://www.hydronic.net
