Originally Posted by OldCoot
I do believe the pressure is the same with or without the air bubble.
Water cannot be compressed. Without somewhere to expand to easily (air bubble), pressure will build high enough to stretch the water heater and the rigid PEX lines if necessary, if it wasn't for the relief valve. I have seen this pressurization effect many times when the water heater is heating up, and the heater has lost it's air bubble, particularly if it is heating from a cold start. Of course, running a little hot water now and then while the tank is heating helps to bleed off this pressure. Usually, unless I forget, I do the air bubble restore trick before each trip, as the bubble apparently disappears fairly quickly.