I recently added a ball valve upstream of my expansion tank, so I could isolate it when it eventually fails (without turning off the house supply). I also have a boiler valve upstream of the ball valve, and have a maximum-reading pressure gauge screwed onto the threads of the boiler valve so I can monitor house pressure.
Just for fun, I did a little experiment. One night, I ran about 5 gallons of hot water before going to bed, and the next morning, the max house pressure overnight was 60 psi, the normal reading I always get.
Another night, I shut off the ball valve to the expansion tank (so it was "off line") and again ran about 5 gallons of hot water before going to bed. That night, house pressure reached 100 psi!
I can only imagine how high the pressure might have been if I had filled up a bathtub with mostly hot water in the evening, without an expansion tank.
Just for fun, I did a little experiment. One night, I ran about 5 gallons of hot water before going to bed, and the next morning, the max house pressure overnight was 60 psi, the normal reading I always get.
Another night, I shut off the ball valve to the expansion tank (so it was "off line") and again ran about 5 gallons of hot water before going to bed. That night, house pressure reached 100 psi!
I can only imagine how high the pressure might have been if I had filled up a bathtub with mostly hot water in the evening, without an expansion tank.