On Sunday, February 5, 2017 at 2:44:15 PM UTC-5, dp3 wrote:
replying to fftt, dp3 wrote:
Even if the system is closed I would ask of the expansion of the steel tank
itself somewhat matches any water expansion? additionally if there is a
anti-water-hammer device (i.e., shock absorber) anywhere in the system (often
found at washing machine or shower or sometimes whole house - simply a
vertical pipe filled with air) then I wuold think a thermal expansion tank is
not actually needed.

A vertical pipe filled with air isn't going to stay filled with air
for long. The air winds up dissolving into the water under pressure.
That's why water storage tanks on pumps need either a mechanism to
inject air as needed or a plastic bladder to separate the air and
water.