"blueman" wrote in message
...


Specifically, we have a 40 gallon gas-fired water heater on city water
with 3/4" inlet and outlet to the water heater and copper plumbing
throughout the house. We have 4 bathrooms and a kitchen. The city
water pressure comes in at about 85 PSI.

85 PSI is pretty high, you may want to invest in a pressure reducing valve.

How important is it to add a thermal expansion tank or is this one of
those "if it ain't broke, don't fix it" kind of things?

Depends on your city water system. Do they have an anti-backflow valve in
the water meter? If yes, then you must have an expansion tank otherwise the
T&P valve on the water heater will get a lot of use. Water expands when it's
heated, it has to go somewhere.

If there's no anti-backflow valve in the system, don't bother with an
expansion tank.