According to Toller :
Your plumbing should be about 50psi. The T&P valve will drip at about
150psi.
Can you see why letting it drip is a bad idea?

Someone recommended either a shock absorber or a capped pipe. The smallest
expansion tank is probably 50 times the size of a shock absorber.
Can you see why a shock absorber probably won't work?

No ;-)

Water is incompressible. That means that _very_ small thermal expansion
leads to very high pressures in a rigid plumbing system. A few cubic inches
worth of expansion room is usually going to be more than adequate to
handle the fractions of a percentage of expansion from the "HWT downstream
of checkvalve" problem.

But if you have the room, an expansion tank would be better. Usually
overkill, but "better".

You might want to check your water pressure. Mine is 50psi normally, and
goes up to 80psi maximum when the water heater does it worst.

You really should regulate that out too. Mine doesn't vary more than a PSI
or two. Important if you're on a well (eg: with poly well lines).
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.