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So. The question is: do I *need* an expansion tank? I was given two
options:
1) Expansion tank.
2) Just run a tube from the valve to the floor drain nearby

My concern is whether option (2) -- easy, cheap, etc. -- is a good idea
long-term. Will constant slight "blowing" of the valve cause it to fail
catastrophically?

Maybe not, but it'll stop closing properly... So (2) is a bad idea

3) a springloaded shock absorber will also work (the expansion is actually
_very_ little), and use up less room.

4) An upwards facing capped off chunk of pipe will work for a few months, but
you have to reconstruct the airpocket every so often.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.