(g) What are the odds that the heater has been damaged
by the overheat/overpressure event? The relief valve tag
specifies release at 150psi/210F, which is pretty impressive.
What's more, this unit has a build tag dated 1985! Assuming
that it was installed at that time (it may be old stock) that is
impressive longevity. My experience has been that modern
water heaters seldom last more than a decade even without
adverse events of this type. And while I would have liked to
eke out another few years, if the heater is likely to have been
compromised, I don't want to risk having it split wide open
a few months later on.

It shouldn't have been damaged; hopefully you won't get the
pressure/temperature up that high again.
Personally I would not want a 20 year old heater in my house. I have a 25
year old heater in my cottage, but a leak won't do anything horrible there.

(h) Even if the tank and heater assembly seem OK, should
I think about replacing the relief valve itself? It was fine before
this, with no drips or hisses. I do want to be quite sure that
the relief will open when it needs to, though. Note that there
is also an unknown in that a new-in-box valve might itself
be junk -- the current valve at least has been demonstrated
to work properly under field conditions.

A 20 year old valve is a little spooky also. Did you test it regularly? If
not, you are darn lucky it worked! Again, I would replace the whole tank.
Comments and feedback actively solicited. I'd prefer to see
them here in the newsgroup rather than mailed to me: this
in-box is utterly soaked with spam.

I wonder if it went because of temperature or pressure. You would think
they design them so that the highest setting isn't too high, but perhaps
your thermostat is off also. Another good reason to get rid of it.
Do you have a closed system without an expansion tank. If so, it was bound
to happen eventually.

I have tested my system by letting it cool down, and then setting it to the
highest temperature, while monitoring the pressure. It doesn't exceed the
street pressure (90psi) so I know the relieve valve on my pressure reducer
is working. Probably should test it again, or at least clean it like the
instructions say.... (or bite the bullet and put in an expansion tank)

And before I close, let me take this opportunity to wish the
top of the season to you all!

You too. Be thankful the valve worked!