Horatio Hornblower wrote:

SNIP,SNIP
I'm due to replace that water heater, cause it's about 12 years old and
an accident waiting to happen according to my brother. I was thinking of
a 50 gallon but now I think I will put another 40 gallon in there
because there's NO room to spare! In fact, I will try to free up a bit
more space. I could get another 2-3 inches by moving it left almost to
the gas connection, but there's a pipe whose function I don't know (I
bet you do). It's sweated copper, and I just noticed it yesterday. The
hot water out and cold water in are galvanized, but this copper comes
from the top of the tank and is connected to a steam blow-off valve. It
goes horizontally to the side of the tank and then goes down through the
floor. What's that?

That's a very old pressure relief valve. It dumps steam/hot water
under the floor (somewhere?) if it ever opens.

Jim