"Matt" wrote in message
I do know that you DO NOT want to add a water heater in series; if you
decide to add another one, it should be in parallel with the existing
one - but it's tricky to do. The lengths of all the pipes between the 2
WH's MUST be IDENTICAL; as I recall a very small variance can really
screw things up.

And how/why do you know this? Series makes more sense to me. Then you can
turn the first one off if you don't need it.

Bob