On Tue, 2 Dec 2014 03:46:21 -0800 (PST), trader_4
wrote:

On Tuesday, December 2, 2014 1:56:35 AM UTC-5, Jim Joyce wrote:

I can't drain the unused heater because its outlet pipe is connected to the
outlet pipe of the other heater. Each heater has a valve on its respective
inlet pipe, but no valve on the outlet pipes, so if I open the drain valve
on either heater, the other heater will simply (try to) reverse fill it
again.


It would not refill, you could drain it, if you just shut off the incoming
cold water. But I agree, I'm not sure that I'd worry about draining it
because of worrying about bacteria growth. Especially if you're not using
the hot water for drinking, and/or as you suggested, flush it for awhile
when returning it to service.

I agree with everything you said, except that it would really try to refill
from the other heater. The only way to drain one and keep it drained is to
shut off the water to both heater units.