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Don Y[_3_]

On 11/20/2015 7:51 PM, wrote:
I'm pretty sure you can install the expansion tank
anyplace in your system as long as it is on the
inside side of the regulator.

There can't be any shutoffs between the tank and the
inlet of the heater. I.e., the tank needs to be
part of the heater at all times.

It is probably better for the
longevity of the tank to install it at a cold water
location.

Because the bladder in the tank will wear at high temperatures,
it goes on the cold water supply. Note that you need to
isolate it upstream of any heat trap you may have on the
water heater.

The size of the tank should be large
enough to absorb the expansion of both water heaters.

Again, that assumes both heaters can freely exchange water
regardless of the positions of any valves in the system.

Shutoffs tend to be local to the water heater -- to isolate
input and/or output in the event of failure, maintenance,
etc. So, it's quite possible that putting the tank "at"
one heater could result in it being "hydraulically isolated"
from the other water heater. The same can also occur if you
site the tank at the PRV (which is probably remote from the
water heater.

You also have to consider what may happen in the future.
E.g., when a water heater fails, will a NEW shutoff valve
be installed NEAR the replacement? So, the lack of a shutoff
TODAY might cause you to make a poor tank site selection
choice that impacts what you can ("legally") do in the
future.

[Don't count on that future plumber "doing things right"!
Many of my neighbors have PRV's and AFAICT, I'm the only
person who bothered to install an expansion tank. None
of them were even offered that option at the time their
PRV was installed! (tanks are a PITA compared to something
simple like a PRV)]