On 11/20/2015 5:48 PM, Walter E. wrote:

I installed a pressure regulation valve in my city water supply because city
pressure is 120 psi. This turned my house piping system into a "closed system"
with no place for the pressure and added volume to go. This results in water
pressure of 150 lbs, which damages my appliances.

This suggests the PRV that you purchased/installed does not have a built-in
bypass (which would ensure your "inside" pressure never exceeded that of
the municipal supply).

Turns out that I now need an
expansion tank to absorb the volume and pressure created by the water being
heated.

Yes. You would be surprised how many folks omit this step -- thereby
negating the benefits of the PRV!

If you have a pressure gauge, the easiest way to see this effect is to
take a long shower (i.e., use lots of hot water so the water heater
ends up with a fair amount of cool water that will later expand, on
heating) and then watch the pressure AFTER you've turned off the
water (and don't use any OTHER water until the experiment is over)

The plumbers I talked to cannot agree on what I should do:

I have one water HEATER each to supply water to the front and rear of our home
(2 heaters). Both water HEATERS seem to be connected by pipes to ONE water
SOFTENER that supplies both areas (front and back of the house) with hot, soft
water.

How do I install the water heater expansion tank(s)?

Expansion tank(s) need to be upstream from the heaters BUT WITH NO INTERVENING
SHUTOFF VALVES!! I.e., the tank(s) only work if water from the cold water
inlet to the heater(s) can flow BACK to the expansion tank!

Typically, you would install a shutoff on the cold inlet and hot outlet
of the water heater to make servicing easier. When you shut the cold water
inlet valve, a path must continue to exist from the heater to the tank.

Can you do this with a single tank? Or, will you only be able to satisfy
that requirement for ONE of the heaters?

[Imagine having the inlet shut to one of your heaters, the heater full of cold
water AND the heater trying to bring that water up to your hot water
temperature. You now have that same "closed system" -- no place for the
water to "expand BACK into". Just like the problem you described in your
opening statement!]

Do I install one small expansion tank at one of the heaters, based on the
specs for that heater only (40 gal and 40 lbs pressure), or should I install a
larger expansion tank at the first water heater to allow for the second and
more remote heater, or should I get two small expansion tanks, one at each heater?

You have to assume, worst case, that both hot water heaters will be full
of cold water (imagine the gas gets shut off at your house for some time;
or, you have house guests who use all the hot water by running multiple
showers concurrently). The expansion tank is sized based on the volume
of water that will be expanding from the heating. So, you have to be
able to accommodate ALL of it.

Both heaters are 40 gal and the house pressure is set for 40 psi.

So, size your tank(s) for 80 gallons total. You can do this as two
tanks (40+40) or one (80). Again, subject to the shutoff valve issue!
If each heater has a shutoff, then treat it as two 40G systems
and provide each with a "local" tank ("local" meaning "within the control
range of that tank's shutoff valve)

I installed pressure gauges on the municipal supply (upstream from the
PRV), the "regulator output" (just downstream of the PRV) and the
load side of the water softener (there is a drop through the softener
and this also lets me monitor for a catastrophic failure of the
softener). These are reassuring -- I can glance over and see that
the pressure is as it should be (and, as our PRV has a bypass, it
is possible for the "inside" pressure to fall BELOW the set point;
but, only if the municipal pressure has also fallen! Hence the desire
to be able to monitor each!)