On Jul 22, 4:27*pm, blueman wrote:
justalurker justalurker writes:
On Jul 21, 10:05*pm, blueman wrote:
"Bob M." writes:
"blueman" wrote in message
...
"Bob M." writes:

"blueman" wrote in message
...

Specifically, we have a 40 gallon gas-fired water heater on city water
with 3/4" inlet and outlet to the water heater and copper plumbing
throughout the house. We have 4 bathrooms and a kitchen. The city
water pressure comes in at about 85 PSI.

85 PSI is pretty high, you may want to invest in a pressure
reducing valve.

How important is it to add a thermal expansion tank or is this one of
those "if it ain't broke, don't fix it" kind of things?

Depends on your city water system. Do they have an anti-backflow valve
in the water meter?

I am not aware that there is an anti-backflow valve though I imagine
it could be built into the meter (which is one of those electronic
ones that they then broadcast wirelessly). Is there any easy way to
check by looking or would I need to call the city and/or try to find
and look up a model number.

If your T&P valve on the water heater hasn't sprayed scalding-hot
water all over by now, then you don't have an anti-backflow valve (or
DO have one, -and- an expansion tank). *You would know by now.

Well the water heater is 5 years old and not a drop has come out of
the T&P valve and we definitely don't have an expansion tank... so I
guess e don't have an anti-backflow valve then. Thanks

Buy this gaugehttp://www.watts.com/pro/_productsFull.asp?pid=647&ref=1
at Home Depot
for about $12.

Put it on the drain of your water heater and open the drain valve. If
the tattletale needle indicates
over 90lbs the next day you have a closed plumbing system and need a
thermal expansion tank.

That's a great idea and I already have one of those gauges anyway
screwed into a garden hose fitting right off my main cold supply so
shouldn't be too hard to move it to the hot supply.

Though based on my understanding, it should be sufficient to keep the
gauge on the cold supply since the piping between the cold inlet
(after the valve) and the water heater is an open system (without
intervening valves) with relatively large caliber pipe (1" and 3/4")
so the pressure on the hot water tank should be pretty close to the
pressure on the T&P valve. In fact, the whole danger of thermal
expansion is presumably that the pressure (and associated wear & tear)
is transmitted to all the fixtures on your system and if the pressure
can be transmitted forward from the hot water tank 50-100 feet to
faucets then the same pressure should certainly be transmitted
backwards 10-20 feet to the pressure gauge on my main water line.

So in summary, I am assuming that if the pressure on my main line
doesn't spike up much beyond the baseline then I probably don't need a
thermal expansion tank.

Does this make sense or am I missing something...

Also, now that I think of it, even if I did have a backflow preventer
without an expansion tank, there probably is a fair bit of reserve
"capacitance" in the overall system given that the house is relatively
large (bathrooms on the 3 floors plus basement) and also there is an
attached in-ground sprinkler system with probably couple of hundred
feet of 1" poly distribution piping (even with all the valves closed)
-- also the poly piping is more flexible/expansible than copper. Of
course during the winter with the irrigation system shut off there is
less reserve capacitance since you only have the rigid copper piping
inside the house.

More info on thermal expansion here...
http://www.watts.com/pro/divisions/w...rol/learnabout...

You can leave that gauge on the cold side as long as that bib has
service pressure on it.
Take a bath or shower before you go to bed. The water heater will
fire and as long as you don't open a
faucet or flush the toilet and relieve the pressure you'll get a
surprise on that gauge in the morning (or just stand there and watch
it till the WH shuts off.

If you have a closed plumbing system you'll see pressures up around
130-140psi on the tattletale needle.

If that is the case you need a thermal expansion tank.

BTW, that excessive pressure of thermal expansion is at every pipe,
fitting, appliance, and valve in your system and most pluming fixtures
are rated to only 80psi... just a thought.

There is NOT sufficient "reserve capacitance" in any plumbing system
to absorb water expansion @ 140psi.
Well, there's some "reserve capacitance"... it's leaking faucets,
running toilets, blown pipes, leaking fittings, and a T&P valve. Water
is not progressively compressible as air is. Double the pressure and
water will go somewhere...
and most of the somewheres waste money or cost lots of money to fix.