I may have placed my original post under the wrong message ... sorry.

Many municipal and rural water systems have placed (or are placing) a check valve at
the meter to eliminate the possibility of backflow into the water system from any
home. Often this change is made without notifying the home owners or in a manner
that the home owner doesn't really understand.

I recall that water heated in a water heater increases in volume about 2%. My T&P
valve was "burping" about 6 ounces of water out the valve when ever the heater fired
to heat the water AND only if no toilet was flushed or faucet opened (which would
have relieved the excess pressure). The usual symptom is that the last person to go
to bed takes a bath or shower, the water heater fires, and then no valve is opened
to relieve the pressure ... the T&P valve burps. I've searched the web and found
MANY people with EXACTLY the same symptoms. My water pressure is 55psi and goes up
dramatically as the water heater burner fires and heats the water. I used a pressure
gauge with a "tattletale" needle to record 150PSI for a split second just as the
burner shut off. Odd, 150psi is exactly where the T&P valve releases. My water meter
HAS a check valve AND a PRV AND I have a water softener (triple threat I guess). The
Watts GOV 80 toilet ballcock that releases pressure at 80psi did the trick and took
all of ten minutes to install.

As far as "he does not need an expansion tank on his domestic hot water system
unless he has a circulating pump" I fail to see the sense in that statement. A
recalculating pump does not increase the pressure or water volume in the system.
Plumbing code (here) mandates at least one and sometimes two (for redundancy)
thermal expansion devices in any new construction where the house plumbing is a
CLOSED system (by check valve or Pressure Relief Valve at the water meter or by a
water softener (recommended by most softener manufacturers). In fact, if you look at
the booklet included with just about any hot water heater it will say EXACTLY that.
It did in detail in the booklet with my A.O. Smith 50 gallon heater.I just never
read it as a licensed plumber installed the heater and according to code should have
added a thermal expansion device of some kind. Well, at least he left me the
booklet.

If you have this problem ask your water supplier or water system IF there is a check
valve or PRV at the meter. If they answer YES then check local plumbing codes and
I'm sure you'll be required to install a thermal expansion device.

If you should have a thermal expansion device GET ONE. It lengthens the life of your
water heater, T&P valve, washing machine, dish washer, and faucet seals. Do a GOOGLE
on "thermal expansion" ... you'll be real surprised at what you find.