Expansion Tank and Water Pressure

About a year ago I installed a pressure reducing valve on the water line coming in to the house. The incoming pressure was fluctuating between 80 - 150 pounds. The PRV is now set to 55 pounds.

Water pressure on the outside faucet now reads 55 pounds. I then checked the hot water tank pressure at the drain valve on the tank and it was reading up to 110 pounds after the burner ran.

So I decided to install an expansion tank on the cold line feeding into the water heater. It is a Watts 4.4 gallon tank which I have pre-charged to 55 lbs pressure.

After reading the water tank pressure over a period of several days I am getting pressure readings up to 70 lbs while the burner is running.

Is this normal? I was expecting to see the pressure in the hot water tank to equalize with the 55 lbs set on the expansion tank.

Thank you for any insight!

Do a right dimension of expansion tank is not simple, there are too many variable from calculate.. But if you have 55 psi of water inlet, and expansion tank was loaded at 55psi(anyhow is better check before install) for me is right.. Usually I charge 3 psi less of water inlet.. (I hope to have converted in exact way).. Anyhow, is right that the pressure increase a little bit.. Most important is that the security valve not drains

In 35 years of home ownership, I've never had an expansion tank at the hot water heater, so this is all new to me. Not sure what to expect in how the expansion tank behaves!

Try to take a picture of the hot water tank, expansion tank and all connecting pipes and post them to the thread.
How thermal expansion is effecting the plumbing system is best determined from another location. Test for expansion by putting the gauge back on the hose connection.
Open the hose valve all the way and record the pressure.
Run only hot water at a nearby sink until the tank turns on then shut off the faucet and use no water at all until the test is done.
Remember to leave the valve to the gauge on the hose connection 'on'.
You can watch the gauge while the tank is heatin or just check it again after the tank stops firing.
If you check the gauge and are still reading excessive pressure there is something being missed in the equation. That's why I ask to post pictures if possible.
I have to also ask if you pressurized the expansion tank before installing it?
You can also test the pressure in the expansion tank with a tire gauge by shutting off the water supply to the house, opening a faucet to relieve pressure and see if it is still holding 55 psi. Do not test the expansion tank pressure with the house water pressure on.

Thanks to all for the responses!

I checked the pressure on the expansion tank and on the hot water heater with everything static...

62 lbs in the expansion tank, and 57 lbs at the water heater.

I then opened the tub faucet in the basement bathroom to full hot. After about 5 minutes, the water heater fired up. At this point I turned off the running water and took these readings...

62 lbs in the expansion tank and 38 lbs at the water heater.

The water heater has been sitting now for 15 minutes with no demand on it with the following readings...

70 lbs in the expansion tank and 68 lbs in the water heater.

I'll test the expansion tank with no water pressure (water line turned off) this afternoon. I charged it prior to installation with 55lbs of air.

Having never had an expansion tank on my water heating system, I have no idea if these readings are within acceptable/desirable limits.

Thank you!

With the posted pic and pressure readings, I would appreciate any input on my installation, as to whether this is a proper configuration and normal operating experience for an expansion tank at the water heater.

Thanks!

In Europe we use this valve.. I don't know in USA if this is avaliable.. Anyhow, I think that the minim request are:valve, no return valve, security valve, and expansion tank

When your heater fires up you are adding heat and expansion to your water system. Checking pressure at heater is not going to give you an accurate reading. Check pressure outside. Pressure regulators are designed so that thermal expansion goes back to the city through the reg. Why did u install expansion tank? Was t and p valve leaking?

I installed the expansion tank at the recommendation of the plumber who installed the PRV. He said the code now is for all systems to have an expansion tank, and that it would prolong the life of my hot water tank and be good for the house plumbing in general.

Before the PRV, I was getting +130 psi, causing the T&P valve on the hot water heater to open.

The PRV was installed and all was good with inside pressure now at 60 psi. Shortly thereafter I installed the expansion tank, setting the tank pressure to 60 psi.

A couple of months later we start noticing low water pressure in the second floor shower. I check the pressure at the outside hose bib and get 110 psi. With the guage on the bib, I open a cold water faucet just inside and watch the guage drop pressure all the way down to 60 psi.

This is going on for a while now. I just checked at the outside bib and pressure is 120 psi! I thought my gauge might be bad so I ran up and bought a new one and am getting identical readings. Opening an indoor cold water tap makes the pressure drop quickly.

SO what am I dealing with now? A bad PRV?

Sure sounds that way.