City Water - Expansion Tank

[not-a-plumber]

Daughter's house built in 2009. Plumbing appears to be untouched and as installed and inspected 13 years ago.
Main line from water meter at the street comes into the crawlspace, runs across the bottom of the joists, 3/4" tee to 2 gallon expansion tank, and then up to the water heater closet on the first floor where there is the main shutoff valve and a pressure regulator. So the expansion tank is on the unregulated side of the pressure regulator and before the main shutoff valve.
The tank is unsupported, just hanging on the PEX tee and feels heavy and full.
If could just be low air pressure, but I am going to replace the tank and support it properly.
I will look and see if any outside faucets are unregulated (probably not) and try to get pressure readings on both sides of the pressure regulator. If no outside faucets are unregulated, I will only be able to get a reading of the regulated side.
Questions:
Is this an acceptable location for the expansion tank?
Since it is on the unregulated supply side of the pressure regulator what pressure should it be set at?
In this location, can the tank serve as a thermal expansion tank for the water heater or does it serve a different purpose having to do with city water pressure?
Thanks

 

[Twowaxhack]

I’d either move the tank or move the valve and regulator.

From the street I’d install the stop valve, the pressure reg. and then the expansion tank in that order.

Set your pressure regulator at 65-75 Psi and set your tank to match before you install it or with the water off and water pressure relieved.

 

[breplum]

Exp. tank needs to be fully supported/strapped.
They only serve to protect water heater on systems that are "closed", meaning that the pressure regulator is not a bypass model, or there is a check valve creating the closed system.
If the water supply system at the dwelling is not closed, then exp. tank is not necessary (no harm either).

 

[not-a-plumber]

Twowaxhack and breplum,
Thanks for the replies.
I don't think that this is a closed water system. I guess it depends on the type of pressure regulator, but there is no back flow preventer and no check valve, so...
The City came out and told us that we are being supplied with 78 psi.
I'm reading 70 psi on the regulated side. Higher than I'd like, but the regulator is pretty crusty on top, so I think I'll wait until I change the water heater and then do everything at once. The water heater is 13 years old, so probably pretty soon.
For now I replaced the tank in the crawlspace, mounted it properly and set the pressure to 78psi. It is rated at 150 psi with a maximum bladder pressure of 80 psi.
In the attached picture you can see the main supply line coming in the back corner going to the main shut off, then the pressure regulator, then it tees to the main cold water line and the cold water feed to the water heater. Above the water heater it tees again and goes up to some device (maybe a vacuum breaker?
What I'd really like to do is move everything to the crawlspace and free up the closet to be a small pantry for the kitchen.
The crawlspace is pretty low, so I'd need to find a pretty low lowboy, do some digging or both.