Thermal expansion tank required by City... no backflow preventer, pressure gauge or PRV? Latter aren't required if the psi doesn't doesn't exceed 80? How do you check water pressure without having to install the pressure gauge? Will the Thermal Expansion Tank work w/out anything stopping backflow? Never had any issue w/ leaking, but tested the T & P valve, didn't reset (water heater is not lit yet, haven't lived here in several years.. need to replace t & p or will lighting tank create pressure for reset?

Hot water heater T&P valves frequently fail to reset once opened. When you opened the valve you did not actually test either the T or the P, you just opened the valve. Try to do it several more time and you may clear the debris from the seal. If it does not seal it should be replaced. I don't really understand the cities requirement for an expansion tank if there is no requirement for a back flow preventer. Can you ask the inspector to fully explain and then come back to us.

Thanks, going to try the t & p before replacing... new user, tried to "Rate this Answer", didn't find it? Here is link to what our city posted, also not shown in illust. Is PRV... maybe this will help? http://ci.alamogordo.nm.us/Assets/COA+Document/Community+Development/Thermal+Expansion+Tank+Information.pdf

What the city is saying is that they have or will be installing back flow prevention valves at the meter. That requires you to install a thermal expansion tank.

When water in the heater is heated it expands and increases the pressure in the tank. Since the hot water or output side is sealed (unless you have a valve open) the increased pressure pushes the water back into the cold water side and pressures are equalized. If a back flow prevention valve is installed the pressure in the tank cannot be relieved by pushing water back into the cold water line. Unacceptable pressures can build in the water heater tank and cause the P&T valve to open to relieve the pressure.

Installation of a pressure reducing valve (PRV) can have the same effect. Apparently the city is saying that a PRV is not a requirement unless the pressure is 80 lbs. or greater.

Personally I would not want more then 60 lbs.

You can purchase a pressure gauge that attaches to your hose bib at Home Depot, Lowe's or other hardware stores. About $10-15.
https://www.askmehelpdesk.com/members/hkstroud-albums-+pictures-picture524-pressure-gauge.jpg

Nice explanation. Can't spread the rep.

Wanted to plus-rate Ballenberg1's response - but got that "...have to spread some more reputation ..." note. So I'll keep spreading... In the interim: very good answer, Ballenberg1 !

Starburst, you are fairly new and they don't allow you to rate us until you have been around for a awhile and understand how the desk works, think you have tohave 10+ posts before you can rate. Milo and Kiss, we got to get out mkore often, rated each other and not enough other folks. Think you must rate 5 new folks before you can go back to someone rated earlier.

Thanks to all! So I installed tank & t/p valve-will tank hold 40 psi w/out lighting the water heater?

Not sure what you just asked here in post #8. Are you asking what T&P the T&P valve will hold? Read the little brass plate, should be 120psi and 210 degrees

I think what starburst is asking is, should he pressurize the tank before starting water heater.

Read the instructions or tag. Tank is probably already pressurized to 40 lbs. Tank should have a bladder to contain the air.

The pressure in your expansion tank should be slightly greater than your incoming water pressure. That way it only accepts any thermal expansion when the pressure starts increasing above normal pressure. Otherwise, it will just be filled at normal pressure and do nothing for the thermal expansion.