Do I need an expansion tank?

[Myhouse]

Some background info:
House was built in 1956. City water, has a Watts 25AUB pressure reducer at the meter. It's set to 50 psi now. (In March of this year I finally noticed the water was blasting out of the shower head, so I checked the pressure - it was at 85 psi! The locknut on the PRV's setscrew came loose. I dialed it back to 50...and tightened the locknut. It's still tight today.)

10 years ago, the boiler and water heater were replaced with a new Burnham ES2 boiler and a Bradford-White 50 gallon indirect water heater. The old (1956 vintage) boiler's expansion tank was full of water so it was replaced. The house's cold water supply has never had an expansion tank for the water heater.

Recently I got curious about this again, and noticed that when a hot water tap is closed after the water heater has been heating, the pressure jumps to 90-100 psi and slowly drops.

I tested the T&P valve, it works, it does not leak.

I can't find anything in the PRV information that says it has a backflow preventer in it. Would the water meter have one?

Do I need an expansion tank for the water heater?

 

[Reach4]

Recently I got curious about this again, and noticed that when a hot water tap is closed after the water heater has been heating, the pressure jumps to 90-100 psi and slowly drops.

Not a problem in itself. With numbers like that I first suspected you had a thermal expansion tank already. But your PRV has a bypass feature, which may be feeding expansion water back into the incoming water line.

If the city puts in a check valve at your meter some day that bypass feature will not be able to give the expansion water back to the city temporarily.

So I would say that you don't appear to need a thermal expansion tank at this side. At the first sign of dribbling from your temperature&pressure valve at the WH, check pressures to see if you need to add the expansion tank. If you put in a new WH, you might plumb in the place for the future expansion tank.

http://media.wattswater.com/es-25aub.pdf

 

[Sponsor]

 

[Jadnashua]

The likelihood is that something is leaking, otherwise, your pressure would get higher than that without an expansion tank. Common things that you may not notice are toilet fill valves.

While things are tested for overpressure, the pressure rising and falling does put more stress on things. Do you reliably turn the valves to your washing machine off, for example? utilities have had a campaign to help protect their system by installing check valves to all of their customers. If you don't have one, at some point, you eventually will. Where I live, because of that, the local code REQUIRES an ET, on any plumbing permit. IMHO, it's better for your hoses, WH, and valves to keep the pressure at a more manageable value.

Buying a PRV and paying extra for an internal bypass is mostly a waste of money because of the safety mandate that's having check valves installed to customers, and, never a great idea...the goal is to keep the pressure down, and without an ET, it will rise in a closed system.

 

[Myhouse]

I never turn off the water to the washing machine.

I do turn off the water at the main valve when I'm on vacation though.