PRV, no expansion tank

I didn’t see any expansion tank anywhere, shouldn’t there be one? I always thought that with a PRV now there wouldn’t be any provision for the expanding water to go back into the city supply. Would I be correct in recommending one to be installed? Thanks so much.

I would recommend an expansion tank.

What does the local AHJ require?

First, that model of Pressure Reducing valve not function as a check valve. They limit inflow by dynamically changing the valve opening so that when less pressure is on the out side, the valve opens up fully, and too much pressure on the out side causes the valve to shut. In a no pressure situation (where water could go backwards), it won’t be impeded at all.

Series LF25AUB-Z3
Water Pressure Reducing Valves**
Sizes: 1⁄2" – 2"
Series LF25AUB-Z3 Water Pressure Reducing Valves are designed to
reduce incoming water pressure to a sensible level to protect plumbing
system components and reduce water consumption. This series is suitable
for water supply pressures up to 300psi (20.7 bar) and may be adjusted
from 25 – 75psi (172 – 517 kPa). The LF25AUB-Z3 features Lead Free*
construction to comply with Lead Free* installation requirements. The
standard setting is 50psi (345 kPa). All parts are quickly and easily serviceable without removing the valve from the line. The standard bypass feature
permits the flow of water back through the valve into the main when pressures, due to thermal expansion

So the bottom line would be, it does not make for a closed system unless there is an RPZ valve elsewhere that would mandate the use of an expansion tank on the cold water side.


I stand corrected.

Thank you! You guys are great and are incredibly helpful. Thank you all for responding, I learn so much from each and every one of you.

I agree that while that particular pressure regulator has a dynamic bypass, the pressure in the home must overcome the incoming street pressure to activate. Where we are, it’s common to see 130 psi street pressures, and with most appliance manufacturers recommending no higher than 80 psi, how does one justify not having the expansion tanks? Looking for more education not picking a fight

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So, you are telling me that that a 90 pound kid isn’t going to push a 130 pound woman out of a doorway? And when he can’t, he might start knocking over small children? Sounds legit. He might need a bounce house to play in.

There was no mention of street pressure by the OP, so your statement is not relevant to the fact that thermal expansion is allowed to go back to the street side with the PRV that was shown.
I was merely pointing out that with this valve shown, it is still an open system. Dependent on the street pressure would decide if an expansion tank is required in conjunction with the PRV.
Also now, most water meters come installed with a check valve which makes it a closed system and an expansion tank would be required.
Checking with the AHJ would be prudent.

I had the same installation yesterday.
Water cooled Trane AHU.

Copper supply.

Marcel brought up the pont that there is a pressure regulator valve. Nice work.

Moot point about upstream water pressure. Water Pressure Valve is 25 to 75 PSI.

Any pipe noise/water hammer/pipe vibration? I did.

Beyond SOP without accurate readings.

If anything, refer to plumbing or HVAC contractor. We are not AHJ or code inspectors. Watchout for walking the fine line or liability.

Forgetting the techie words, in my area we have another problem that we always address

FYI - The plumbing supply system has a PRV (pressure reducing valve) in the water lines. This often means you have high water pressure in the building (usually a good feature). In the past PRV’s were not always used, however newer plumbing codes, building standards and most water heater manufacturers recommend that when a PRV is present - you install an expansion tank or similar. A recent story in the local newspaper reported that some warranty companies have denied coverage to damaged water heaters or supply lines when this condition exists AND there is no expansion tank, etc present.

No visible expansion tank, etc was present on this system. We recommend that a new owner check with a plumbing specialist to see if this type of device is recommended. If so a plumber can easily install this for the new owner as a safety improvement.

or a check valve on its own. I wonder if the WH is an on demand type if an expansion tank would be required being that the temperature with those are usually preset at 120F. Last 2 homes had a check valve, on demand WH and no expansion tank.

Thank you Dan.