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Expansion Tank vs. Pressure-Reducing Valve: Which Should Come First?
I’m comparing two common fixes for a closed-loop residential water system: installing a thermal expansion tank at the water heater or adding/replacing a pressure-reducing valve (PRV).
My understanding is:
- The PRV controls excessive incoming municipal pressure.
- The expansion tank absorbs pressure increases caused by heating water.
- A PRV does not replace an expansion tank when a check valve or backflow preventer makes the system closed.
- An expansion tank will not correct continuously high static pressure.
- A pressure gauge and pressure readings at different times seem important before choosing either accessory.
For landlords and rental properties, the trade-off appears to be:
- PRV: Better protection if supply pressure is consistently high, but it may need adjustment or eventual replacement.
- Expansion tank: Usually simpler and less expensive, but it must be correctly sized, precharged, supported, and installed on the cold-water line.
- Both: Potentially the best protection, though it adds cost and another component to inspect.
A few questions for the group:
- What pressure readings would convince you that a PRV is needed?
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Replies
I agree - measuring both static and dynamic pressure is the best way to separate these problems.
1. Attach a reliable pressure gauge to a hose bib or the water-heater drain valve.
2. With all fixtures off, record the static pressure. If it is consistently high - commonly above about 80 psi, subject to local code and equipment limits - a PRV is generally warranted.
3. Open a faucet or hose bib and note the dynamic pressure while water is flowing. A major drop can indicate restricted flow or an undersized/failing PRV, while a fairly normal flowing pressure does not rule out thermal expansion.
4. For a closed system, record the pressure again after the water heater has run through a heating cycle. A substantial rise or recurring spikes after heating point toward a properly sized, supported expansion tank, with its air charge set to the system’s incoming pressure.
If the supply pressure is high before the heater operates, the PRV addresses the underlying issue. If the pressure is acceptable cold but rises only as the water heats, the expansion tank is the more direct solution. In some homes both are needed: the PRV controls the baseline pressure, and the expansion tank manages the resulting thermal expansion.
Check an expansion tank’s air charge only after isolating and draining the tank so the water side is at zero pressure. Otherwise, water pressure can make the Schrader-valve reading appear higher than the actual precharge.
A tire-pressure gauge can check the air side, and a hand pump can bring it to the measured cold system pressure. Check it again after a few minutes; a leaking Schrader valve can let the charge drift.
For readers checking this, compare the system’s actual cold pressure with a known-good gauge rather than relying on an old hose-bib gauge. A 60 psi precharge is not useful if the cold system pressure is really 45 or 75 psi.
The PRV’s location matters when both components are used. It should generally be upstream of the expansion tank, with the tank connected on the regulated side of the PRV. That way the tank absorbs thermal expansion at the controlled house pressure, rather than being exposed to the higher and possibly fluctuating municipal pressure.
So the sequence is typically supply, PRV, then the branch or cold-water connection for the expansion tank and water heater. The tank’s precharge should match the measured cold pressure on that regulated side, not the street pressure. This also makes the 60 psi example useful only if 60 psi is actually the system’s cold regulated pressure.
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