Expansion Tank vs PRV Adjustment

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Lowering the PRV Really Does Shrink the Required Tank

Acceptance factor is 1 minus fill-absolute over T&P-absolute. Drop fill and the air cushion has more room to compress before 150 psig. Required tank volume falls. That is not folklore; it is the same Boyle’s-law step the calculator uses.

That is why the calculator asks for incoming and PRV as two numbers. If incoming is 85 and the PRV is 50, fill is 50. If you raise the PRV to 70 to “get better showers,” fill becomes 70 and the tank you already hung may start dripping. If you lower the PRV from 70 to 50, a borderline tank may stop dripping and showers may feel worse. Both outcomes are the same formula.

There is a floor. Residential fixtures and some tankless heaters want a minimum flowing pressure. A PRV at 35 psig can make a 2.0 gal tank look brilliant on paper and make the upstairs shower die when the washer fills. Do not size expansion by ruining the house. Size the tank at a PRV you can live with. Upstream low pressure is supply pipe sizing, not this page.

A Closed System Still Needs the Tank — PRV Setting Does Not Waive That

The PRV (or the backflow preventer) is usually why the tank is required. It is the check. Expanded water cannot leave. IPC 607.3 wants thermal expansion control on a closed water-heating system. UPC 608 is the same idea in different clothes. Neither section says “unless you set the reducer to 45 psi.”

You can lower the PRV until the required V_t is 1.1 gal. You still hang a listed potable tank, precharged to that new fill, because the system is still closed. The T&P is still the emergency, not the daily expansion device.

ASSE 1017 mixing valves are not a substitute either. They change outlet temperature. They do not give the heater a path back to the main. The devices that close the system are typically ASSE 1003 PRVs and ASSE 1013 / 1015 backflow preventers. The tank is the expansion control. Keep the jobs straight. Walkthrough of the formula: sizing guide.

When Adjusting the PRV Is a Valid Move (and When It Is Not)

Valid: you already have a tank, fill is high, showers can spare a few psi. House static is 75 because someone opened the PRV years ago. The 2.0 gal tank weeps on recovery. Precharge and bladder check out. Dropping the PRV to 55 (and recharging the tank to 55) can put V_t back inside that 2.0 gal cushion. Confirm upstairs flow after. Then stop. You did not delete the tank; you resized fill.

Valid: you are choosing fill before you buy the tank. Street is 90. You are installing a PRV anyway. Setting it at 50 instead of 70 is how you buy ST-5 class instead of ST-8, and how fixtures still see a number the WSFU tables understand. That is design, not a workaround.

Not valid: skipping the tank because “I turned the PRV down.” Closed is closed. The inspector is not signing off on a dripping T&P and a story about 48 psi.

Not valid: dropping the PRV to hide a waterlogged tank. If the Schrader spits water, replace the tank. A lower PRV will not reinflate a torn bladder. Diagnosis: troubleshooting.

Not valid: raising the PRV to fix weak showers when the branch is 1/2 in. That is pipe. The expansion tank will then need to be larger, and the T&P may start dripping. Measure, then size the supply: supply troubleshooting.

Open Main vs Closed Main — The Actual Split

No PRV, no check, no RPZ, meter that will let water reverse: expanded volume can leave the house. Many jurisdictions still like a tank. Some do not require one until the system is closed. Confirm the adopted code. Do not assume a 1970s open main stays open after a meter swap.

The day the utility installs a dual-check or you add a PRV for 90 psi street pressure is the day this worksheet becomes mandatory, even if the heater is the same 40 gal that never dripped. That is the replacement-day trap in the guide.

Well systems with a pressure tank already have a large air cushion on the well side. Domestic expansion on the house side of a check is still a closed bottle. Do not point at the well tank and skip the potable expansion tank on the heater if a check separates them.

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