Expansion Tank Sizing Guide

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The Formula: Expand the Water, Then Size the Air Cushion

Two physics problems are glued together. First, water gets bigger when you heat it. Second, the only spare room in a closed house is the air on the dry side of an expansion tank. You calculate how many extra gallons appear, then you calculate how big a tank must be so that air can shrink enough to swallow those gallons before pressure reaches the T&P.

V_s is the heater tank gallons you typed. Density ρ is saturated liquid water at atmospheric pressure, interpolated against temperature. Absolute pressures add 14.7 psi to the gauge readings. P_fill is house static when the heater is cold — the calculator uses the lower of incoming pressure and the PRV setting. P_max is 150 psig, the standard water-heater T&P.

That second line is Boyle’s law on the air cushion, assumed isothermal: P₁V₁ = P₂V₂. The tank starts full of air at fill pressure. As water enters the wet side, the air is compressed toward T&P pressure. The fraction of the tank that can accept water is 1 minus the ratio of those absolute pressures. Divide the expansion volume by that fraction and you have minimum tank volume.

The calculator does that interpolation and the Boyle’s-law step every time. It does not invent piping gallons you did not type, and it does not pretend a 2.0 gal carton has 2.0 gal of acceptance at every pressure.

Incoming Pressure and the PRV Are Two Different Levers

Fill pressure is the pressure the closed system sits at when nothing is running and the heater is cold. That is also the precharge you put on the tank. Get it wrong and the tank is already half full of water before the first recovery.

Incoming is the street or well static. A hose-bibb gauge at 2 a.m. on the meter side of the PRV is the usual field number. Cities at 70–90 psig are common. Some wells and some rural mains sit lower.

PRV setting is house static after the reducer. 50–60 psig is a typical residential setpoint. If the PRV is actually reducing, the heater never sees the street pressure. Fill is the PRV number. If there is no PRV, or the PRV is set above the street, fill is incoming.

Higher fill shrinks acceptance. The air is already compressed, so a smaller fraction of the tank is left to swallow expansion, so V_t goes up. That is why the same 50 gal heater that is happy with a 2.0 gal tank at 50 psig may want the next size at 80 psig with no reducer.

Lowering the PRV therefore reduces required tank size. That is a real lever. It is not a free pass to skip the tank on a closed system. That split is the whole point of the comparison.

A tired or clogged PRV is a pressure problem at the fixtures, not this worksheet: water supply pipe troubleshooting.

When Adding a PRV or Backflow Preventer First Makes the Tank Code-Required

An open system can push expanded water back into the main. The street is a giant expansion tank you do not own. Many older houses ran that way for decades: no PRV, no check at the meter, T&P never weeps.

Install a pressure-reducing valve, a dual-check, an RPZ, or any device that will not let water reverse, and you have bottled the house. The next recovery has nowhere to go except the T&P. That is the usual origin story of a dripping relief valve “that started after the city changed the meter.”

IPC 607.3 (thermal expansion control) and the UPC 608 family require a means to control that pressure on a closed water-heating system. The means in residential work is a potable diaphragm expansion tank on the cold inlet, precharged to the static fill, with the air valve left accessible. A water-hammer arrester is not that tank. A mixing valve is not that tank.

Tankless heaters store almost nothing. Expansion is smaller and some manufacturers use a different accessory. Do not hang a 2.0 gal potable tank on a tankless and call the storage-tank math done. Storage vs tankless demand is tank vs tankless; this page is the cushion on a storage tank.

How to Apply the Result When You Replace a Water Heater

Match the new heater’s actual tank gallons, not the old carton and not the first-hour rating. A 50 gal that replaced a 40 gal expands more water. A 140°F thermostat instead of 120°F is a bigger rise. If the plumber (or the city) added a PRV or a check while they were in the pit, fill pressure may have changed on the same afternoon.

1. Read tank gallons on the new rating plate.
2. Gauge street static and house static. Fill = the lower number if a PRV is reducing; otherwise incoming.
3. Use inlet temperature if you know it; 50°F is the planning default. Rise = thermostat minus inlet.
4. Run the calculator. Buy the next common size at or above V_t.
5. Precharge the new tank dry, to that fill pressure, then install it on the cold inlet, upright, Schrader accessible.
6. Do not reuse a ten-year-old tank whose bladder you have not checked. Waterlogged tanks are why T&P valves weep after a “perfect” heater swap: troubleshooting.

Heater selection itself — people, baths, peak hour, gas vs electric — stays on the water heater sizing calculator and sizing guide. This page does not change first-hour rating.

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