Know the conditions.
Check the numbers.

What this worksheet can establish

Water expands as it warms. When an arrangement of check or backflow devices prevents that water returning toward the supply, an expansion cushion may be needed on the heater side. The useful starting question is which circuit is isolated and what conditions actually apply. The boundary check helps you record that distinction without inferring it from a valve’s shape.

Record conditions before calculating

This worksheet uses the Amtrol MC10007 page 2 (opens source in a new tab) expansion factors for an initial water temperature of 40°F. It does not infer your incoming temperature, recommend a thermostat setting or select a tank model. A result is withheld until the recorded circuit, pressure limits and matching pre-charge assumption are supplied. If your starting temperature or system differs, use the applicable manufacturer method instead.

• Stored water

  Use the recorded water volume, in US gallons. A model name or nominal marketing size may not be the stored volume needed by the applicable method.

• Pressure conditions

  Keep static pressure, the chosen design ceiling, relief rating and component limit separate. Record where each came from.

• Pre-charge condition

  The equation assumes pre-charge equals static pressure. An air-side reading on a connected pressurized tank does not establish its unpressurized pre-charge.

• Unknown values

  Leave them unknown. The summary can record what needs to be confirmed; do not change equipment to fit an accepted input range.

A worked example you can trace

For an illustrative 50 US gal storage volume, 40→140°F, matching pre-charge, 60 psig static pressure and an independently chosen 80 psig design ceiling: expanded water is approximately 0.75 US gal. DPF is 4.735, so the ideal minimum total tank volume is approximately 3.55 US gal. These conditions are an example, not measurements of your home.

The required expanded-water acceptance and the calculated total tank volume are separate checks. A retail tank must meet both under the actual manufacturer conditions; rounding to a familiar “2 gallon” or “4.5 gallon” label is insufficient. The manufacturer comparison guide shows which fields must agree before two results are comparable.

A relief rating is not an operating target

For its own potable sizing method, Watts ETP · source record tells users to keep final pressure at or below 85% of the relief rating, and specifies starting-temperature conditions. This site uses that pressure margin as a conservative worksheet restriction alongside Amtrol’s 40°F table; it is an explanatory hybrid, not an implementation approved by either manufacturer. We limit this form to 20–80 psig static pressure and 50–125 psig design ceilings. Those are scope limits, not national code assertions.

Use the right next check

If you are comparing an installed tank’s factory label with a supply reading, start with pre-charge conditions. If the relief pipe is discharging, use the observation and service guide; a sizing number cannot diagnose a fault. If your question is slow hot-water recovery, the cold-start worksheet answers a different energy-balance question.