Water Heater Thermal Expansion Tank Calculator
Calculate required plumbing thermal expansion tank size in gallons, water expansion volume from heating, and bladder pre-charge air pressure settings.
Calculated Results
Frequently Asked Questions
Why is a thermal expansion tank required on a water heater?
When water is heated from 50°F to 140°F, it expands by approximately 2% to 3% in volume. In modern closed plumbing systems (mandated by backflow check valves or pressure-reducing valves PRVs), expanding water has nowhere to escape, creating dangerous pressure spikes that rupture water heater tanks and drip the T&P relief valve.
How must the air bladder pre-charge pressure be calibrated?
The expansion tank's Schrader valve air pressure MUST be pressurized with a bicycle pump or tire gauge to exactly match your home's incoming static water pressure (e.g. 60 PSI) BEFORE connection to the water supply pipe.
What size expansion tank is needed for a 50-gallon water heater?
A standard 50-gallon water heater with incoming water pressure under 60 PSI typically requires a 2.0-gallon expansion tank (e.g., Amtrol Therm-X-Trol ST-5). If water pressure is 70 to 80 PSI or the heater is 75–80 gallons, a 4.5-gallon tank (ST-12) is required.
How can you tell if an expansion tank has failed?
Tap the top and bottom of the tank with a coin. The top half should sound hollow (pressurized air), while the bottom sounds solid (water). Alternatively, press the Schrader valve pin: if water squirts out, the internal butyl diaphragm has ruptured and the tank must be replaced.