What sizeexpansion tankdo I need for a 40 gallonwater heater

In the last R. L. Deppmann Monday Morning Minutes we used the Bell & Gossett Systemwize selection program to size and select a water heater thermal expansion tank. Safety factor when sizing is always on the mind of the engineer or plumbing contractor. Where do I apply safety factors and what happens if the tank is undersized? Today we address these concerns.

Expansion tank sizingrule of thumb

One final note about thermal expansion tanks. As mentioned above, the result of any undersizing is increased pressure. If the pressure is getting so high that the relief valve is opening, there are two places to look first.

You can visit last week’s Monday Morning Minute, Thermal Expansion Tank Sizing: Bell & Gossett Systemwize, for more information on this formula.

Expansion tank sizingcalculator

The purpose of the thermal expansion tank is to limit the pressure rise when the volume of water in the hot water system is heated from the initial temperature to the design temperature when there is no draw on the system. Last week we introduced the formula:

Reference the Manufacturer IOM for instructions on properly charging the thermal expansion tank as well as troubleshooting recommendations.

Temperature and pressure relief valves are used in residential and commercial water heater and hot water storage tank applications to provide automatic temperature and pressure protection to hot water supply tanks and hot water heaters. We have a full-line of products in multiple sizes, pressure ratings, and inlet/outlet types.

We can see that missing the volume has a huge effect on the tank size, so safety factors should be applied to the volume. Your choice of safety depends on the reliability of your calculations.

In all cases shown above, a missed selection will cause the pressure in the system to be higher than design. If that pressure gets close to the pressure relief setting, the relief valve will open. We never want to use a safety device as a control mechanism. That is the job of the thermal expansion tank.