Thermal Expansion Tank

Overview

A thermal expansion tank protects a closed water-heating system from the pressure spike that occurs when water is heated and expands but has nowhere to go. In a system with a check valve or pressure-reducing valve at the meter, heated water cannot push back into the main, so without a tank its expansion would lift the relief valve or stress the plumbing. The tank gives that extra volume somewhere to expand into.

Inside the Steel Shell, a flexible Bladder / Diaphragm divides the vessel into a water side and an air side. The Air Pre-Charge Valve sets the air pre-charge, and the Connection Fitting ties the water side into the heating loop. A Internal Liner keeps potable water off the steel, and the Mounting Bracket bracket carries the filled weight so the piping is not loaded.

How it works

The tank is shipped with an air pre-charge set through the Schrader Valve valve, and the installer matches it to the system's cold static pressure. With the system cold and at that pressure, the EPDM Diaphragm sits against the water-side wall and the tank holds almost no water.

When the water heater fires, the heated water expands. Because it cannot return to the main, the surplus flows through the Connection Nipple into the tank, pushing the EPDM diaphragm back and compressing the trapped air. The air acts as a spring: it accepts the expansion volume while the pressure rises only modestly, staying well below the relief-valve setting. As taps are opened or the water cools and contracts, the compressed air pushes the stored water back out, so the diaphragm returns toward its rest position.

The diaphragm keeps the air charge and the system water permanently separated, so the air cannot dissolve into the water and waterlog the tank. The External Coating protects the outside of the shell, the polypropylene liner protects the inside, and an O-ring at the connection seals the water side. A tank sized and pre-charged correctly absorbs every heating cycle silently and lasts for years.