An expansion tank is a small, pressurized metal vessel installed in a home’s plumbing or heating system. This tank is designed to manage pressure fluctuations that occur when water is heated or cooled within a closed system. Its primary function is to act as a shock absorber, protecting pipes, fixtures, and appliances from damage caused by excessive pressure buildup. The tank ensures the system maintains a stable, safe operating pressure, which is necessary for the longevity and efficiency of water heaters and boilers.
Why Expansion Tanks Are Necessary
The need for an expansion tank stems from the physical property of water known as thermal expansion. When water is heated, such as inside a water heater or boiler, its volume increases significantly. For example, heating 40 gallons of water from 40°F to 120°F causes the volume to expand by approximately half a gallon.
Modern plumbing systems are often considered “closed” due to the installation of backflow preventers or pressure-reducing valves on the main water line. In a closed system, this expanded volume of water has nowhere to go, leading to a rapid spike in pressure. Without the tank, pressure would quickly exceed safe limits, stressing pipe joints and fixtures.
This excess pressure would force the Temperature and Pressure (T&P) relief valve on the water heater to open repeatedly, releasing water. The expansion tank prevents this constant pressure relief, protecting the T&P valve from premature failure and avoiding potential water damage. By absorbing the extra volume, the tank safeguards the entire plumbing network.
How an Expansion Tank Works
The internal mechanism of a modern expansion tank is based on a two-chamber design. The tank is divided into two sections by a flexible barrier, typically a butyl rubber diaphragm or bladder. One side of the barrier is connected to the home’s water system, while the other side is pre-charged with compressed air or an inert gas like nitrogen.
The air-filled chamber acts like a spring, providing a cushion against incoming water. Before installation, the air side is charged to match the system’s static cold water pressure, often between 40 and 60 pounds per square inch (psi). When the water in the system heats up and expands, the excess volume flows into the tank’s water chamber.
This incoming water pushes against the flexible diaphragm, which compresses the air on the opposite side. The compression of the air absorbs the increased volume, preventing the pressure from rising excessively. When a hot water faucet is opened, the system pressure drops, and the compressed air pushes the stored water back out of the tank and into the plumbing lines, maintaining pressure stability.
Types and Installation Locations
Expansion tanks are categorized based on their application: domestic hot water or closed-loop heating systems. Thermal expansion tanks are used with water heaters to manage the pressure created by heating potable water. These tanks are typically installed on the cold water supply line near the water heater, often mounted vertically or horizontally.
Hydronic expansion tanks serve a similar function but are used in closed-loop heating systems, such as those connected to a boiler for radiant heat or baseboards. In these systems, the tank manages the expansion of the non-potable water circulating through the heating loop. Modern hydronic tanks use a diaphragm design similar to the thermal type.
For both applications, the tank must be installed on the cold side of the system, where the water is entering the heating appliance. This placement ensures the tank absorbs the volume increase before pressure builds up. Proper installation also requires the tank to be adequately supported, preventing the weight of the water-filled tank from stressing the connecting pipes.
Identifying a Failing Expansion Tank
A failing expansion tank, often described as “waterlogged,” can be identified by several clear symptoms. The most common sign is the Temperature and Pressure (T&P) relief valve on the water heater constantly dripping or releasing water. This indicates the tank is no longer absorbing thermal expansion, forcing the safety valve to relieve excess pressure.
Another simple check involves tapping the tank with a small metal object, such as a wrench. A functional tank will sound hollow on the air-charged side, typically the top, and solid or dull on the water-filled side, usually the bottom. If the entire tank produces a dull, solid sound, it suggests the diaphragm has failed and the air cushion is lost.
To confirm a failure, the air pressure can be checked using a standard tire gauge on the Schrader valve located on the tank’s air side. Before checking, the water supply must be shut off and the pressure relieved to ensure an accurate reading. If the gauge reads zero or significantly less than the system’s cold water pressure, the tank has lost its air charge and needs replacement.