Expansion Tanks: The Basics

Expansion Tanks: The Basics

Expansion tanks are an important component to ensure that hydronic heating systems perform efficiently and provide reliable heat for many years. Circulating hot water through radiators has provided safe, effective heating in homes for well over 100 years.

Hot Water Expands

One challenge of hydronic heating is the basic fact that water volume expands as it’s heated. For effective heating performance, the system should be entirely full at all times with no voids containing air. As water heats and volume increases, however, excess pressure that could damage components may result within the closed system. That’s where expansion tanks perform a critical function.

Pressure is Relieved

The tank is divided into two segments separated by a flexible rubber diaphragm. As water heats and system pressure rises, water enters the upper portion of the expansion tank. The rubber diaphragm expands downward into the lower, empty portion of the tank that contains pressurized air, allowing more water to enter the upper portion. This continuous expansion moderates system pressure, keeping it within proper specifications. When the system cycles off and water cools, the rubber diaphragm slowly pushes water back into the system.

What Can Go Wrong?

An expansion tank is simple and reliable. However, the internal rubber diaphragm is subject to wear and tear over the years and may eventually rupture, filling the entire tank with water and no longer providing system pressure relief. A simple check can determine the state of the diaphragm.

  • While the system is running and heating water, feel the sides of the expansion tank from top to bottom.
  • If the upper portion of the tank feels definitely warm while the lower portion of the tank feels more like room temperature, the diaphragm is still intact and the tank is still functional.
  • If the entire tank feels warm or hot from top to bottom, this likely indicates that the diaphragm is leaking or ruptured and qualified professional service to replace the tank is required.

Ask the pros at Aggressive Mechanical for more about the proper operation and maintenance of hydronic system expansion tanks.

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