Exploring Types of Water Heater Expansion Tanks and Their Functions

Discover the different types of water heater expansion tanks and their important roles in maintaining optimal pressure levels in your heating system.

Introduction of Thermal expansion tanks

In closed-loop heating boiler systems, an expansion tank is a crucial requirement. As the boiler heats the system water, the water undergoes thermal expansion, increasing volume. Unlike air, water cannot be compressed. To address this thermal expansion, an expansion tank is employed, utilizing air or an inert gas like nitrogen, which acts as a cushion or spring to counteract the force created by the expanding water.

The primary function of an expansion tank is to prevent the pressure within the water heater system from approaching or exceeding the relief valve setting of the boiler. If the system pressure surpasses the relief valve’s threshold, the valve may open, causing potential damage and compromising the safety near the water heating system. Therefore, the presence of an expansion tank is vital to maintain optimal pressure levels and ensure the system operates within its designed parameters.

Types of Expansion Tanks

Water heater expansion tanks are essential to maintaining the safety and efficiency of heating systems. They come in various types, each designed to handle specific requirements. In this detailed guide, we will explain the two main types of expansion tanks: Compression Tanks and Bladder/Diaphragm Tanks. Additionally, we will discuss their unique features, installation considerations, troubleshooting, and the importance of air management in the system.

Explaining different types of expansion tanks

Compression Tanks:

compression expansion tanks for hot water heaters

Compression or plain steel tanks are horizontally mounted above the boiler. These tanks are ideal for one or two-story buildings with no internal parts. They typically feature a gauge glass on the side to monitor the air level inside the tank. The higher the tank’s location, the smaller its size. Compression tanks are generally less expensive but require careful engineering and secure installation due to their weight.

Bladder/Diaphragm Tanks:

Bladder or Diaphragm expansion Tank showing internal working


Bladder or diaphragm tanks are easier to install as they have a ring base that sits on the floor. These tanks feature an internal butyl rubber bladder or diaphragm attached to the system water connection. The bladder is surrounded by air that is pre-charged with compressed air with a pressure which is equal to the highest static water pressure in the system plus 5 PSI. Bladder tanks are advantageous as they allow for pre-charging, have replaceable bladders, and are suitable for systems with operating temperatures below 240°F. These are also used as an expansion tank for tankless water heater.

Installation Considerations:

When installing an expansion tank, it is crucial to consider the tank’s location and connection points. The expansion tank should always be connected on the suction side(cold water inlet side) of the system pump, never on the discharge side. Expansion tanks should be connected above the air separator or the air elimination fitting on the boiler. Bladder/diaphragm tanks require pre-charging with air before the system is filled and pressurized. The air pressure in the tank should be adjusted to 5 PSI over the maximum static pressure in the system. It is recommended to valve off the tank from the system during air pressure testing.

Sizing Recommendation:

Proper sizing of an expansion tank is vital for optimal performance. Factors such as the total system gallons, fill water temperature, maximum hot water operating temperature, relief valve setting, and static pressure at the expansion tank location are considered for accurate sizing. Consultation with a professional or utilizing a sizing program is recommended to determine the appropriate tank model.

table of what capacity expansion tank needed for what capacity of water heater

  1. For water heaters up to 50 gallons, a 2-gallon expansion tank is typically sufficient.
  2. Water heaters between 50 and 100 gallons may require a 5-gallon expansion tank.
  3. Larger commercial water heaters with capacities exceeding 100 gallons may need larger expansion tanks, such as 10 gallons or more.

Troubleshooting Expansion tanks

To ensure the expansion tank is working correctly, monitor the boiler pressure gauge as the system heats up. If the pressure rises significantly, it may indicate an undersized expansion tank, waterlogged tank, ruptured bladder (bladder/diaphragm tank), inadequate pre-charging (bladder/diaphragm tank), or a closed valve isolating the expansion tank from the system.

Air Management in the System:

Air can cause noise, inefficiency, and reduced boiler performance. Compression expansion tanks have air in direct contact with water, which allows air to get back into the main piping and eventually reach the expansion tank. Automatic air vents should not be used in compression tank systems as they may reduce the air volume in the tank or cause waterlogging. On the other hand, bladder/diaphragm tanks are part of air elimination systems that utilize air separators and automaic or manual air vents to remove air from the system.


Understanding the different types of water heater expansion tanks is essential for maintaining the safety and efficiency of heating systems. Compression tanks and bladder/diaphragm tanks offer distinct advantages and are suitable for various applications. Proper installation, sizing, and air management play significant roles in optimizing the performance and longevity of expansion tanks. Consultation with professionals and adherence to manufacturer guidelines are recommended for accurate installation, troubleshooting, and system maintenance. By implementing these practices, you can ensure the smooth operation of your water heater and heating system while minimizing potential issues associated with thermal expansion and air accumulation.

TanklessHelp Staff

TanklessHelp Staff has an experienced writers and researchers in the field of water heating technology. With a background in HVAC and plumbing, Tanklesshelp staff has in-depth knowledge in many types of waterheaters in America and Canada. They have a passion for educating homeowners and businesses on solving easy to hard repairs and guide them on how to maintain them to save money.

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