The Basics of Aquaflex Expansion Tanks

An expansion tank is one of the least understood products in the HVAC world.

While they look simple on the outside, the inner workings of these tanks can be complex, and your understanding of the basics is crucial to their smooth operation.  

The Purpose of an Expansion Tank 

The function of an expansion tank is to accommodate the expansion of fluid in a closed-loop system. Liquid expands as it increases in temperature, so this always occurs in HVAC systems. 

How it Works 

Expansion tanks are always connected to the return line. As temperature increases, the volume of the liquid expands and needs somewhere to displace to. 

The expansion tank’s empty membrane provides this needed space. The bladder may get partially or completely filled, depending on which temperature the liquid reaches.

This is why it’s important to size your tank correctly. An undersized expansion tank will result in routine spillage and an unnecessary loss of liquid from the system. 

As the liquid fills the membrane, it presses against the pre-charged air inside the tank. If the pre-charge is set too high, there may be no effective expansion capacity available, resulting in spillage. 

Diagram A demonstrates how the Aquaflex Expansion Tank performs through various stages of the cycle. 

Diagram A: 

Expansion Tank Componentry 

An Aquaflex Expansion Tank is a steel canister with a synthetic rubber membrane or “bladder” inside. 

The bladder is flexible and swells to accommodate the expansion of the fluid as it changes temperature. Each tank’s bladder is secured by the flange at the base of the vessel. The legs of the tank provide sufficient height to allow the removal of this membrane if necessary. 

Water enters the vessel through the bottom connection, which is usually ½ ,3/4 or 1 inch. Water never touches the expansion tank unless the bladder ruptures. 

A Schroder Valve is included with all Aquaflex Expansion Tanks to allow the injection of air. These tanks are charged with air before they’re dispatched from the factory. This is called “pre-charge”. 

Smaller tanks will have 2bar of pre-charge and tanks larger than 200L will have 4bar of pre-charge. 

Diagram B explains the various parts of an Aquaflex Expansion Tank. 

Diagram B: 

Materials of Construction 

Vessel + Legs: Cold-pressed carbon steel (stainless steel available upon request) 

Bladder: Natural rubber or EPDM membrane 

Connection Flange: Stainless steel 

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