As water changes temperatures, its volume can expand or contract. This physical characteristic is important when designing closed recirculating hydronic systems without open sumps/tanks. These closed systems have specific volumes with pressure relief valves that will release water if the pressure set point is exceeded to avoid bursting pipes or damaging other equipment.
Expansion tanks are pressure vessels that provide a space for the water to expand into when its volume increases due to a rise in temperature. This maintains the loop pressure and prevents water loss due to over-pressurization. When the temperature of the loop eventually decreases, the contracting water volume reduces the pressure within the loop, and the water that flowed into the expansion tank pushes back into the system.
Without an expansion tank, the water in the loop would have nowhere to go during thermal expansion cycles, resulting in the loop pressure exceeding its pressure setpoint. The loop would then relieve pressure by opening a valve to send water to drain, requiring fresh makeup water to be added back into the loop once the temperature falls. The fluctuating temperature can lead to uncontrolled water loss without a properly functioning expansion tank. Failed expansion tanks can also contribute to uncontrolled water loss and should be monitored on a routine basis.
Bladder Expansion Tank
Compression Expansion Tank
There are two main types of expansion tanks commonly used: compression expansion tanks and Bladder/diaphragm expansion tanks. Compression expansion tanks have direct contact between the air in the tank and the water from the loop whereas bladder/diaphragm expansion tanks have a rubber diaphragm separating the air and water.
The compression expansion tanks are equipped with a sight glass to show the internal water level. These site glasses can often fail over time, allowing air to escape the tank. This can lead to the tanks becoming water-logged (flooded with no compressible air space) leaving no room for the expanding water to go. As a result, the excess pressure in the loop is relieved by bleeding water, which increases water usage. Similarly, in bladder expansion tanks, the rubber diaphragm can sometimes rupture, flooding the air space with water and causing water loss due to loop over-pressurization.
Expansion tanks are often overlooked and ignored; however, they are essential in ensuring proper operation of true closed hydronic systems. Routine inspection should be carried out in order to identify any flooded tanks or vessels that are no longer airtight. If your system is experiencing uncontrolled water loss, the expansion tank may be the source.
For help troubleshooting closed recirculating hydronic systems for water loss, corrosion, and other problems, contact Chem-Aqua today.