How to Set Up a Bladder Expansion Tank

Hydronic hot water systems require an expansion tank in order for the system to work properly and to keep the relief valves on the boilers from opening. The purpose of an expansion tank is to provide a place for water to go when it’s heated in the system. A hydronic system is a closed loop, ideally 100% full of water with no air anywhere in the system, and so the water needs to have a place to expand when heated. Since water is a liquid and cannot be compressed, the expansion tank provides that extra space and has an air cushion that can compress, allowing the bladder inside the tank to expand to maintain proper system pressure.  

What Is a Bladder Expansion Tank?

A bladder expansion tank is a cylindrical tank containing a rubber bladder with an air pressure charge occupying the space around the bladder within the metal tank. The expansion tank is attached to the system through a single piping connection. As it’s only one pipe between the tank and system, there is no circulating flow through the tank. The bladder from the tank holds the water from the system and will expand and contract based on the forces exerted on the bladder. When the system pressure increases, the bladder will expand and compress the air inside the tank. The air acts as a spring on the bladder pushing back to counter the force of the expanding water in the system.

Correctly Piping a Bladder Expansion Tank

It’s important that a bladder expansion tank be piped properly for initial installation as well as future service. If the tank is piped incorrectly, this will inhibit operation and testing of the bladder tank in the future. Below are the steps to properly pipe an expansion tank. (Refer to the piping diagram as well.)

  1. The pipe from the expansion tank should be the same size as the tank connection, and it will usually be connected to the system at the top of the air separator port with a tee. There needs to be a valve directly off the tee that will isolate the expansion tank piping from the rest of the system. This valve should remain closed until the bladder tank is fully commissioned. Once the system is operational, the isolation valve should remain open.
  1. Near the expansion tank piping connection there needs to be a union that will allow the tank to be disconnected from the system piping, should the bladder need to be serviced or the tank replaced. The bladder inside the tank can fail and may need to be changed, and if there isn’t a union or flange, the piping will have to be cut and a union added after the fact. Thus, it’s best to start out with one.
  1. Between the union and the expansion tank piping connection will be a tee with a valve on the branch. This valve has a standard garden hose connection, and when opened it will expose the inside of the bladder to atmospheric pressure, which is required to properly set up and service the tank. This valve is normally closed and only used for service of the tank. There are also special expansion tank valves that combine a tee branch valve connection into one fitting to shorten install time.

Commissioning an Expansion Tank

What’s the most important part of setting up a bladder expansion tank for a system? Always check the air charge. On a new tank, the charge will be written on the side of the tank from the factory, but you’ll have to check to make sure it is what you need. The air charge of the tank should be the same as the desired system pressure. Once the air charge is set, open the system isolation valve and the pressure from the system and expansion tank will equalize, filling the bladder with water. Ensure that the pressure reducing fill valve is set for the same pressure as the air charge in the expansion tank. Once the system is at the desired cold fill pressure, check the tank fittings for any leaks. Once you have determined there are no leaks, the expansion tank is ready for service in the hydronic system.

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