Are you having frequent problems with large fluctuations in closed loop hydronic system pressure? Does your system frequently pop safety relief valves one day and then need to have fluid added they next day? Is your pump cavitating at times? If your pump sound like it is pumping marbles, it could be cavitation.

If the answer is yes to these any of these questions, your expansion tank is not performing as it should for some reason. Sometimes we find this is related to an undersized tank caused by the designer, or installer, not taking into account the proper system volume and temperature change when sizing the tank. In those cases, the only solution is to replace the tank with a larger one.

But more often, it is caused by one or more of the following:

  • The tank not being installed in the proper location (See our other October 2, 2019 insight for further explanation)
  • Air not being properly bled from the system
  • The air-side of the tank not being properly pre-charged to the pressure required for that system
  • The installation circumstances of the tank into the system.

To properly pre-charge the air side, you need to know the static pressure (with the pumps off) at the elevation of the expansion tank, or the elevation difference between the highest point in your hydronic system and your expansion tank. Once you know the elevation difference, convert that elevation difference to static pressure by dividing by 2.31 to get pressure in psi. That’s the absolute minimum pressure you need to keep the system under positive pressure. Add about 3-5 psi to that figure to give your system a little buffer, and now you have the proper pre-charge pressure for the air side. The key here is pre-charge pressure. That pressure needs to be adjusted when there is no fluid pressure on the other side of the tank.

If not specified differently during submittal or ordering, many tanks are supplied with a 12 psi air-side factory charge. This may work on many systems of 3 stories or less, but if your building has more floors and your tank is in the lowest level, you’ll need more air pressure in the tank. Too little air-side pressure in the tank will cause the fluid static pressure to push the bladder or diaphragm well into the tank and not allow room for additional thermal expansion.

Now that you have the air-side of the tank properly charged, before you open the valve between the system and the fluid side of the tank, the fluid should be at the coolest temperature that you expect it to be. Keep in mind that this may be when the system is off, like in a heating system that normally operates with temperatures of 120-180°F, the lowest temp might be summer time when the system is off and the fluid is at room temperature of 72-75°F.