Best Practices for Hydronic Systems Part 3: Where to Locate the Expansion Tank
/Air management is an important part of the functionality of a hydronic system. Correct placement of the expansion tank can literally make or break the pumping performance. The rule for placement of the expansion tank is pretty simple: Always pump away from the expansion tank. In other words, make sure the expansion tank piping connects to the system piping on the suction side of the pump like this,
and NOT like this,
Otherwise, the pump could cavitate due to a negative pressure in the suction and we can have difficulty getting rid of unwanted air in the system.
To understand why this could happen, let’s look at a system where the pump is correctly installed after the expansion tank and throw in some numbers to reflect the operating pressure of the system.
Notice the point of no pressure change between the expansion tank piping and system piping. Regardless of whether the pump is on or off, the pressure at this point always remains the same. This is thanks to the air cushion inside the expansion tank, which is selected to absorb any water expansion that occurs as a result of temperature increases in the circulating water. When water volume increases due to thermal expansion, it enters the tank. Even when the pump turns on—increasing the differential pressure to the system to establish flow—the pressure at the point of connection between the system piping and the expansion tank piping does not change. This is because the pump cannot add or take water out of the tank.
CORRECT: Pumping away from the point of no pressure change.
Let’s say, as Figure 1 above shows, that we have a minimum system pressure of 12 PSI and a pump designed to create a 14 psi differential. When the pump comes on, it will boost the pressure to 26 PSI to serve the system, gradually returning to 12 PSI as a result of friction losses in the piping. In a properly installed system, the pressure will remain at or near 12 PSI at the pump suction, regardless of whether the pump is on or not.
INCORRECT: Pumping toward the point of no pressure change.
Now let’s look at what can happen when the pump is (incorrectly) installed upstream of both the expansion tank and point of no pressure change (Figure 2). When the pump starts up, it will develop pressure differential based on the resistance in the system. But because the pump is discharging towards the point of no pressure change, the pressure at the pump’s suction will be reduced by the total friction loss of the system. The pressure then at every point in the system is less with the pump operating than when it is when the pump is off. If the existing non-operating pressure acting on the system is not enough to offset the negative pressure produced by the pump, cavitation can occur.
