All closed-loop hydronic systems require an air space to absorb the increased volume of water as it warms during system operation. In most hydronic systems, this task is handled by an expansion tank. In most modern hydronic systems, it’s handled by a diaphragm-type expansion tank.

The intended benefit of a diaphragm-type expansion tank is that it can be prepressurized with air before it is installed. The air is held captive in the space between the steel tank shell and flexible rubber diaphragm, as shown in Figure 1.

Having the proper air-side prepressurization prevents system water from entering the expansion tank, until the water begins expanding due to heating. This benefit cannot be achieved using a nondiaphragm-type “standard” expansion tank. Because the air pressure in a standard expansion tank is zero when the fill process begins, water flows into the tank until the air is pressurized to match the static water pressure in the system.

In some systems, this initial inflow could fill half the tank’s volume and occurs before the water is heated. This is undesirable because it reduces the tank’s ability to accept the additional water volume when it’s heated.

Unfortunately, improper air-side prepressurization also “wastes” some of the internal volume of a diaphragm-type expansion tank. During initial filling, if the air-side pressure is less than the static water pressure at the tank’s entrance, some water will force its way into the tank until the air-side pressure-balances the water-side pressure. This situation, illustrated in Figure 2, reduces the tank’s available acceptance volume.

## Calculate it

It’s not hard to get the air-side prepressurization correct. First, determine the height of the system’s highest piping location above the expansion tank connection, as illustrated in Figure 3.

Next, determine what static water pressure you want at the top of the system when the circulator is off. I suggest 3 to 5 psi static pressure at the top. This allows float-type air vents, mounted at the top of the system, to eject any air that accumulates within them. It also helps suppress boiling if the system operates at higher water temperatures.

Once you have these values, just plug them into Formula 1 to calculate the proper air-side prepressurization.

## Formula 1

Where:

Pa = air-side pressurization of the tank (psi)

H = distance from inlet of expansion tank to top of system (ft.)

Dc = density of the fluid at its initial (cold) temperature (lb./ft.3)

5 = assumed 5 psi at top of system (this can be changed to other values)

Water, at a typical cold fill temperature of 60º F, has a density of 62.4 lb./ft.3. A 40% solution of propylene glycol antifreeze has a density of 64.90 lb./ft.3 at 60º. For other fluids, look up their density at the expected cold fill temperature.

## Measure it

Once this air-side pressure is calculated, use an accurate 0-30 psi pressure gauge to check the air pressure in the tank. Ideally, you should do this before installing the tank in the system. However, you can check the air pressure after the tank has been connected to the piping, provided that no water has been added to the system.

If you’re lucky, the air-side pressure will match the value calculated using Formula 1. If not, add or release air from the tank, just like adjusting the pressure in a tire, until you get the calculated pressure.

Assuming you do this correctly, the air pressure in the tank will match the water pressure at the tank’s connection. When the system is filled with fluid and the pressure at the top of the system is at the value appearing at the end of Formula 1 (e.g., 5 psi or whatever other pressure you set), then the diaphragm will fully expand against the shell of the tank, as shown in Figure 4.

This preserves the full tank shell volume to absorb the increased volume of water, once it is heated.

This is how diaphragm-type expansion tanks are supposed to work. Verifying that the expansion tank has the proper air-side prepressurization is something every hydronic heating pro should be doing on every installation.