The cold static fill pressure (CSFP) in a closed hydronic system has to be high enough to accomplish three things.

1. Overcome the static head (height) between the fill point and the highest point in the system.Provide adequate pressure (minimum 4 psig) at the top of the system for proper air venting.
2. Maintain adequate pressure to prevent cavitation in pumps and control valves.

The formula for calculating the required CSFP to satisfy points 1 & 2 is:

(Static height in feet H/2.31) + 4 = CSFP in psig.

As an example, the CSFP for a system with a static height above the fill point of 40 feet would be:

(40/2.31) + 4 = 21.3 psig.

For most closed hydronic systems operating below about 210 F, maintaining the minimum top-of-system pressure at 4 psig will be adequate to prevent cavitation even if the pumps are at or near the top of the system as well.

IMPORTANT NOTE - For pre-charged diaphragm or bladder tanks, the air charge in the tank must be adjusted to equal the cold static fill pressure before it is connected to the system.