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