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The Basics of Air Purging in your Hydronic HVAC System

Hydronic HVAC systems use water to transport energy throughout a building to create a comfortable environment for everyone in the building. The water that fills the pipes during the initial installation will remain in the hydronic system and be used continually for its entire life. The only time this water could be lost is during equipment maintenance or if the system has an occasional leak. Most buildings will get their water through the municipality system; this could require chemical treatments depending on the water chemistry. With EVERY hydronic system, the system fill pressure must be set and the air must be purged. This will require four components:

  1. Air Vents: These can be automatic or manual, and should be installed at the air separator and every high point in the piping system. Check out TACO Comfort Vents and MetraFlex Vents for several vent options.

  2. Air Separator: Installed at the lowest pressure point of the system, this is usually on the suction side of the system pumps. Check out TACO Comfort air separators.

  3. Make-Up Water (MUW) Assembly: As the link between the hydronic system and the domestic water feed, the MUW assembly includes a Pressure Reducing Valve (PRV), a Pressure Relief Valve, and full-size bypass piping for use during initial system fill. This normally connects to the hydronic system at the air separator inlet, which is the point in the system with the lowest pressure. Our expert team at Pump'nFlo can create an MUW assembly to fit your needs.

  4. Pressure Reducing Valve (PRV): The PRV must be set to a pressure that ensures 5-psig of positive pressure at the top of the system. This keeps the system full of water, purges out the air, and ensures the pumps have a constant pressure at their suction.


Air in a hydronic system will corrode any piping or equipment made of ferrous materials and it can partially or completely prevent water flow. Dissolved air also hinders the heat transfer properties of water! Overall, air does not belong in a closed hydronic system.


A real-world example of troubleshooting the system fill pressure:

A common mistake we see, especially in variable speed (VFD) pumping systems, is increasing the pump speed to ensure flow at the highest point in the system. (Keep in mind, the “highest point” does not necessarily mean the “farthest point.") In one example, there was an air handling unit (AHU) on the roof of a three-story school building, located above the mechanical room. The contractor increased the pump speed to force water through the starved coil, but it did not work. The AHU still had insufficient flow, and the VFD began “hunting” (oscillating speeds). The reason it did not work was the PRV in the first-floor mechanical room was set to 12-psig. For this three-story building, the vertical height to the AHU on the roof was 40-feet. So, 17.3-psig was needed to get water to the top of the system. Another 5-psig was added to ensure positive air purge pressure. The PRV setting was adjusted to 23-psig and the flow issue at the AHU disappeared.



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