Hydronic System Air Purging Procedure

Why this matters

Air trapped in a hydronic loop causes uneven heat (cold zones), pump cavitation that destroys circulator bearings inside a season, banging on system startup, and oxygen corrosion that pits steel boiler heat exchangers and cast-iron radiators. A system that was bled with an old-school radiator key once a year on a forced-water gravity system is not the same problem as a closed-loop modulating-condensing boiler with three zone pumps and a radiant manifold; the modern system needs an actual purge, not a bleed. Purging done at the right pressure with the right valve sequence finishes in under an hour. Done with shortcut tricks, the system never quite runs right and the homeowner calls every cold snap about that one bedroom that will not warm up.

Tools and prep

• Hose bibb adapter that mates the boiler drain to a garden hose, plus a clean garden hose long enough to reach a floor drain or 5 gal bucket.
• Pressure gauge confirmed accurate. Most installed boiler gauges drift 3 to 5 psi over time.
• Inhibitor (Fernox F1, Sentinel X100, Rhomar Pro-Tec 1820) per system volume; mix in a separate pail for injection.
• Floor drain or sufficient buckets for the system volume (a typical residential hydronic loop holds 15 to 40 gallons).
• Combination wrench set and bleeder screw key for any high-point manual vents.

Identify in advance: the fill connection (PRV / autofill), the boiler drain at the system low point, the purge ball valve on each zone, the air separator location, and the system isolation valves.

Purge sequence (zoned system with purge valves)

This is the standard piped-for-purge configuration. Every zone has a ball valve on the return and a boiler drain just below the ball valve.

1. Confirm boiler is OFF and cool. Do not purge a hot system; water at 180 F entering the floor drain via the hose is a burn risk and a thermal shock to the heat exchanger if cold makeup hits it suddenly.
2. Close the system isolation valve (typically just upstream of the air separator or just downstream of the autofill).
3. Open the autofill / PRV bypass to allow city water to pressurize the fill line.
4. Pick the first zone to purge. Close that zone's return ball valve. Connect the hose to the boiler drain on that zone. Open the boiler drain.
5. Open the autofill / PRV fully. Water flows in through the supply, through the heat emitter (radiator, radiant loop, fan coil), out the boiler drain through the hose. Watch the discharge.
6. Discharge starts as foam (air-water mix), transitions to spurts, then to steady clear water. Continue running until the discharge is clear and bubble-free for 30 seconds. On a long radiant loop, this can take 5 to 10 minutes per loop.
7. Close the boiler drain. Open the zone's return ball valve. The first zone is now purged.
8. Repeat for each remaining zone, one at a time. Purging multiple zones simultaneously splits the flow and air migrates between loops.

Purge sequence (system without purge valves)

Older or retrofit systems may lack zone purge valves. Air must be moved to a high-point manual vent on each radiator or to the air separator at the boiler.

1. Fill the system to 12 to 15 psi static pressure (residential, single-story). Add 0.5 psi per foot of static head above the boiler for multi-story.
2. With the boiler off, open each high-point manual bleeder one at a time. Bleed until water (no spit, no air) comes out. Top-floor radiators always need to be done last because air migrates up.
3. Run the circulator(s) for 5 to 10 minutes. Air migrates to the air separator (Spirovent, Caleffi Discal) where the float vent releases it.
4. Re-bleed each high point.
5. Repeat until the system holds steady pressure with the circulators running and no further air discharges at the float vent.

This sequence takes 30 to 60 minutes longer than a system piped for purge. It is the cost of cheap original install.

After purging, before lighting the boiler

• Confirm fill pressure: 12 psi typical for residential 2-story, 15 psi for 3-story, expansion-tank precharge matches.
• Confirm expansion tank is sized and precharged correctly. A waterlogged or under-precharged tank causes pressure swings and air re-entrainment; see the expansion tank sizing reference.
• Inject inhibitor per manufacturer dose rate. Inhibitor protects the heat exchanger from the oxygen that came in with the makeup water.
• Cycle each zone valve and confirm flow at each loop with the boiler still off. Air pockets in a radiant loop show up as a cold spot you can feel through the slab.
• Light boiler and ramp through one full cycle. Watch the system pressure as it heats. A 30 F rise typically adds 4 to 6 psi. Excess rise means undersized expansion tank.

Persistent air symptoms

If air keeps coming back after a clean purge, the source is one of:

• Active makeup water (PRV stuck open, slow leak somewhere in the loop). Bring the makeup to zero by closing the autofill manual valve and watching system pressure for 24 hours. Loss of pressure = leak.
• Oxygen ingress through non-barrier PEX. Original radiant PEX without an EVOH oxygen barrier continuously absorbs oxygen. Solution is a heat exchanger isolating the radiant loop from the boiler loop, or full repipe with PEX-AL-PEX or barrier PEX.
• Undersized or failed air separator. A coin-vent at the boiler is not an air separator; a Spirovent or Caleffi Discal centrifugal-separator is.
• Expansion tank failure: water in the air bladder. Pressing the Schrader on the tank should release air, not water. Water means a ruptured bladder, replace the tank.

References

• ASHRAE Handbook, HVAC Systems and Equipment 2024, Chapter 13 (Hydronic Heating)
• Caleffi Idronics 15: Hydraulic Separators (air separation principles)
• Taco FloPro 101: Hydronic System Design
• ASTM F876/F877: Crosslinked Polyethylene (PEX) Tubing
• Lochinvar Knight Boiler Installation Manual: Initial Fill and Purge Procedures
• Hydronics Institute Section IV: System Air Removal