Purging and filling a hydronic circuit: the clean method

A poorly purged hydronic circuit doesn’t fail the same day. It runs “well enough”: a lukewarm radiator on the top floor, a circulator that gurgles, a loop that never quite reaches temperature. Then, a few weeks later, the service call comes. In the commercial and institutional buildings of Greater Montréal — where risers run tall and loops number in the dozens — filling a circuit cleanly is not a end-of-job formality. It decides whether the system will deliver its output for ten years or call you back all winter.

Here is the method we apply, on mechanical plumbing and piping work as much as on recommissioning a boiler room, to get the air out of a circuit and keep it out.

Tools required

• A reliable gauge on the make-up line and at the low point of the circuit.
• An adjustable make-up pressure-reducing valve and a fill hose.
• Purge valves at the return end of each loop (or temporary purge hoses).
• A compressor or tire gauge to check the expansion tank precharge.
• Treated make-up water — see the note below on water quality.

⚠ Safety

Before any work, lock out the burner and circulator supply. Filling is done on a cold circuit: never introduce cold make-up water into a still-hot heat exchanger, or you risk thermal shock. And never exceed the relief valve’s set pressure while purging — an “aggressive” purge that makes the relief valve spit is not a success, it’s a stop signal.

1. Map and isolate the loops

The first mistake is trying to fill the whole circuit at once. Water takes the easiest path, traps air in dead branches and leaves you with half-full loops. First identify each loop, its return-end purge valves and its high-point vents. Then purge one loop at a time, isolating the others: slower to describe, much faster to execute, and far more reliable.

2. Check the expansion tank before filling

Check the expansion tank with the circuit empty, on the air side. Its precharge must equal the cold-fill pressure you’re about to target. If the precharge is too low, water enters the tank as soon as you fill and there’s no volume left to absorb expansion: pressure will climb abnormally when hot and the relief valve will eventually spit. It’s a two-minute adjustment that belongs before the fill, never after.

3. Calculate the cold-fill pressure

Forget the universal “12 psi.” Cold-fill pressure is set by building height: take the height of the highest point in the circuit above the gauge, divide it by 2.31, then add about 4 psi to guarantee a slight positive pressure at the top — enough for the high vents to release air instead of drawing in parasitic air. On a tower, that value can far exceed a residential figure. Set the make-up PRV to that number, not to habit.

4. Fill from the bottom, one loop at a time

Fill from the low point and push water upward: rising water drives the air ahead of it toward the vents and purge valves instead of trapping it. Open the make-up water, direct flow into the first loop, and let it run at the return purge valve until you get a clear, continuous, bubble-free stream. Close it, move to the next loop. Take the chance to start with properly treated make-up water: a fill is the ideal moment to address the circuit’s water treatment, because raw water introduces oxygen and minerals that will later attack heat exchangers and circulators.

5. Drive out the free air with the pump off

The bulk of the purge happens with the pump off. With it off, air uses its natural buoyancy to rise to the high points, where vents can release it; the pump running, by contrast, churns the bubbles and scatters them through the network. Hold a starting pressure slightly above target, since it will drop as air leaves. Forced purging removes over 95 % of the free air this way. Start the circulators only once most of the air is out — and remember that a circulator running in air degrades fast, all the more reason to confirm pump and circulator maintenance after any refill.

6. Stabilize, deaerate, then document

Forced purging does not remove the air dissolved in the water. That’s where the microbubble separator works, ideally placed on the hot supply at the boiler outlet: as it heats, water holds less gas and releases its bubbles, which the separator captures continuously. This deaeration takes time — often several days, not an afternoon. So bring the circuit up to temperature, let it run, then come back to check the cold-fill pressure once the system has deaerated: it will almost always have dropped. Readjust make-up to the calculated value, log the cold-fill pressure, the tank precharge and the readings to the file. That record is gold at the next visit.

The mistakes that bring the technician back

Three faults come up again and again: filling the whole circuit at once (instead of loop by loop), skipping the tank precharge check before the fill, and confusing forced purging with deaeration — declaring the job “done” the same day when dissolved air will take days to leave. A fourth, quieter one is filling with untreated water: the circuit starts clean… and degrades from the inside by the first winter.

That’s the level of rigour the Montréal Combustion team brings to every job.