Air and dirt separation in hydronic systems

Closed hydronic loops are supposed to be clean, airless and stable. In practice, every new system starts full of air and construction debris, and many existing systems carry a steady load of entrained air, magnetite and scale for years. The results are familiar to any facility manager: gurgling terminals, cold radiators at the top of the building, noisy pumps, stuck control valves and coils that lose capacity.

Air and dirt separation equipment is inexpensive relative to the systems it protects. Getting it right is mostly about understanding where air and dirt come from and putting the right device in the right place.

Where air comes from

Air enters a hydronic system in three forms:

• Free air pockets trapped at high points during filling.
• Entrained bubbles carried along with flowing water.
• Dissolved air held in solution, invisible until temperature rises or pressure falls.

The amount of air water can hold in solution decreases as temperature rises and as pressure falls, a relationship described by Henry's law. That is why air tends to come out of solution in boilers and at the top of buildings, and why the best place to remove it is the point in the system with the highest temperature and lowest pressure.

Ongoing sources of new air include makeup water (fresh water carries dissolved oxygen and nitrogen), leaks at high points where pressure falls below atmospheric, and automatic air vents that let air in when system pressure at their location is too low. Correct expansion tank placement and system pressurization is the first line of defence.

Where dirt comes from

• Construction debris: welding slag, pipe dope, cutting chips, sand.
• Corrosion products, mostly magnetite (black iron oxide) and hematite (red rust), produced when oxygen reacts with steel piping.
• Scale from hard makeup water.
• Biological growth and degraded glycol in poorly maintained systems.

Fine magnetite particles are a particular nuisance. They are magnetic, abrasive and small enough to pass through ordinary strainers, and they accumulate in pump motors with wet rotors, in control valve seats and in small passages of pressure-independent control valves.

Air removal equipment

Locating the air separator

The main air separator should be at the point of lowest solubility that is practical, which in a heating system usually means:

1. On the supply leaving the boiler or heat exchanger, where water is hottest.
2. On the suction side of the main circulating pump, near the expansion tank connection, so it sees low pressure.

In chilled water systems, water is coldest at the plant and holds more dissolved air there, so the separator is often placed on the return, upstream of the pump, where water is warmest. In tall buildings, the low-pressure top floors are where air naturally collects, so good high-point venting matters even with an effective central separator.

Separators must be sized for the flow rate. Most manufacturers recommend inlet velocity limits, often around 1.2 m/s (4 ft/s) or lower for coalescing types, and sizing to pipe size alone can be wrong.

Dirt removal equipment

Strainers are still needed upstream of pumps, coils and terminal units for initial protection, but they become a maintenance burden if they are the only dirt removal. A full-flow dirt separator in the main return lets strainers stay clean longer.

Flushing and cleaning new systems

The best time to deal with dirt is before it reaches equipment. A sound start-up sequence typically includes:

1. Flush mains and branches with terminal units and coils bypassed, at velocities high enough to move debris.
2. Clean with a compatible pre-commissioning cleaner to remove oils and mill scale, following the water treatment specialist's procedure.
3. Drain, refill and flush until the discharge runs clear.
4. Clean all strainers and dirt separators.
5. Add treatment chemicals and establish baseline water quality.
6. Vent all high points and coils, then re-vent after the system reaches operating temperature.

Skipping steps here is one of the most common sources of deficiencies at turnover. The water treatment for closed and open loops guide covers chemistry and monitoring.

Signs of trouble in existing systems

• Gurgling or rushing noises in piping and terminals.
• Upper floors or high coils that never heat or cool fully.
• Frequent manual venting by maintenance staff.
• Pump noise, cavitation damage or short bearing life.
• Black or red water when draining a sample.
• Strainers that clog repeatedly.
• Steady makeup water use, which brings fresh oxygen.

Persistent air despite good separators usually points to a pressurization problem, a leak drawing air in, or a vent at a location with negative pressure.

Frequently asked questions

Should I install automatic air vents everywhere?

No. Automatic vents at points where pressure can drop below atmospheric will admit air. Use them where pressure is reliably positive and use manual vents or vents with isolation elsewhere.

Is a magnetic separator worth it?

In systems with older steel piping or a history of magnetite, magnetic separators often capture far more debris than conventional strainers. They are also useful protection for electronically commutated pumps, whose permanent magnet rotors attract iron particles.

Key takeaways

• Air comes out of solution where temperature is high and pressure is low; locate the main separator there.
• Correct pressurization keeps high points positive and stops air being drawn in.
• Strainers protect equipment; low-velocity and magnetic dirt separators actually clean the system.
• Thorough flushing and cleaning before start-up prevents most debris problems.
• Steady makeup water use signals a leak and ongoing oxygen ingress; verifying flows and venting is part of water balancing work.