Air Separators

A well-designed and properly functioning hot water (hydronic) heating system should be whisper quiet, so if you hear noise in your pipes, something’s amiss. A common cause of noisy pipes is “entrained air,” tiny bubbles racing around your system at high speed. There are many ways to describe the noise created by these bubbles: it can sound like whooshing, or rushing, or even rattling ball bearings!

Aside from making noise, entrained air acts as a subtle, unwanted insulator, actually preventing heat from transferring to radiators. Also, if too much air gets trapped in the furthest radiators, system flow can stop altogether and the radiators can go cold.

Where does this air come from? It’s dissolved in the cold water used to fill and pressurize your pipes. When cold water is heated, the air comes out of solution in the form of bubbles that are whipped around the pipes by the circulator.

Traditional remedies seldom work. Venting, either at the top of the system or near the boiler, simply can’t get rid of the smallest bubbles, and every time you vent, system pressure is lowered and water must be added to re-pressurize. Purging with cold water only exacerbates the problem. In both cases, adding more cold water means adding more dissolved air.

The correct remedy is to catch the tiny bubbles before they get into the system. That’s what an air separator does. Xylem RCW engineers two kinds of air separator: the Enhanced Air Separator (EAS) and the Inline Air Separator (IAS). Both remove most entrained air on the first pass, and practically all air once water has circulated multiple times.

The EAS captures air with an element that looks like a tiny wire brush. As water passes through the separator, bubbles are caught on stainless steel bristles inside a diffuser. This trapped air then rises through a baffle to an air vent at the top of the unit, where it’s released. The water, minus the air, then exits the valve.


How It Works

1 As system fluid
enters through the inlet, (either straight or angle) the diffuser distributes flow evenly
across the stainless steel, wire brush-like medium.



2 Air
bubbles, even micro air bubbles, stick to the brush filaments.

3 Trapped air
rises above the dif-fuser through a baffle (not pictured), where the air is then released
through an opening on top.

4 Deaerated
water then goes back into the system.

Another way to catch the entrained air is to slow it down. That’s how an IAS works. If you’ve noticed how debris in a fast-flowing stream collects in a slow-moving eddy, then you’ve seen the principle behind the IAS. This two-chambered component is wider than the pipes, so water slows down inside it and air bubbles are released. These bubbles then rise to the top of the first chamber. Once captured, air is either vented through an automatic air vent, which is installed on top of the IAS, or sent into a steel compression tank.

What happens to the water? After entering the initial chamber and releasing its bubbles, the water passes through a small orifice (ensuring that bubbles can’t find their way back into the system) and then into the second chamber. The second chamber prevents a whirlpool from forming that might suck the bubbles back in. From there, the water re-enters the regular piping. What an elegantly simple solution!

Always make sure the air separator is installed in the correct position. Air wants to come out of solution where the temperature is highest and the pressure is lowest—in a typical home heating system, that’s after the boiler and before the circulator.