How To Remove Air From A Closed Loop System?

Trapped air is the most common reason a closed-loop heating or cooling system underperforms. The fix starts with the highest point in the loop, not the lowest. Air rises, so you purge from the top down, using a purge valve, an air separator, or a manual bleed port at each high point while the circulator runs. Below is how each method works and when to use it.

Why Does Air Get Trapped in a Closed Loop System?

A closed loop is sealed, but it is not perfectly sealed. Air enters in several predictable ways, and knowing the source often matters more than the bleeding technique itself.

When a system is first filled, water carries dissolved air with it. As that water heats, its ability to hold dissolved gas drops, and the gas comes out of solution as bubbles. This is the same principle behind bubbles forming in a pot of water before it boils. New systems and systems that have just been drained and refilled are the worst offenders.

Air also gets pulled in through small leaks on the suction side of the pump. A pinhole leak under negative pressure will draw air in rather than push water out, so you may never see a puddle. Failing seals, loose fittings, and worn pump gaskets all do this.

Finally, some air is simply left behind after any service that opens the loop. It collects at the highest points and at any place the pipe changes direction upward.

What Are the Signs of Air in a Closed Loop System?

Air in a hydronic loop shows up as noise, poor heat transfer, and erratic flow. The symptoms are usually obvious once you know what to listen for.

• Gurgling, bubbling, or rushing water sounds from pipes or radiators
• Radiators or baseboards that stay cold at the top and warm at the bottom
• A pump that runs hot or sounds like it is moving gravel
• Pressure that drifts or fluctuates more than usual
• Uneven heating between zones or rooms

The cold-at-the-top pattern is the clearest tell. Air floats, so it collects at the highest point of a radiator and blocks water from reaching that section. A radiator that is warm at the bottom and cold at the top almost always has air in it.

Pump cavitation is the more serious symptom. When air reaches the pump, the impeller churns a water-and-gas mix instead of solid water. That reduces flow and can damage the pump over time. A pump that sounds like it is grinding or moving marbles needs attention quickly.

How To Remove Air From A Closed Loop System Using a Purge Valve

Purging is the standard method for a system that has just been filled or drained. It uses the fill pressure to push water through the loop and carry air out through a purge point.

Here is the general sequence. Setups vary, so follow the manufacturer’s instructions for your specific equipment.

• Locate the purge valve, usually a hose bib or drain valve near the boiler or the lowest point of the loop.
• Attach a hose and run it to a drain or bucket.
• Close the isolation valve on the return side so water is forced through the supply side of the loop.
• Open the purge valve and let water flow until it runs steady with no sputtering or bubbles.
• Close the purge valve, then reopen the isolation valve.
• Repeat for each zone if the system is zoned.

The key is to purge with pressure behind the water. If you open a purge valve with no fill pressure, you get a trickle and the air stays put. You want a strong, steady stream that pushes the air ahead of it.

Purge from the lowest point and let the air escape at the highest point. This is the opposite of what people often assume. Air rises, so the purge valve at the bottom pushes water up and forces air out at the top of the loop.

How Do Air Separators and Automatic Vents Work?

Manual bleeding is a one-time fix. Air separators and automatic vents handle the air that keeps showing up.

An air separator is installed at the point in the system where water is hottest and pressure is lowest, which is where dissolved gas comes out of solution most readily. It uses a spinning or coalescing element to gather small bubbles into larger ones, then vents them. Some separators use a microbubble design that removes dissolved gas more aggressively than older styles.

An automatic air vent is a small float-operated valve that opens when air collects and closes when water reaches it. They are common at high points and on the top of boilers. They work well but can fail. A vent that sticks open will leak water and drop system pressure. A vent that sticks closed will let air build up again.

If your system keeps getting airlogged after you have purged it properly, the problem is usually not the vent. It is a leak or a pump issue pulling air in. Fixing the vent alone will not solve it.

What Pressure Should a Closed Loop System Run At?

Most residential hydronic systems run at 12 to 18 psi when cold, and pressure rises as the water heats. The exact target depends on the height of the system and the manufacturer’s specification, so check your equipment’s manual rather than guessing.

Pressure matters for air removal because of a simple relationship. Higher pressure keeps dissolved gas in solution. Lower pressure lets it come out as bubbles. A system running too low on pressure will constantly release air, and no amount of bleeding will keep up.

This is why the fill valve and expansion tank matter. The expansion tank absorbs the pressure change as water heats and expands. A failed or waterlogged expansion tank causes pressure swings that drive air out of solution and into the loop.

If your pressure gauge swings widely as the system heats and cools, the expansion tank is worth checking before you bleed anything.

When Bleeding Does Not Fix the Problem

If you have purged and bled repeatedly and air keeps returning, the loop is drawing air in from somewhere. Bleeding treats the symptom, not the cause.

The most common culprits are leaks on the suction side of the pump, a failed pump seal, a waterlogged expansion tank, or a fill valve that is letting in too much fresh water. Fresh water carries dissolved air, so a system that constantly takes on new water will constantly make new bubbles.

Some systems also have a design problem. If the pump is mounted at a point where pressure drops below the vapor pressure of the water, the pump itself will create air. This is called cavitation, and it is a placement issue, not a bleeding issue. A qualified technician can check the pump location and the system’s pressure profile.

One point worth knowing: automatic air vents can hide a leak. If a vent is constantly releasing air, it is often releasing air that is being pulled in somewhere else. A vent that never seems to stop working is a signal to look for the source, not to add another vent.

Can You Prevent Air From Getting Back In?

You cannot keep a closed loop completely air-free forever, but you can slow the process considerably.

• Fix leaks promptly, especially on the suction side of the pump.
• Keep system pressure in the correct range so dissolved gas stays dissolved.
• Check the expansion tank periodically for waterlogging or failure.
• Service automatic air vents and replace ones that stick.
• Avoid unnecessary draining and refilling, since each refill adds fresh dissolved air.

Each refill introduces a new batch of dissolved air, so every drain-and-refill cycle means you will need to purge again. Minimizing how often the system is opened is one of the simplest ways to keep air out.