How to Bleed Air from Radiators? Automatic Air Venting Methods

Why Does Air Build Up in Radiators?

System water contains dissolved air. As the water heats up, this air separates out as bubbles and collects at the highest points of the system, which are mostly the tops of radiators and the radiators on upper floors. The following increase air formation:

• Filling the system with new water or topping it up,
• Starting the system for the first time, or refilling it after maintenance, repair or radiator replacement,
• Small leaks at unions, valves or air vents.

How to Tell There Is Air in a Radiator

• The top of the radiator is cold and the bottom is hot.
• You hear water noise, gurgling or hissing from the radiator or the pipes.
• The room heats slowly or insufficiently even though the boiler is running.

Not every cold radiator has air in it. If the whole radiator is cold, a closed valve, a blocked line or an imbalance in the system may also be the cause. If a radiator is cold at the bottom and hot at the top, the problem is usually not air but sludge collected at the bottom of the radiator.

How to Bleed a Radiator Step by Step

1. Turn off the boiler and wait for the radiators to cool. This reduces the risk of scalding from hot water; when the pump stops, the air also collects at the top of the radiator.
2. Check that the radiator valves are open.
3. Put a cloth or a small container under the air vent. The air vent is the small bleed screw at the top corner of the radiator.
4. Open the air vent slowly. Depending on the model, use a radiator bleed key or a flat screwdriver. A quarter or half turn counterclockwise is usually enough; do not remove the screw completely.
5. Wait for air to come out first, followed by a steady flow of water. When the hissing stops and water flows without bubbles, the radiator has been bled.
6. Close the air vent. Tighten it, but do not overtighten; the screw and seal can be damaged.
7. Repeat on the other radiators. In multi-story homes, pay particular attention to the radiators on upper floors; air collects at the high points of the system.
8. Check the system pressure and top up with water if necessary.

Note: If your radiator has a hygroscopic automatic air vent (e.g. the CALEFFI 5080 series), no key is needed to bleed it: turn the cap back 1 turn from the fully closed position and close it fully again when water appears. The cap must be fully closed for the vent to release air on its own.

What should the boiler pressure be after bleeding?

Because some water also leaves the system during bleeding, the pressure drops. The pressure on the boiler gauge should be within the range given in the boiler manual; on most combi boilers the recommended value with the system cold is between 1 and 2 bar. If the pressure is low, top up slowly with water through the boiler’s filling valve. If you overfill, you can lower the pressure by releasing a little water from a radiator’s air vent.

After topping up, run the boiler and check the radiators again after a while; you may need to repeat the process, as new air separates from the heated water. If the pressure drops frequently, there may be a leak in the system; in that case call a service technician.

In centrally heated buildings

In a central system, the radiator in the apartment is bled from its air vent in the same way, but the system pressure is managed from the boiler room. If air keeps coming from a radiator or the radiators do not heat at all, report it to the building management or the company maintaining the system. If the heat meter at the apartment inlet shows an “air” or “no flow” warning, see our Heat Meter Error Codes guide for the meaning of the codes.

What Is an Automatic Air Vent?

An automatic air vent (automatic air release valve) is a small valve that releases air collected in the system without user intervention. In float-type models, while the body is full of water the float stays up and keeps the outlet closed. As air collects, the water level drops, the float goes down and the outlet opens to release the air; then the water level rises and the outlet closes again.

Some small models fitted to radiators use discs that swell with moisture (hygroscopic) instead of a float: the discs swell when wet and close the outlet, and when air arrives they dry, shrink and let the air out. For models, see our automatic air vent category.

Air vent for radiatorsCALEFFI 1/2" Automatic Hygroscopic Air Vent for Radiators

• Automatic operation: Works with the cap in the fully closed position. When the cellulose-fiber discs inside are wetted by water, their volume increases by about 50% and they close the outlet; when air collects in the radiator, the discs dry, shrink and the air is released.
• Manual bleeding: The cap is turned back 1 turn from the fully closed position and air is released until water appears. This is mostly used when filling the system.
• Maintenance: The manufacturer states that hard or unfiltered water can damage the discs over time and recommends replacing the cartridge at least every 36 months. The cartridge can be replaced without draining the water from the radiator.
• Connection size: The 5080 series is made with 1/8", 1/4", 3/8" and 1/2" connections; choose according to the size of the air vent port on your radiator.

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Where Are Automatic Air Vents Installed?

• Underfloor heating manifolds: There is an automatic air vent at the end of the manifold’s supply and return bars; air collected in the long floor pipes is released here. For the role of the manifold, see our What Does a Manifold with Flow Meters Do? page; for adjustment steps, see our Manifold with Flow Meters Adjustment guide.
• High points of the system: Tops of risers, upper ends of vertical pipes and bends where air collects.
• Around the boiler: At the boiler or combi boiler outlet, together with an air separator.
• Radiators that often trap air: A suitable automatic air vent can be fitted instead of the manual one. For radiators, the CALEFFI 1/2" automatic air vent with hygroscopic discs can be used; it is fitted in the radiator’s air vent port in place of the manual vent.

Installation tips

• Float-type air vents are installed upright; a vent installed sideways or upside down cannot release air and may leak water.
• If a shut-off valve is fitted below the air vent, the vent can be removed and replaced without draining the system.
• On many models the small cap on top is left loose during operation; if it is fully tightened, the vent cannot release air. Follow the product’s installation instructions.
• Sediment in the system water can make the air vent leak over time; a leaking vent should be cleaned or replaced.
• Have installation and system pressure adjustment done by a qualified installer.

Air Separator: The Companion of the Automatic Air Vent

An automatic air vent releases free air collected at certain points of the system. To catch the micro-bubbles circulating with the water, an air separator is installed at the boiler or combi boiler outlet. The air separator separates the bubbles from the water flow and releases them through the vent on top. In large and central systems, an air separator and a dirt (sludge) separator are used together to reduce both air and dirt problems. For models, see our air and dirt separator category.

Benefits of Automatic Air Venting

• Radiators and floor heating pipes work without air, so heating is more even.
• The need for manual bleeding is reduced.
• Noise and strain on a pump running with air are reduced.
• Air-related water noises in the system are reduced.

Automatic air venting is not mandatory in every system. However, it makes maintenance easier in systems where manual bleeding is difficult, such as underfloor heating, large apartments and central heating. For plumbing products, see our mechanical plumbing, technical products and underfloor heating products categories; for temperature control per radiator, see our Thermostatic Radiator Valve guide.