Bleeding a radiator: when and how to do it

In short: a radiator that is warm at the bottom and cold at the top, or that gurgles, contains air. To bleed it: heat up the system, switch off the heating and wait about an hour for the air to rise, then open the bleed screw on each radiator until water flows without bubbles, starting on the lowest floor. Then check the pressure on the gauge and top up with water if necessary.

Why is there air in radiators?

Mains water contains dissolved air. When it heats up, this air is released and collects at the highest point of the radiators. Air also gets in every time the system is filled, after draining or after work on the system. An air pocket stops hot water from reaching the top of the radiator: the room heats less well and the system runs longer for the same comfort.

Signs that a radiator needs bleeding

• The radiator is warm at the bottom and cold at the top.
• You hear gurgling, splashing or whistling in the radiators or pipes.
• One radiator stays lukewarm while the others heat well.

Beware of misdiagnosis: a radiator that is cold at the bottom and warm at the top is more likely clogged with sludge (it needs flushing), and a radiator that is cold all over often has a stuck valve pin. See our guide Thermostatic radiator heads: how to choose, set and replace them.

When should you bleed your radiators?

• In autumn, at the start of the heating season.
• As soon as one of the signs above appears.
• After filling, draining or work on the system.

Even without symptoms, the Swiss Federal Office of Energy recommends bleeding all radiators about every three years.

What you need

• A bleed key: the usual model has a 5 mm square end, for example our standard bleed key or its long version for hard-to-reach screws. Some screws can also be opened with a flat screwdriver.
• A cloth and a small container, or a bleed key with a drip tray that catches the water directly.
• Gloves: the water that comes out can be hot.

Bleeding a radiator, step by step

1. Heat up the system and open all thermostatic heads to 5.
2. Switch off the heating, or at least the circulation pump, and wait about an hour. This gives the air time to rise to the top of the radiators and lets the water cool down a little.
3. Start with the lowest radiator, in the basement or on the ground floor, then work your way up floor by floor. Otherwise, air from the lower floors rises into radiators you have already bled.
4. Find the bleed screw: it is at the top of the radiator, usually on the side opposite the thermostatic head. Place the cloth and container underneath.
5. Open it slowly, a quarter to half a turn. The air escapes with a hiss. Do not open it further: the screw or its seal could be damaged.
6. Close it as soon as water flows steadily, without bubbles or spluttering. Tighten without forcing.
7. Move on to the next radiator. At the end, check the pressure (see below), then switch the heating back on.

Allow a few minutes per radiator. If the noises come back after a few days, bleed again: after refilling, two rounds are sometimes needed.

Checking the pressure after bleeding

Each time you bleed, a little water comes out and the system pressure drops. Read the gauge on the boiler or heat pump with the system cold:

• In a single-family house, the cold pressure is usually between 1 and 1.5 bar. In a building with three or four storeys, expect around 2 bar.
• The rule: the cold pressure must be at least 0.3 bar above the pre-charge pressure of the expansion vessel. See our guide How to choose an expansion vessel.
• Many gauges have a green zone or a reference needle set by the installer: that is your target value.
• If the pressure rises close to 3 bar while the heating is running, the safety valve may open: have the expansion vessel checked.

If the pressure is too low, top up through the system's filling valve, slowly and while watching the gauge, then close it again. The connection must protect the drinking-water supply (backflow preventer). If in doubt, ask your heating engineer to show you how to do it once.

As for water quality, the Swiss SICC BT 102-01 guideline requires demineralised water, including for top-ups. The simplest way is to fill through a demineralisation cartridge. See our guide Topping up your heating system with water.

Mistakes to avoid

• Bleeding with the pump running: the water comes out hot and under pressure, and the air has not had time to rise.
• Opening the screw all the way, or even unscrewing it completely.
• Bleeding only one radiator: air moves through the whole circuit.
• Forgetting to check the pressure at the end.
• Overtightening the bleed screw: its seal gets damaged and it ends up dripping.

Automatic air vents: bleeding without thinking about it

If you often need to bleed, an air vent can do the job for you:

• On a radiator: a Caleffi Aercal automatic air vent fits in place of the bleed plug (check the thread) and releases air continuously. The Caleffi 508 with hygroscopic cartridge lets air out and closes by itself on contact with water. The manual air vent with handwheel opens by hand, without a key.
• At the high points of the system (risers, boiler, manifolds): automatic air vents such as the Caleffi Minical or the Spirotech Spirotop. Mount them vertically, preferably with a shut-off valve so they can be replaced without draining.

If air keeps coming back despite bleeding, look for the cause: pressure too low, a small leak, a faulty automatic air vent or a failed expansion vessel. In that case, have the system checked by a professional.

Frequently asked questions

Do you need to switch off the heating to bleed radiators?

Yes, or at least stop the circulation pump. Wait about an hour: the air rises to the top of the radiators and the water that comes out is less hot.

Which radiator should you start with?

The lowest one, usually in the basement or on the ground floor, then floor by floor up to the top.

Black water is coming out of the radiator: is it serious?

Dark water is common in older systems: it contains corrosion particles. If it is very dirty, or if several radiators heat poorly at the bottom, having the system flushed by a professional can help.

The bleed screw is stuck or leaking: what should you do?

Don't force it. A leaking screw is replaced with a new bleed valve, but first the radiator must be isolated (valve and return fitting closed) and the pressure released. If you are not sure, leave this job to a heating engineer.

What about underfloor heating?

Underfloor heating loops are bled at the manifold, which is usually fitted with air vents. The method depends on the model: follow the instructions or ask your installer.

Equipment for bleeding and maintaining your radiators

Browse our bleed keys, air vents and radiator plugs, our automatic air vents and our pressure gauges.