Why You Should Bleed Your Radiators

A simple task, but often overlooked

Every winter, DPS Chauffagiste receives the same calls: "my radiator isn't heating properly", "there's a strange noise in the pipes", "one room stays cold even though the heating is on". In many cases, the cause is neither the boiler nor the thermostat, but simply air trapped in the heating circuit. The solution is then very straightforward: bleed the radiators.

This is a basic maintenance task that many homeowners and tenants in Brussels either don't know about or keep putting off. Yet it plays an important role in the proper functioning of the whole system, from the boiler right through to the last radiator in the house.

Where does the air in radiators come from?

A central heating circuit is meant to be closed and permanently filled with water. In practice, however, air always ends up getting in, for several reasons:

• Regular water top-ups. Every time water is added to the circuit (via the filling valve), a small amount of dissolved air is introduced along with it.
• Micro-leaks. A slightly porous seal, a connection that's no longer perfectly airtight, an automatic bleed valve that lets a little air through: all of this is enough to feed the phenomenon over time.
• Temperature changes. Water expands when heated and contracts when it cools, which encourages the formation of air bubbles, particularly at the high points of the system.
• Internal corrosion. A chemical reaction between the water, residual oxygen and the metal of the radiators can produce hydrogen, which also builds up in the circuit.

Air, being lighter than water, naturally rises to the top of radiators and collects there. This is why the most classic symptom is a radiator that's cold at the top and warm at the bottom.

Warning signs to watch out for

You don't need to be a heating engineer to spot that a radiator needs bleeding. Here are the most common signs:

• The top of the radiator stays cold while the bottom heats normally.
• One radiator heats up noticeably less well than the others in the house.
• Gurgling, sloshing or whistling noises can be heard in the pipes or inside the radiator itself.
• The boiler seems to run longer than usual for the same result, or even a worse one.
• The pressure shown on the boiler's gauge drops faster than normal.

If you notice one or more of these signs, there's a good chance that a proper bleed will be enough to solve the problem.

Why trapped air really is a problem

You might think a small pocket of air in a radiator is nothing to worry about. In reality, its effects build up over time and end up costing you, both in comfort and in money.

Less efficient heating

Air acts as an insulator. Wherever it collects, hot water can no longer circulate and transfer its heat to the radiator's metal. The result: part of the heating surface becomes useless, and the room takes longer to reach the desired temperature, or never quite gets there at all.

Higher energy consumption

To make up for this loss of efficiency, the boiler has to run longer and more often. You end up using more energy for the same result, or even a poorer one. Over a full heating season, this extra consumption clearly shows up on your gas or heating oil bill.

Premature wear of the circulation pump

The circulation pump, which pushes water around the whole circuit, has to work harder when pockets of air disrupt the normal flow. Over time, this wears the pump out faster and can lead to avoidable breakdowns.

Increased risk of corrosion

Air contains oxygen, which encourages the formation of rust and sludge inside radiators and pipework. These deposits eventually build up at the low points of the circuit, where they can partially block the flow of water and, in time, damage the boiler's own heat exchanger.

Unstable pressure in the system

A circuit containing too much air loses its stability. Pressure fluctuates, the boiler may go into safety shutdown more often, and you find yourself having to top up the water regularly, which, as mentioned above, introduces yet more air. It's a cycle best broken at the source.

How to bleed a radiator safely

Bleeding a standard radiator remains a task most households can manage themselves, provided a few simple precautions are followed.

1. Turn off the heating and let the radiator cool down a little before starting, to avoid any risk of scalding from hot water.
2. Locate the bleed valve, usually found at the top of the radiator, to one side. It looks like a small screw or a square-shaped valve.
3. Get a bleed key (or a flathead screwdriver, depending on the model) and place a small container or cloth underneath to catch any water that comes out.
4. Open the valve gently, about a quarter turn. You'll first hear a hissing sound: this is the air escaping. Let it out until the water starts flowing steadily, with no bubbles.
5. Close the valve immediately as soon as the water flows smoothly, to avoid draining too much water from the circuit.
6. Check the pressure on the boiler's gauge once all the radiators have been bled. If it's too low, you'll need to top up the water via the filling valve, following your boiler manufacturer's instructions.

If several radiators need bleeding, it's best to start with those on the lowest level of the house and finish with the highest ones, following the natural direction in which air rises through the circuit.

How often should you bleed your radiators?

There's no universal rule, but a good habit is to check your radiators at the start of each heating season, before the cold really sets in. If a particular radiator turns cold at the top again just a few weeks after being bled, this could point to a micro air leak somewhere in the system, or a deeper issue that a professional should look into.

When to call in a heating engineer rather than bleeding it yourself

Manual bleeding has its limits. Some situations call for a professional's help:

• Frequent, repeated bleeding, a likely sign of a leak in the circuit or a faulty air separator.
• Pressure that keeps dropping despite regular top-ups.
• Radiators fitted with automatic bleed valves that appear stuck or leak continuously.
• Blackish sludge coming out when bleeding, a sign of advanced corrosion in the circuit, which may require a full power flush of the system.
• A boiler that regularly shows a pressure fault despite your efforts.

In these cases, it's best not to keep struggling on your own: a power flush or a full inspection of the circuit by an approved technician will identify the actual source of the problem, rather than just treating the symptoms.

Bleeding radiators: part of your annual maintenance routine

At DPS Chauffagiste, we regularly remind our customers in Brussels that the annual boiler service, which is compulsory for both gas and heating oil systems in the Brussels-Capital Region, is also the ideal time for a full check-up of the entire heating circuit, including the bleed valves. During this visit, a technician checks the pressure, the condition of the water, any presence of sludge, and makes sure the whole system, from the boiler to the radiators, is working optimally and safely.

This regular follow-up helps avoid unpleasant surprises in the middle of winter, limits the extra energy use caused by a poorly bled circuit, and extends the lifespan of your boiler and radiators. If you notice signs of air in your system, or if you haven't yet booked your annual service, feel free to contact DPS Chauffagiste: our technicians work throughout the Brussels Region to keep your heating efficient, economical and safe all winter long.