Radiator Not Heating? Bleeding, Flushing or a Fault

Look at where the radiator is cold. Cold at the top means trapped air: bleed it yourself in five minutes. Cold at the bottom means sludge: the circuit needs a professional flush (désembouage). One radiator fully cold points to a closed or stuck valve; all of them lukewarm points to the boiler, the pressure or the pump.

A radiator that is cold at the bottom and hot at the top, a room that never warms up, gurgling in the pipes: every winter the same symptoms come back. A radiator that heats poorly is not just uncomfortable. It pushes up the bill, because the boiler makes up for it by running longer.

Need a heating engineer? Boiler repair, annual servicing and replacement across Paris and the inner suburbs: see our heating engineer service in Paris.

The good news is that in many cases the fix is simple and free. You do need the right diagnosis first. This guide helps you tell apart a radiator that needs bleeding, a system that needs flushing, and a genuine fault, and shows what you can do yourself.

The best reflex is to observe before you act. A radiator tells you a lot through its symptoms: which part stays cold, what noises it makes, whether it is the only one affected. A few minutes of observation is often enough to decide.

Reading the symptoms

• Cold at the top, warm at the bottom: Air is trapped in the radiator: bleed it
• Cold at the bottom, warm at the top: Sludge has settled at the bottom: the system needs flushing
• One radiator completely cold: Valve closed, lockshield valve or thermostatic head stuck
• All radiators lukewarm: The problem is the boiler or the pressure
• Gurgling, knocking: Air in the circuit or disrupted circulation

Note what you see before touching anything. This saves you bleeding a radiator that does not need it, or calling a professional for a valve that was simply left shut.

Bleeding: the first thing to try

Bleeding lets out the air trapped in the radiator so hot water can reach every part of it again. It is free, takes five minutes per radiator, and solves a large share of cases.

Step 1: turn the heating off

Switch the boiler off or turn the heating down, and wait for the radiators to cool. Bleeding when cold avoids sprays of scalding water and works better: the air gathers more easily when the circuit is at rest.

Step 2: open the bleed valve

Find the small screw, usually at the top of the radiator on the opposite side from the main valve. You need a radiator key, which costs very little at any DIY shop, or a flat screwdriver depending on the model. Put a container and a cloth underneath, then open gently: a quarter turn is almost always enough. You will hear the air hiss out.

Step 3: close it when water runs

Let the air out until a steady trickle of water comes through without bubbles. Then close it without forcing, so you do not damage the screw.

Repeat on each radiator, starting with the one lowest down or closest to the boiler, then working up through the floors. The order matters.

Step 4: check the pressure

Bleeding always lowers the pressure in the circuit, because a little water comes out with the air. Look at the gauge and top up the pressure if it has fallen below 1 bar. Our guide to heating breakdowns explains how to repressurise the boiler step by step.

Four classic mistakes

Bleeding while hot: scalding water spurts out and the result is worse. Opening the screw too far: once the air is out, water goes everywhere. Forgetting to check the pressure: the boiler locks out a few hours later and you do not make the connection. Bleeding in the wrong order: starting upstairs leaves air at the bottom of the circuit.

Flushing (désembouage): when bleeding is not enough

If your radiators stay cold at the bottom even after a proper bleed, the problem runs deeper. In France this cleaning of the heating circuit is called "désembouage", literally "de-sludging".

What the sludge actually is

It is not dirt. It is a corrosive mixture that forms inside the pipes:

• iron oxide, in other words rust, as the iron in radiators and pipes corrodes on contact with the oxygen dissolved in the water;
• micro-organisms and algae, which grow in warm, still water;
• scale residue, especially in Paris, where the water is hard.

This sludge circulates and then settles where the flow is slowest: the bottom of the radiators, pipe bends, the boiler's heat exchanger. Hence the typical cold patches along the bottom.

Four warning signs

Flushing is worth doing every five to ten years depending on the age of the system, but it becomes urgent if:

• cold patches persist at the bottom of radiators despite bleeding;
• the water comes out blackish when you bleed;
• the boiler is struggling: it overheats or locks out because water is not circulating fast enough;
• you are planning a heat pump: flushing is then essential, as sludge destroys the new heat exchanger. Our article on whether a heat pump is feasible in a Paris flat covers this prerequisite.

If your system is more than ten years old and has never been flushed, it is worth having it checked before winter.

How it is done

Chemical flushing, the preventive option: a pH-neutral cleaning product is added to the circuit. The water circulates and the product dissolves sludge and rust over several days. This is the option for regular upkeep.

Hydrodynamic flushing, the curative option: the heavy-duty method for badly clogged systems, which is often the case in older Paris buildings. A professional pump is connected to the circuit and sends a mix of water and air in pulses, creating a mechanical action that loosens stubborn sludge, radiator by radiator. The sludge is collected in a tank and the system is rinsed.

Protection, always done after the operation: a corrosion inhibitor is added, which forms a protective film inside the pipes and slows down the formation of new sludge.

What you gain

A flushed system can cut heating consumption by 15 to 25 %, extends the life of the boiler (or heat pump), and brings back even, fast heat in every room.

It is also a job that standard maintenance contracts almost never include. Check what yours covers: our guide to annual boiler servicing sets out what is compulsory and what is not.

Can you flush the system yourself? No. It needs specific equipment and a good understanding of the circuit. Done badly, it moves the sludge around without removing it, and the problem comes back faster than before.

Only one radiator stays cold

A single radiator that is completely cold while the others heat almost always has a simple mechanical cause.

First check that its valve is open: it is sometimes closed by mistake, often at the end of the previous season.

Then check the thermostatic head, which often sticks in the closed position after the summer. To free it, unscrew the head completely: you will see a small pin in the middle. Press it gently a few times. If it was stuck, this alone usually frees it.

If the radiator is still cold after that, the lockshield valve or the radiator itself may be at fault, and a visit becomes necessary.

The thermostatic radiator valve: stuck, badly set or due for replacement

The thermostatic valve adjusts the flow of hot water to the temperature of the room. The numbered head (1 to 5, or with a frost setting) unscrews; underneath, the valve body holds the pin. Three situations come up:

• The head is simply set too low: a low or frost setting gives a barely lukewarm radiator. The middle settings are enough for most rooms.
• The pin is stuck after the summer: the trick described above usually frees it.
• The valve leaks or stays stuck: the head or the body is replaced. Changing the valve body means isolating or draining the circuit, so a plumber, and on shared heating the building manager's (syndic) agreement.

Do not put furniture or a curtain in front of the head: it then reads the wrong temperature and closes the radiator too early.

All the radiators are lukewarm

Do not look at the radiators: the problem lies with the boiler, the circuit pressure, or two less familiar causes.

The circulation pump. This pump moves hot water around the whole system. When it is tired or clogged with sludge, it no longer pushes far enough, and the radiators furthest away stay lukewarm. Diagnosing it is a job for a professional.

Balancing, often overlooked. In a badly balanced system, the radiators nearest the boiler take all the heat while those furthest away struggle. A professional adjusts the lockshield valve on each radiator to share the flow. It is a discreet job, but it transforms comfort in a large Paris flat.

If your building has shared heating, the problem may affect the whole building: our article on inadequate shared heating explains how to check and what your options are.

Decoding the noises

The noises a radiator makes tell you what is happening inside.

Gurgling, the sound of running water: air in the circuit. Bleeding fixes it. Whistling: often a valve closed too far or a badly set flow rate, forcing water through too narrow a gap. Ticking or clicking: metal expanding and contracting as it heats and cools. Usually harmless; if it becomes violent, it can point to a mounting problem or water circulating too fast. A dull thud in the pipes: this is water hammer, and it deserves a professional's attention because it wears the system over time. Our article on water hammer explains what causes it.

Decide at a glance

• Air at the top of the radiator: bleed it yourself in a few minutes.
• Sludge at the bottom of the radiator: flushing, by a professional.
• One dead radiator: the valve or the thermostatic head.
• All radiators cold: boiler, pressure, circulation pump or balancing.
• Radiator leaking, corroded through or very old: consider replacing it.

Not sure who to call? Our article on the difference between a plumber and a heating engineer explains who does what. And if it is the boiler itself that is showing its age, see when you should stop waiting and replace it.

A healthy circuit saves money

A radiator that heats poorly costs you every day: the boiler runs longer and uses more, and you are no warmer for it.

Beyond bleeding and flushing, a few simple habits help. Dust the back and the fins of your radiators. Do not cover them with laundry or block them with furniture. Turn the heating down in rooms you rarely use.

Above all, watch the thermostat setting: lowering it by a single degree noticeably reduces consumption over the whole season, without any real loss of comfort. Programme lower temperatures at night and when you are out.

Frequently asked questions

How often should I bleed my radiators? Once a year, at the start of the heating season, is enough in most cases. Also bleed a radiator whenever it goes cold at the top or starts making noise during the winter.

Is flushing really worth it? Yes, especially once a system is more than ten years old. Sludge reduces efficiency, creates cold patches and eventually damages the boiler and the circulation pump. Flushing restores performance and extends the life of the system.

What is the difference between bleeding and flushing? Bleeding removes air; flushing removes sludge. If the top of the radiator is cold, it is air. If the bottom is cold, it is sludge. Bleeding is free and immediate; flushing is a professional job.

How long should a radiator take to heat up? A radiator in good condition is hot across its whole surface fifteen to thirty minutes after the heating comes on. If part of it stays cold well beyond that, or it is much slower than the others, it needs bleeding, flushing or adjusting.

The bleed valve is broken or leaking: what now? Close it gently without forcing and put a container underneath. If the screw turns without tightening, the head is broken or water keeps running, turn off the heating and have the bleed valve replaced: a plumber changes this small part by isolating the radiator, without draining the whole system in most cases.

Radiators that actually heat

Réseau Tubulure services, bleeds, flushes and balances heating systems in Paris and the inner suburbs: Paris (75), Hauts-de-Seine (92), Seine-Saint-Denis (93) and Val-de-Marne (94) (see our service area).

We take calls 7 days a week, 24 hours a day. Many of our plumbers speak English. Boiler off or every radiator cold? See our guide heating breakdown in Paris: what to do.

01 89 70 15 36 (from abroad: +33 1 89 70 15 36). Request a diagnosis · See our prices · Heating emergency