In short: bleed with the system switched off and cold, never while it is running. Put a shallow tray under the bleed valve (top of the radiator, on one side), open it a quarter of a turn with the key until the hiss of air turns into a trickle of water, then close it straight away. Do every radiator in the same session, starting on the lowest floor. At the end, check the boiler pressure gauge: cold, it should read between 1 and 1.5 bar.
September is the right month. Not because it is cold, but because it is the only time of year when you can find out that a radiator is not heating up without having to call someone in a hurry and spend two days freezing. After five months of standing still, air has collected at the high points of the circuit: that is the number one reason why, on the first firing, a radiator stays lukewarm at the bottom and cold at the top.
Here is how to bleed radiators, step by step: the standard procedure, the right order in a house with more than one floor, what to do if you have lost the key, and the three cases that catch nearly everyone out — old cast iron radiators, radiators with thermostatic valves and communal heating systems. If a radiator has instead reached the end of its life (leaking, full of sludge, undersized for the room), you will find tubular steel and cast aluminium models in the steel and aluminium radiators collection.
How to tell there is air in your radiators
There are three symptoms and they are unmistakable:
- The radiator is warm at the bottom and cold at the top. Air is lighter than water, it rises and sits in the upper part of the sections. That is the classic sign.
- You hear gurgling or running water when the boiler fires up, especially in the first few weeks of use.
- The house takes far longer than usual to warm up even with the boiler running constantly: with half the radiator surface out of action, real output collapses.
THE SYMPTOM THAT IS NOT AIR: if the radiator is cold at the bottom and warm at the top, bleeding will achieve nothing. That is the signature of sludge settled at the base, and the fix is a power flush carried out by an engineer. Persisting with the bleed valve in that case only loses you water and pressure.
What you need (five minutes of preparation)
- The right bleed key: a 5 mm square key on most modern radiators, a flat-blade screwdriver or an Allen key on others, a triangular key on the oldest ones. It costs a couple of euros at any hardware shop. Do not use pliers: they round off the spindle and next time the valve will not close properly.
- A shallow tray (it needs to fit under the radiator) and a couple of old cloths.
- Your boiler manual, so you know the correct pressure for your system and where the filling loop is.
How to bleed radiators: the 6-step method
1. Switch the boiler off and let the system go cold
Turning the thermostat down is not enough: switch the boiler off properly (or set it to hot water only) and wait at least an hour. There are two reasons and neither is optional. The first is safety: with the system hot, water comes out of the valve at 60-70 °C. The second is technical: with the pump stopped, air has time to rise and gather at the high points, which is exactly where you are going to collect it.
2. Open every valve fully
Turn the knobs (or thermostatic heads) on every radiator to maximum. If a valve is closed, that radiator does not fill and bleeding it achieves nothing.
3. Find the bleed valve
It is always at the highest point of the radiator, on one of the two sides, usually opposite the control valve. It is a small plug with a square, hexagonal or slotted socket.
4. Open slowly, a quarter of a turn
Tray underneath, key in the socket, turn anticlockwise just a little: you will hear a hiss, that is the air escaping. Do not open the valve wide, it serves no purpose and you end up with water everywhere. Keep your hand steady and wait.
5. Close as soon as water appears
When the hiss stops and a steady trickle of water starts, the air is out: close it clockwise without forcing it. Cranking it tight with the key ruins the washer and creates the leak you did not have before.
6. Check the pressure and switch back on
Look at the boiler pressure gauge: with the system cold the correct value is normally between 1 and 1.5 bar (1.2 bar is the most common reference; the exact figure is in the manual). Bleeding has removed water, so it is normal for it to have dropped. If it is below 1 bar, open the filling loop slowly and close it firmly as soon as you reach the right value.
THE MOST EXPENSIVE MISTAKE: leaving the filling loop open. The system goes over-pressure, the safety valve discharges (usually at 3 bar) and you find water on the floor of the plant room — in the worst case with the boiler locked out. Check twice that it is closed before you walk away.
Switch back on, let the system run for an hour and run your hand over every radiator: they should be evenly warm from top to bottom. If one is still cold at the top, bleed that one again: on the first pass a bubble is often left behind.
What order to bleed radiators in a house with several floors
Order matters, because every bleed shifts a little water around the circuit. The rule most commonly given, including by boiler manufacturers, is this:
- Start on the lowest floor and work your way up floor by floor.
- On each floor, start with the radiator closest to the boiler and follow the run of the pipes, finishing with the furthest.
- Do every radiator in the same session, then check the pressure once, at the end.
In fairness, the opposite school also exists — starting with the radiator furthest from the boiler, or with the top floor — and it is widely used. Both work. What actually matters is following a consistent order, not jumping from room to room at random, and not stopping halfway round the house.
How to bleed a radiator without a key
The key has vanished and the shops are shut. What works, and what does not:
- Works: if your valve has a slotted or cross socket, an appropriate screwdriver will do the job. If it is hexagonal, an Allen key of the right size. Some modern radiators also have a knurled knob you can turn by hand.
- Works with care: a 5 mm square socket from a socket set, if it sits with no play at all.
- Does not work: pliers, mole grips, adjustable spanners. They bite on the corners, round off the spindle after two or three attempts and turn a five-minute job into a plumber's visit. The key costs less than two euros: better to wait until tomorrow.
One special case is radiators fitted with an automatic air vent screwed into the top: those need no key at all, they release air on their own. If one is fitted and the radiator still stays cold at the top, the vent itself is usually limescaled up and needs replacing.
How to bleed radiators with thermostatic valves
Thermostatic heads complicate things for a simple reason: in summer, with a warm room, the head is closed even if the dial looks like it is in a normal position. A radiator with a closed valve does not fill and cannot be bled properly.
So before you start: turn every head to maximum (position 5, or the flame symbol), or unscrew the collar and remove the thermostatic head completely, leaving the valve pin exposed — that is the surest way to have the valve fully open. Refit the heads when you have finished.
If you find a pin has seized (common after months of inactivity), press it gently a few times with your fingers or a pair of flat-nose pliers, using very little leverage: it almost always frees off. It is also the reason why, at the end of the season, it pays to leave the heads in the open position all summer.
How to bleed cast iron radiators
On column cast iron radiators, often installed decades ago, one rule outranks all the others: never force it. Bleed valves from that era are brass and oxidise into their seat; a key used with too much force does not open them, it snaps them — and at that point the problem is no longer an air bubble, it is a radiator to be removed and a system to be drained.
Do it this way: clean the socket with a cloth, then try to open it with the correct key applying progressive pressure with no jerking. If it will not budge, do not persist: a drop of penetrating oil, half an hour's wait and a second attempt. If it still resists, stop and call a plumber. A planned job in September costs a fraction of an emergency in December.
Consider the output too: a cast iron radiator with seventy years of deposits inside puts out far less than the plate says. If your system has several in that condition, before spending on repeated repairs it is worth doing the sums on replacement: in the central heating radiators collection you will find tubular steel and cast aluminium models in standard pipe centres (500, 600, 700 and 800 mm), which in most cases fit the existing connections.
How to bleed a radiator with no bleed valve
Some old radiators have a plain blanking plug screwed into the top instead of a valve. The honest answer here is that this is not a DIY job: loosening that plug means opening the circuit with no way of closing it precisely, and on a pressurised system the difference between a trickle and a flood is half a turn.
The correct fix is to have a bleed valve fitted by a plumber: it is a short job, it needs the system partially drained and it solves the problem for good. An alternative many engineers suggest at the same time is an automatic air vent, which releases air by itself with nothing for you to remember — particularly useful on radiators that are awkward to reach and at the high points of the system.
Communal heating and rented flats: what you can and cannot do
With a communal or district heating system the mechanical part does not change: you can bleed your own radiator, with the system off, exactly as described above. Everything else does change:
- You cannot (and must not) top the water up. There is no filling loop in your flat; it is in the plant room, and topping up is the job of the maintenance contractor. If your flat is still cold after bleeding, the issue is system pressure and it needs reporting to the building manager.
- Tell the building manager or the maintenance contractor. If lots of people in a block bleed their radiators and nobody tops up, pressure falls and the first to lose heat are the flats on the top floors. One message avoids a problem for the whole building.
- Do not touch the lockshield valve or any calibration seals. In buildings with heat cost allocation the valves are set and sealed: altering them distorts the readings and the billing.
When bleeding is not enough
There are three situations where carrying on bleeding solves nothing and you should call an engineer:
- The air comes back every few days. If you have to bleed constantly, air is getting in somewhere: a flat expansion vessel, a faulty automatic vent, a micro-leak. The symptom is not the problem.
- Pressure drops on its own without you having bled anything: there is a leak, and it needs finding.
- The radiator stays cold at the bottom even after bleeding: that is sludge, and the system needs flushing.
One last useful check: if a radiator heats up well but the room never gets to temperature, the problem is not air, it is output. An undersized radiator works flat out and never gets there. How to check is explained in what size radiator do I need.
FAQ: common questions about bleeding radiators
Do you bleed radiators with the heating on or off?
Off, and with the system already cold. With the boiler running the pump keeps circulating hot water: you risk scalding yourself and the air has no time to gather at the top, which is where you go to remove it.
How much water comes out when you bleed a radiator?
Very little: air first, then a few drops and a trickle of water, which is your signal to close. A shallow tray is more than enough. If it runs for more than a few seconds you have opened too far: close it and reopen a quarter of a turn.
How often should radiators be bled?
Once a year, before the first firing, is enough for a healthy system. They also need bleeding every time work is done on the system (radiator replacement, draining, topping up) and, obviously, whenever the symptoms appear.
My boiler locked out after bleeding: why?
Almost always because of low pressure: bleeding removed water and the value fell below the minimum threshold. Top up slowly from the filling loop to 1-1.5 bar cold, close it, and reset the boiler. If it locks out again, call your engineer.
Does bleeding radiators really save money?
Yes, but the saving does not come from bleeding itself: it comes from the fact that a radiator full of air puts out far less than its rated output, so the boiler stays on longer to reach the same temperature. Bringing the whole radiator surface back into use shortens the firing cycles.
Is «purging» the same as bleeding?
Yes: bleeding, purging and venting all describe the same operation, just as radiator and heater describe the same object. Only the word changes from place to place.
In conclusion
Bleeding radiators is half an hour of work a year that buys you months of full output: system off and cold, all valves open, a quarter of a turn at a time, every radiator in the same session, a look at the gauge at the end. The only two situations where you should stop are the cast iron valve that will not budge and the radiator that stays cold at the bottom: in both cases, the phone call to a plumber costs less than the damage.
If your round of the house tells you that one or more radiators need replacing, the steel and aluminium radiators collection has tubular steel models in 2, 3 and 4 columns and cast aluminium models in standard pipe centres, built up to the number of sections you need.
And if the one you need to sort out is the bathroom radiator, the procedure differs on a few points — starting with the fact that electric models are not bled at all: see how to bleed a heated towel rail.
