Do it in this order
- Run the heating for ten minutes and feel every radiator. Worcester Bosch's own instruction is to warm the system first so you can identify which radiators actually need attention, then switch it off. Feel each one top, middle and bottom, and write down what you find. That map is worth more than the bleeding itself, because it tells you whether you have air at all.
- Turn the heating off and let it cool. Vaillant suggest about an hour, two to be cautious. A hot radiator under pressure will spit water hot enough to scald, and a hot system gives a false pressure reading afterwards.
- Find the bleed valve. It is the small round recess with a square spindle inside, at the top of one end of the panel. Most take the flat clock-type bleed key. Some newer panels use a slotted screw and want a flat-bladed screwdriver instead, and a number of towel rails use a hex bleed plug. Buy the key rather than gripping the spindle with pliers, which rounds the brass off and turns a five-minute job into a valve replacement.
- Hold a cloth under the valve and put a small container on the floor. What comes out is black, it does not wash out of carpet, and it will run down the panel rather than drop cleanly.
- Turn the key anti-clockwise a quarter to half a turn. That is Vaillant's figure and it is enough. Opening the spindle fully risks pulling it out of the valve body with the system still pressurised, and then you are holding your thumb over a hole.
- Wait for the hiss to stop. Air first, then spitting, then a steady dribble of water with no air in it. Close the valve clockwise as soon as the water runs clean and steady. You are not trying to drain anything.
- Work the ground floor first, then upstairs. Air rises, so clearing the lower radiators first stops you pushing air back into ones you have already done. On a three-storey London house, that means the whole ground floor, then the first floor, then the top.
- Check the boiler pressure and top it back up. Worcester Bosch are explicit that pressure is lost when you bleed radiators, so plan to repressurise after. The target on a sealed system is 1.0-1.5 bar cold. There is a separate walkthrough of that on how to repressurise a boiler.
- Run the heating again and re-feel everything. A radiator that is now warm across its whole face is done. One that is still cold in the same place as before was never an air problem.
What comes out tells you what you have
| What the valve gives you | What it means |
|---|
| A short hiss, then clean water | Ordinary trapped air. Job done. |
| A long hiss that keeps going | A lot of air in one radiator. Usually a high point on the circuit, or a system that has been drained or topped up recently. |
| Dark, gritty water that stains the cloth | Magnetite. The system has corroded internally. Bleeding is fine, but the system wants cleaning and an inhibitor dose. |
| Nothing at all | See below. It is usually pressure, paint, or the wrong valve. |
| Air again within days or weeks | The system is drawing air in. That is not a bleeding problem, it is an ingress problem, and it is the paragraph at the end of this page. |
When no water comes out
Four causes, in the order they are worth checking.
The system has no pressure to push with
Look at the boiler gauge before you blame the valve. If it sits at or near zero, there is nothing to expel. Worcester Bosch treat anything below 0.5 bar as water having been lost from the system. Repressurise to 1.0-1.5 bar cold first, then bleed, then top up again.
The port is blocked
Decorators paint over bleed valves, and the hole itself silts up with magnetite on an old system. The spindle turns freely, and nothing happens. A pin or a paperclip will often clear the outlet with the spindle open a quarter turn. If it clears suddenly, expect a squirt.
You are on the wrong fitting
The bleed valve sits at the top of the panel. A blanking plug looks similar and is solid. On a double panel radiator there is sometimes only one bleed point, on one end, and turning the blank on the other end achieves nothing.
The radiator is isolated
If both valves at the bottom are shut, or a thermostatic valve pin has seized down, the radiator can be cold, full, and at no meaningful pressure. Pull the TRV head off and look at the pin. If it does not move when pressed, it is stuck, and that radiator never needed bleeding at all. Freeing a seized pin, or replacing a valve, is covered under radiator and valve leak repair if the valve then weeps.
When bleeding makes the pressure worse
Every bleed takes water out as well as air, and on a sealed system nothing replaces it. Do one radiator and the gauge barely moves. Do twelve in an afternoon and you can drop the system below its minimum, at which point most boilers lock out and you get a low-pressure fault instead of heat. Worcester Bosch mark below 0.5 bar as the red zone; Vaillant's own bleeding guide notes the F22 code appearing when pressure is wrong.
The fix is simply to top up afterwards, cold, to 1.0-1.5 bar. If you overshoot, bleed a radiator briefly to come back down — Worcester Bosch give the same instruction for an overfilled system at around 2.75 bar or more.
What matters more is the pattern. Bleed, top up, and if within a fortnight you are bleeding and topping up again, you have not fixed anything. A sealed system that keeps producing air is either corroding and generating hydrogen, or pulling air in somewhere while losing water somewhere else. Worcester Bosch put the normal top-up frequency at once or twice a year, and say to contact your installer if you are doing it much more often.
Cold at the top, cold at the bottom, cold all over
Bleeding fixes exactly one of these. The others waste an afternoon.
| What you feel | Usual cause | What actually fixes it |
|---|
| Hot at the bottom, cold across the top | Trapped air sitting at the high point of the panel | Bleeding. This is the one. |
| Hot at the top, cold along the bottom | Sludge. Worcester Bosch say a radiator colder at the bottom than the top points to a build-up needing the system cleaned, and Ideal describe the same settled residue blocking flow | A system clean or power flush by a heating engineer, not bleeding |
| Cold in the middle, warm at both ends | Debris or a partial blockage across the lower waterway | Flush or replace that radiator |
| Stone cold all over, the rest of the house fine | A closed lockshield, a seized TRV pin, or the radiator isolated after decorating | Open the valves, free the pin |
| Cold at the far end horizontally, warm at the valve end | The circuit is unbalanced and that radiator is being starved | Balancing the lockshields across the house |
| Every radiator lukewarm | Pump, diverter valve, or the boiler not making temperature | A heating engineer. Nothing on this page helps |
The bottom row of that table is the honest one. If the whole house is tepid, bleeding twelve radiators will change nothing, and the problem sits inside the boiler or at the pump. For the full order of checks on a cold radiator, valves and pressure before air, see radiator not heating up.
When a radiator that "needs bleeding" is actually a leak
A sealed heating system is closed. Air should not be arriving in it. When it keeps arriving, there are only a few explanations, and two of them are expensive to ignore.
The first is corrosion. Oxygenated mains water reacts with steel panels and produces hydrogen, which collects at high points and feels exactly like trapped air. Where that water comes from is the clue: every top-up introduces a fresh dose. Ideal make the point that repeated topping up also dilutes the inhibitor that protects the metal, so the system corrodes faster, generates more gas, and needs more bleeding.
The second is a leak that lets water out under pressure and air in as the system cools and the pressure falls. A pinhole in a 15mm pipe under a screed floor can be small enough to produce no visible damp for months, because the screed absorbs it and the heating dries it, while the gauge drops a little each week and the top radiator needs bleeding every month.
The specific pattern that means stop bleeding and start looking: you are bleeding the same radiator more than about twice a year, the boiler gauge is dropping between top-ups, and nothing under the boiler, behind the radiator valves, or at the outside discharge pipe is wet. Air that returns on a schedule, with pressure that falls on the same schedule, is water leaving the system somewhere you cannot see. The usual places are under a floor, inside a wall chase, or in the ceiling void below a bathroom.
Two other giveaways worth knowing. A warm patch on a floor with no pipe visibly running there, and a boiler that fires up briefly when nothing is calling for heat. See also boiler pressure keeps dropping with no visible leak.
What we do, and when not to call us
Do not call anyone to bleed a radiator. It is a five-minute job with a key that costs less than a coffee, and the same is true of freeing a stuck TRV pin or topping the pressure back up. A sludged system that needs a power flush is a heating engineer's job, not a leak detection job, and you should get it priced as a clean rather than as a hunt.
Call us when the air keeps coming back and the pressure keeps falling, because at that point something is open to the outside. The visit isolates and pressure-tests the system to confirm the loss is real and to size it, then traces the run with thermal imaging and, on a drained circuit, tracer gas, so the floor is opened at one point rather than searched. That is charged at our hourly rate, fixed before we attend, and under our no find, no fee terms you do not pay for the time spent looking if we do not find it. See boiler and heating leak repair for what the visit covers, or pricing for how the rate works.
