DIY GUIDE · PLUMBING & HEATING · CHECKED 2026-10
How to bleed your radiators and re-pressurise the system
Ten minutes a radiator. €10–32 for the tools, once.
Cold at the top means air. Bleed it, then put the pressure back. Nothing on the gas side.
- Every step, start to finish
- Checked 2026-10 — sources at the end
- Honest about when to call a pro
FIRST, THE HONEST ANSWER
Do it yourself, or get a fitter?
Do it yourself if…
- Radiators are cold at the top and warm at the bottom, which is trapped air
- You can find the filling loop and the pressure gauge, and read the boiler's manual
- Nothing is leaking and the boiler is not showing a fault code
- The system is a normal sealed system with a pressure gauge, or an open-vented one with a tank
Get a pro if…
- Anything involves the gas side, the burner, the flue or the inside of the boiler case
- The pressure drops again within days — that is a leak or a failed expansion vessel, not air
- The water coming out is black and sludgy, which means the system needs a proper flush
- You cannot identify the system or find the filling loop, or the boiler is locked out on a fault
Safety first
- Turn the heating off and let the system cool first. Water in a radiator is hot enough to scald and it comes out under pressure.
- A quarter turn on the bleed valve, no more. The plug can come out entirely, and a radiator will empty into the room.
- Nothing on this page involves the gas side of the boiler, the burner, the flue or the inside of the case. That work is restricted by law to a registered installer in both markets, and there is no small exception.
- If you smell gas, leave the gas alone, ventilate, and call the emergency line — not an engineer, and not later.
- Keep the bowl and cloth under the valve, and keep children out of the room: the water is hot, dirty and stains.
Stop here: anything on the gas side of the boiler
Bleeding a radiator and bringing the pressure back up are yours. The boiler case, the burner and the flue are not: that work is restricted by law to a registered installer and there is no householder allowance in it, however small the job looks. How to hire a radiator fitter →
| MATERIAL | COVERAGE | FOR THIS JOB |
|---|---|---|
| Nothing consumable | This job uses no materials | The tools are a few euro and last for ever |
| Inhibitor | Only if the system has been drained or topped up repeatedly | Adding it properly is an engineer's job on most systems |
How long it takes
Fittings you need: 76 items
about 76 minutes — a bleed key, and nothing else
Expect to re-check the pressure once the system has run and cooled again — one pass is rarely the end of it.
A sealed system has a pressure gauge on the boiler and a filling loop — a braided silver hose with a valve at each end. An open-vented system has a small header tank in the attic and no gauge. Bleeding is the same; re-pressurising applies only to the sealed kind, because the tank does it automatically on the other.
Use the figure in YOUR boiler's manual. Most domestic sealed systems sit around 1 to 1.5 bar when cold, and the gauge itself usually marks its normal band — often a green segment or two needles. Read the gauge cold, not after an hour of heating, because the pressure rises as the water warms.
Most radiators take a standard square bleed key, which costs almost nothing. Some modern ones take a small flat screwdriver or have a key integrated into the valve. Check before you start rather than while water is coming out.
Start downstairs and work up, and within each floor start furthest from the boiler. Air collects at the highest points, so working bottom-up and outward-in pushes it where you want it rather than around in circles.
Clear or slightly cloudy water is normal. Black, gritty water is magnetite sludge, which means the system wants a chemical clean or a power flush and an inhibitor — that is a job for a heating engineer, and bleeding will not fix the cold radiators it causes.
THE PART THAT DECIDES THE FINISH
Which case you are in
Four of these are householder work and two are not. Identify yours before you pick up a key.
Cold at the top, warm at the bottom
Trapped air. The textbook case, and the one this guide is for.
- Turn the heating OFF and let the system cool before bleeding — bleeding hot is how people get scalded
- Start downstairs, furthest from the boiler, and work towards it and then upwards
- Open the valve a quarter turn only, until air hisses; close it as soon as water runs steadily
- Check the pressure afterwards and put it back to the figure in the manual
Cold at the bottom, warm at the top
Not air — sludge.
- This is magnetite settling in the bottom of the radiator, and bleeding does nothing for it
- A single radiator can sometimes be removed and flushed, but that means draining down
- A whole house of it means the system wants a chemical clean or a power flush
- Get a heating engineer; the inhibitor needs replacing afterwards and that is part of the job
Whole radiator cold, others fine
Probably a valve rather than air.
- Check both valves are open — a thermostatic head can seize with the pin stuck down
- Take the head off and press the pin; if it is stuck, freeing or replacing the head is a small job
- If both valves are open and the pin moves, it may be a balancing problem across the system
- Do not start closing other radiators' valves to force flow; balancing is methodical and easy to make worse
A sealed system with a pressure gauge
Re-pressurising is yours to do.
- Find the filling loop: a braided silver hose under the boiler with a valve at each end
- Read the cold pressure before you bleed, so you know the target
- Open both loop valves slowly, watch the gauge, and close them as soon as you reach the figure
- Close BOTH valves and, on many systems, disconnect the loop afterwards — the manual says which
An open-vented system with an attic tank
No re-pressurising needed.
- There is no gauge and no filling loop; the header tank keeps the system topped up
- Bleed exactly the same way
- If the tank is empty or its float valve has stuck, that is the fault to look at
- Do not add pressure to an open-vented system; there is nothing to pressurise
Anything involving the boiler itself
Stop — this is the line.
- The gas side, the burner, the flue and the inside of the case are restricted to a registered installer by law
- A fault code, a lockout, a smell of gas or a leak from the boiler is not a bleed job
- Repeated pressure loss usually means a leak or a failed expansion vessel, both of which need an engineer
- There is no small version of this exception; the restriction has no householder allowance
THE JOB
How to bleed a radiator and re-pressurise
Turn the heating off and let it cool
20–30 minSwitch the heating off at the thermostat or programmer and let the system go cold. Bleeding a hot system sprays hot water, and the pressure reading you need is the cold one.
- Cold means genuinely cold, not merely off — give it half an hour
- Note the cold pressure on the gauge now, before you change anything
- Find the boiler manual while you wait; the figure in it is the one that matters
Mistake to avoid: Bleeding a hot system. The water that comes out is at radiator temperature and it comes out faster than you expect.
Find the pressure gauge and note the reading
5 minThe gauge is on or under the boiler, sometimes behind a flap. Note the cold reading. Most domestic systems sit around 1 to 1.5 bar cold, and the gauge usually marks its own normal band — but the manual's figure wins.
- Use a torch; these gauges are always in the dark
- Photograph the gauge, so you can compare it later without remembering
- If the reading is already near zero, do not bleed — go to the stop list
Mistake to avoid: Reading the gauge after the heating has been on. Warm water reads higher, and you will top up to the wrong figure.
Work out the order
5 minDownstairs first, starting with the radiator furthest from the boiler, then work towards it, then upstairs the same way. Air collects high, so this sequence drives it ahead of you.
- Furthest first on each floor, then inwards
- Count your radiators so you do not lose track halfway
- Towels down before you start, not when water appears
Mistake to avoid: Starting with the upstairs radiator that is obviously cold. You bleed it, then bleeding downstairs pushes more air back up to it.
Open the bleed valve a quarter turn
2 min per radiatorTowel underneath, bowl in one hand, cloth around the key. Turn the valve anticlockwise about a quarter turn — no more. You will hear air hiss.
- A quarter turn is enough; a full turn can take the plug out
- Hold the cloth around the key so the first spit of water goes into it
- Keep the bowl under the valve, not on the floor
Mistake to avoid: Opening the valve too far. The plug can come right out, and then you are holding back a system's worth of water with a thumb.
Close it as soon as water runs steadily
1 min per radiatorThe hiss stops and water runs rather than spits. Close the valve just firmly enough to seal — these are brass and over-tightening deforms the seat.
- Steady water, not a dribble; a dribble means more air behind it
- Snug, not tight. A weeping valve afterwards is usually an over-tightened one
- Wipe the radiator and the wall; the water stains both
Mistake to avoid: Over-tightening the valve to be safe. It damages the seat and then it weeps permanently.
Repeat across the house
30 minWork through the order. Keep an eye on the pressure gauge as you go — each bleed drops it, and on a small system several bleeds can take it below the minimum.
- Check the gauge every few radiators rather than only at the end
- If the pressure hits zero mid-way, stop and re-pressurise before continuing
- Note which radiators had air; a pattern tells you something
Mistake to avoid: Bleeding the whole house without watching the gauge. Running a sealed system at zero pressure can lock the boiler out.
Re-pressurise the system
10 minOn a sealed system: open both filling-loop valves slowly and watch the gauge rise to the manual's figure. Close both valves. On many systems the loop is then disconnected — the manual says whether yours is.
- Slowly. The gauge moves faster than you expect and overshooting means draining some back out
- Close BOTH valves, and check neither is weeping afterwards
- An open-vented system needs none of this — the attic tank does it
Mistake to avoid: Leaving the filling loop open. The system then sits connected to the mains, which it is not designed to do, and the pressure climbs as it heats.
Run the heating and re-check
20 minTurn the heating on, let everything warm up, and feel each radiator top and bottom. Then let it cool and check the cold pressure again.
- The pressure reads higher hot — that is expected; the cold figure is the one to judge
- Any radiator still cold at the top has more air; bleed it again
- If a radiator is cold at the BOTTOM, that is sludge and a different job
Mistake to avoid: Judging the pressure while the system is hot, then 'correcting' it. You end up with a system that is under-pressured cold.
Check again in a week
5 minLook at the cold pressure a week later. A system that has lost pressure again has a leak or a failed expansion vessel, and both need an engineer.
- Photograph the gauge each time so you are comparing readings, not memories
- Look under the boiler and around each radiator valve for damp patches
- Topping up more than once or twice a year is a fault, not maintenance
Mistake to avoid: Topping it up repeatedly and treating that as normal. Each top-up adds fresh oxygenated water, which corrodes the system from inside.
The mistakes that matter
- Bleeding a hot system — the water comes out at radiator temperature
- Opening the valve more than a quarter turn, so the plug comes out
- Leaving the filling loop open after re-pressurising
- Treating repeated top-ups as maintenance rather than as the symptom of a leak
Stop here and get a registered installer
- Anything on the gas side, inside the boiler case, the burner or the flue — restricted by law to a registered installer, with no householder allowance
- The pressure drops again within days, which means a leak or a failed expansion vessel rather than air
- The water is black and gritty, or radiators are cold at the BOTTOM — the system needs cleaning, not bleeding
- The boiler is locked out, showing a fault code, or you cannot find the filling loop or identify the system
| WHAT YOU SEE | WHY, AND THE FIX |
|---|---|
| The radiator is still cold at the top after bleeding | There is more air, or air is being drawn back in. Bleed it again after the system has run; if it keeps filling with air, the system is drawing it in somewhere and needs an engineer. |
| The radiator is cold at the BOTTOM | Sludge, not air. Bleeding will not touch it; the system needs a chemical clean or a power flush, and an inhibitor afterwards. |
| The pressure keeps dropping | A leak or a failed expansion vessel. Look for damp around valves and under the boiler, then get an engineer — repeated topping up corrodes the system. |
| The bleed valve weeps now | It was over-tightened and the seat is damaged, or the plug is cross-threaded. A new bleed plug is cheap; fitting it means draining that radiator down. |
| The boiler has locked out with a fault code | Stop. Note the code and look it up, but a locked-out boiler is an engineer's job and nothing on it is householder work. |
| Water came out black | Magnetite sludge. Bleed as little as you can, and get the system cleaned — the black water is why radiators are cold and why pumps fail. |
| The pressure went too high after filling | The loop was left open, or it was overshot. It needs bleeding back down carefully to the manual's figure — and the loop closed and checked. |
| Nothing comes out at all when I open the valve | The valve is blocked or the radiator is isolated. Check both valves are open; do not force the bleed plug. |
WHAT YOU SAVE
What it costs to do it yourself
| Radiator bleed key | €2–6 |
|---|---|
| Cloth, bowl and an old towel | €0–8 |
| Torch, if you do not have one | €8–18 |
| Total | €10–32 |
| DIY total | €10–32 |
Guide prices compiled by the Locufy team from published Irish and UK provider price lists and 2026/27 cost guides. Updated 2026.
This is the cheapest job on the site to do yourself, and the one where knowing where to stop is worth the most.
RATHER NOT?
Radiator fitters near you, with prices shown
The ones who cover your area, nearest first, with one score from every review. No account needed to look.
Or post the job free and let them come to you.
On a label inside the boiler cupboard. It turns 'is this normal?' into a comparison.
Once the air is out, uneven heat is a balancing job — methodical, and worth doing before you blame a radiator.
A slow weep is cheaper to fix than the floor under it.
Air accumulates over a summer standing idle. It takes ten minutes in October.
Bleeding radiators and re-pressurising: questions people ask
It is cold at the TOP and warm at the bottom. That is trapped air. Cold at the bottom and warm at the top is sludge, which bleeding will not fix, and a whole cold radiator with both valves open is usually a valve or a balancing problem.
The figure in your boiler's manual. Most domestic sealed systems sit around 1 to 1.5 bar when cold, and the gauge usually marks its own normal band — but read it cold, because the pressure rises as the water heats.
Downstairs first, starting furthest from the boiler and working towards it, then upstairs the same way. Air collects high, so that order pushes it ahead of you instead of around the system.
Air lost through bleeding accounts for one drop, not a repeated one. Repeated loss means a leak or a failed expansion vessel, and it needs an engineer — every top-up adds oxygenated water that corrodes the system.
No. The gas side, the burner, the flue and the inside of the case are restricted by law to a registered installer, and there is no exception for a small job. Bleeding radiators and using the filling loop are the householder parts.
No. Black, gritty water is magnetite sludge. It is why radiators go cold at the bottom and why pumps fail, and it needs a chemical clean or a power flush with a fresh inhibitor afterwards.
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