DIY GUIDE · PLUMBING & HEATING · CHECKED OCTOBER 2026
How to bleed your radiators and re-pressurise the system yourself
About an hour for a house. £1–£4 for a bleed key, bought once.
Cold at the top means air. Let it out, put the pressure back, and leave the boiler's insides alone.
- Every step, start to finish
- Checked October 2026 — 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…
- The radiators are cold across the top and warm lower down, which is trapped air
- You can find the pressure gauge and the filling link, and you have the boiler's own instructions or can download them
- Nothing is dripping, the boiler shows no fault code and has not locked out
- The system is either sealed, with a gauge on the boiler, or open-vented, with a small tank in the loft
Get a pro if…
- The job touches the gas side, the burner, the flue or anything behind the boiler's front panel
- The pressure falls again within days of topping it up, which points to a leak or a tired expansion vessel
- The water that comes out is black, or the radiators are cold along the bottom, which is sludge
- There is no filling link you can find, or the hot water comes from an unvented cylinder you would need to touch
Safety first
- Switch the heating off and let the radiators go cold before you open a single valve. Hot system water comes out under pressure and scalds.
- Open a bleed valve a quarter turn and no further. Unscrew it too far and the little plug drops out, and the radiator empties onto the floor.
- Leave the gas side alone. Working on a gas fitting or appliance for pay needs a Gas Safe registered engineer — regulation 3 of the 1998 Gas Safety (Installation and Use) Regulations in Great Britain, and of the Northern Ireland Regulations of 2004. Nothing in this guide is gas work.
- If you smell gas, do not touch a switch. Open the doors and windows, turn the gas off at the meter if you know where the lever is, leave the building and ring the gas emergency line on 0800 111 999 from outside.
- Headaches, dizziness or sickness that ease when you leave the house, sooty marks around the boiler or a lazy yellow flame are warnings of carbon monoxide. Get out, get air and call the emergency line; it is not a bleeding problem.
- Keep children and pets out of the room while valves are open. The water is dirty, it stains, and the floor gets slippery.
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 | Bleeding uses no materials | Water from the mains replaces what you let out, through the filling link |
| Spare bleed plug | Only if a valve has started weeping | Fitting one means draining that radiator, so it is a job for when the heating is off for the season |
How long it takes
Total time: 70 minutes
about 70 minutes for eight radiators
Read the cold pressure again the next morning; that reading is the one that counts.
Look at the boiler. A pressure gauge on its front or underneath, and a braided hose or a small lever below it, means a sealed system, which loses pressure every time you bleed. No gauge, and a small plastic tank in the loft fed by a ball valve, means an open-vented system, which tops itself up. Bleeding is identical on both. Re-pressurising only exists on the sealed kind.
One widely fitted boiler maker sets the working band at 1 to 1.5 bar, shown as a green zone on the gauge, and most combis are similar. Your own boiler's instructions are the authority, so download them by model number if the booklet has gone. Read the gauge when the system is cold. Warm water expands and the needle climbs, so a hot reading tells you nothing about whether you need to top up.
There are three common kinds. An external loop is a braided hose between two small valves under the boiler, often left disconnected with caps on. A keyed link is built into the underside of the boiler and turns with a plastic key kept behind a clip-off panel. A keyless link has a lever or a blue tab you pull. Find yours and read its page in the instructions before you bleed anything, because it is the tool that undoes the bleed.
Most radiators take the standard square bleed key, which costs pennies and fits nine in ten valves. Some newer valves have a slot for a flat screwdriver instead, and a few towel rails use a small hex. Try the key on each radiator before you start, while the system is cold and nothing is dripping.
Air rises to the highest points of the pipework and gets carried round by the pump. Bleed the ground floor first, starting with the radiator furthest from the boiler, then work back towards it, then go upstairs and repeat. Doing it the other way round chases air from one radiator to the next.
Clear, slightly grey or rusty-tinted water from a bleed valve is normal. Thick black water is iron oxide sludge, and it settles in the bottom of radiators, so the cold patch is low down, not at the top. Bleeding cannot clear it. A system like that wants a chemical clean or a power flush and fresh inhibitor, which is a heating engineer's job.
THE PART THAT DECIDES THE FINISH
Before you start — which case you are in
Six starting points. The first four are yours to deal with; the last two are where the householder's part ends. Find yours before you pick up the key.
Cold at the top, warm below
Trapped air. The case this guide is written for.
- Turn the heating off at the programmer or thermostat and give the system time to go fully cold
- Read and photograph the cold pressure on the gauge
- Put a folded towel under the bleed valve end of each radiator you will open
- Plan the route: ground floor, furthest from the boiler first, then upstairs
Cold along the bottom, warm at the top
Not air. Sludge has settled in the radiator.
- Do not bleed to fix it; letting water out only adds fresh oxygen-rich water when you top up
- Check whether one radiator or several are affected — one is a radiator problem, several is a system problem
- Note the colour of any water that does come out of the bleed valve
- Book a heating engineer for a clean or flush and a fresh dose of inhibitor
The whole radiator is cold, others are fine
Usually a valve, not air.
- Check the lockshield cap at one end and the thermostatic head at the other; both should be open
- Unscrew the thermostatic head and press the pin underneath; it should spring back up
- If the pin is stuck down, work it gently with a cloth-wrapped pair of grips until it moves freely
- If both valves are open and the pin moves, the system may need balancing — leave the lockshields alone until you have read the balancing note under Afterwards
Sealed system, pressure low
Re-pressurising is your job.
- Find the filling link and identify which of the three kinds it is
- Read the section of the boiler's instructions for that link
- Have the cold target figure written down before you open anything
- Make sure the bleed valves you have opened are all closed before you start filling
Open-vented system with a loft tank
No pressure to add.
- There is no gauge and no filling link; the small tank in the loft refills the system by gravity
- Bleed exactly as for a sealed system
- If radiators keep filling with air, look in the loft: the tank may be empty because the ball valve has stuck
- Do not connect anything to pressurise an open-vented system; it is not built for it
Anything involving the boiler itself or the cylinder
This is the line.
- The gas valve, burner, flue and the inside of the case are gas work and belong to a Gas Safe registered engineer
- A boiler showing a fault code or locked out is not a bleeding job; note the code and call an engineer
- An unvented hot water cylinder, the tall pressurised one with a cluster of valves and a pipe to a drain, is maintained only by someone qualified for unvented systems
- Replacing or adding a filling loop is a change to a water fitting connected to the mains, which is covered by the water fittings rules — not a householder adjustment
THE JOB
How to bleed the radiators and put the pressure back
Set up: heating off, system cold, gauge read
30 minTurn the heating and hot water off at the programmer and leave everything to cool. Go through the case cards above and decide which one is yours. While you wait, find the pressure gauge, shine a torch on it and photograph the reading, then find the filling link and the page for it in the boiler's instructions. Lay the towels, put the key, cloth and bowl in your pocket, and write the target figure on your hand if you have to.
- Feel the top of a radiator before you start; if it is warm, wait longer
- A cold reading inside the green zone means you probably only need to bleed, not fill
- If the needle is already near zero before you have bled anything, the system has lost water somewhere — note it, then read the stop list
Mistake to avoid: Starting while the system is still warm. You get hot water on your hand, and the gauge reading you note is a warm one, so you top up to the wrong figure later.
Prepare each radiator and plan the route
10 minThis is the prep from the first case card, done radiator by radiator. Find the bleed valve on each: a small square or slotted nub at one top corner, at the opposite end from the pipe with the thermostatic head on most radiators. Check the key fits it now. Put a folded towel on the floor under that corner. Then number the radiators in the order you will do them, ground floor first, furthest from the boiler first.
- Old valves are sometimes painted over; a gentle tap with the key handle usually cracks the paint free
- A towel rail's bleed point is often on the top cap, facing up
- Count the radiators so you know when you are halfway and can check the gauge
Mistake to avoid: Forcing a key that does not quite fit. You round the square on the valve, and then nothing will turn it short of a new bleed plug.
Open the first valve a quarter turn
2 minHold the cloth around the key and the bowl just under the valve. Fit the key and turn it anticlockwise slowly, a quarter of a turn. You will hear a hiss as air escapes. Keep the key in place and your hand steady. If nothing at all happens, give it another eighth of a turn, never more than half a turn in total.
- Turn slowly; the first movement often releases a spit of water before the air
- Keep your face away from the valve, especially on a radiator above head height in a hallway
- On a stiff valve, steady the radiator with your other hand so you do not twist it on its brackets
Mistake to avoid: Opening the valve a full turn or more to speed it up. The bleed plug can unscrew completely, and a radiator then drains through a hole you cannot easily block.
Close it as soon as water runs
1 minListen. The hiss changes to a gurgle, then a thin, steady stream of water replaces the air. That is the moment to close the valve, turning clockwise until it stops, then a little more to seal. These valves are small and made of brass. Snug is enough. Wipe the valve, the radiator and the wall straight away.
- A steady stream, not drips and spits; spitting means there is still air behind it
- If water dribbles from round the key after closing, nip it up a fraction more
- Dab the wall behind the valve; system water marks paint and wallpaper brown
Mistake to avoid: Cranking the valve shut as hard as you can. It squashes the seat, and an over-tightened valve is the one that weeps from then on.
Work round the house, watching the gauge
30 minRepeat for each radiator in your order. After every two or three, go back and look at the gauge. On a sealed system each bleed takes a little water out, and on a small system several radiators can take the pressure down into the red zone. If it gets near the bottom of the green zone before you have finished, stop, re-pressurise with the next step, and then carry on bleeding.
- Write a tick against each radiator so you do not bleed one twice and miss another
- Note which radiators had a lot of air; the same one every time is a clue for an engineer
- Leave the heating off until every valve is closed
Mistake to avoid: Bleeding every radiator in the house without once checking the gauge. A boiler that senses low pressure locks out, and you will then be resetting it as well.
Re-pressurise through the filling link
10 minWith every bleed valve closed and the system still cold, open the filling link exactly as your boiler's instructions show. On an external loop that means connecting the hose hand-tight and turning both valves until their slots or handles line up with the pipe. On a keyed link, push the key in and turn it. On a keyless link, pull the lever. Water flows in from the mains and the needle rises. Keep your eyes on the gauge the whole time and shut the link as soon as the needle sits inside the green zone, between 1 and 1.5 bar on the guide we checked.
- It can fill quickly; be ready to close the moment you reach the band
- Close both valves on an external loop, then take the hose off and put the caps back if that is how it was fitted
- If you overshoot, bleed a radiator a little until the needle drops back into the band
Mistake to avoid: Leaving the filling link open, or both ends of an external loop connected and open. The system stays tied to the mains, the pressure creeps up as it warms, and the safety valve starts discharging outside.
Run the heating and feel every radiator
30 minTurn the heating on and let it run until the radiators are fully hot. Then walk round and run your hand down each one from top to bottom. Hot all over is the result you want. A cool band along the top means more air has collected; mark it and bleed it again once the system has cooled. A cool band along the bottom is sludge and goes on the engineer's list.
- Expect the gauge to read higher now; that is the water expanding, not a fault
- The pump moves air around for a while, so a second bleed is normal on the worst radiators
- Listen for gurgling in the pipes; it usually fades once the last air is out
Mistake to avoid: Reading the gauge while the system is hot and letting water out to bring it down. When it cools you are then below the band and the boiler may lock out overnight.
Cool down and take the reading that counts
20 minTurn the heating off again and let it cool. Read the gauge cold and compare it with your photograph and the target. If it has dropped below the green band, top it up through the filling link exactly as before, a little at a time. Close the link, check there are no drips at the link or at any bleed valve, and wipe everything dry so a new drip will show.
- Lay a sheet of kitchen roll under each bleed valve you opened; a damp mark by morning gives away a weep
- Put the filling key back in its clip, or the caps back on the loop
- Write today's cold reading and the date on a label inside the boiler cupboard
Mistake to avoid: Skipping the cold re-check because the radiators feel hot. The reading after cooling is the one that tells you whether the system is sound.
Check again in a week
5 minA week later, with the heating off for a while, read the gauge cold again and compare it with the label. A steady reading means the job is done. A reading that has fallen means water is leaving the system somewhere, through a weeping valve, a pinhole under a floorboard or the boiler's own safety valve, and the boiler maker's guidance is to get a plumber or heating engineer to look for it rather than keep topping up.
- Look under the boiler and along exposed pipe runs for drips or green staining
- Look outside where a small copper pipe leaves the wall near the boiler; drips there mean the safety valve is discharging
- One top-up a year after the summer is normal; one a fortnight is a fault
Mistake to avoid: Topping up every few days and treating it as routine. Each top-up brings in fresh mains water and oxygen, which corrodes radiators from the inside and makes more sludge.
The mistakes that matter
- Bleeding while the system is hot, then topping up to a figure read on a warm gauge
- Opening the bleed valve too far, so the plug drops out
- Leaving the filling link open, or the external loop connected at both ends
- Treating a pressure that keeps falling as maintenance instead of the sign of a leak
- Bleeding radiators that are cold at the bottom, which is sludge and needs cleaning, not air
Stop here and get a heating engineer
- Anything on the gas side, the burner, the flue or behind the boiler's front panel — gas work for pay is restricted to Gas Safe registered engineers
- The pressure keeps dropping after you have topped it up, or water drips from the small pipe outside — a leak, a failing expansion vessel or a discharging safety valve
- The water is black and the radiators are cold along the bottom — the system needs cleaning and fresh inhibitor
- The boiler is locked out or showing a fault code, or you cannot find a filling link at all
- The hot water comes from an unvented cylinder and something on it needs attention, or the filling loop itself needs replacing
| WHAT YOU SEE | WHY, AND THE FIX |
|---|---|
| Radiator still cold at the top after bleeding | More air has arrived since you bled it, carried round by the pump. Let the system run, let it cool, and bleed that radiator again. If it fills with air week after week, air or gas is getting in somewhere and an engineer needs to look. |
| Radiator cold at the bottom, hot at the top | Sludge, not air. Bleeding will not shift it. The system needs a chemical clean or a power flush, and fresh inhibitor afterwards, from a heating engineer. |
| Boiler pressure keeps dropping | Water is leaving the system. Check every radiator valve, the pipes you can see and the small pipe outside by the boiler for drips. If you find nothing, a pinhole leak under the floor or a failing expansion vessel is likely, and both are an engineer's job. |
| Boiler pressure too high after filling | The link was left open or you overshot. Close the link fully, then let a little water out of a radiator bleed valve, a bowl at a time, until the cold reading is back in the green band. |
| Bleed valve dripping after bleeding | It was over-tightened, or a speck of grit is on the seat. Open it a fraction to flush the grit and close it snugly. If it still weeps, the plug needs replacing, which means draining that radiator, so plan it for when the heating is off. |
| Nothing comes out when I open the valve | The valve's outlet may be blocked with paint or debris, or the radiator is isolated. Check both valves at the bottom are open. Close the bleed valve and do not keep opening it further, or the plug will come out. |
| Water coming out of the bleed valve is black | Iron oxide sludge from inside the radiators. Bleed as little as you can and book a system clean; black water is what blocks radiators along the bottom and wears out pumps. |
| Boiler showing a fault code after bleeding | Usually low pressure. Read the cold gauge and top up through the filling link if it is low, then reset the boiler as its instructions show. If the code returns, stop and call an engineer — the inside of the boiler is not householder work. |
WHAT YOU SAVE
What it costs to do it yourself
| Radiator bleed key, single or a pair | £1–£4 |
|---|---|
| Towel, cloth and bowl | from the kitchen |
| Total | £1–£4 |
| DIY total | £1–£4 |
Guide prices compiled by the Locufy team from published Irish and UK provider price lists and 2026/27 cost guides. Updated 2026.
A radiator fitter's guide price covers fitting a radiator, not bleeding one; the comparison shows that this is the job where doing it yourself costs almost nothing, and where knowing when 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.
Write the cold reading and the date on a sticker inside the boiler cupboard. Next time the needle looks low, you are comparing two numbers instead of a memory.
Once the air is out, a house where the rooms near the boiler roast and the far ones lag is a balancing job. It means adjusting the lockshield valves methodically from the boiler outwards; read up on it or have it done when the boiler is next serviced.
Kitchen roll under each bleed valve and a glance at the pipes under the boiler. A drip found now is a small repair; a drip found after months is a ceiling.
Nothing to dispose of from a bleed except a few cupfuls of dirty water, which can go down an outside drain or the toilet. If you replace a bleed key, valve or plug, small metal items go in the metals skip at your council's household waste recycling centre; find yours through the council website or the recycling pages for England and Wales, Scotland and Northern Ireland.
Air builds up while the system stands idle through the summer. A bleed and a cold pressure check before the first cold week takes under an hour.
Bleeding radiators and re-pressurising: questions people ask
It is cold across the top and warm lower down, and you may hear gurgling in the pipes. That is trapped air. Cold along the bottom is sludge, and a radiator cold all over while others are hot is usually a stuck valve pin or a balancing problem.
Whatever your boiler's instructions say. One major boiler maker gives 1 to 1.5 bar, marked as a green band on the gauge, and many others are similar. Read it when the system is cold, because the needle rises as the water warms.
Ground floor first, starting with the radiator furthest from the boiler and working back towards it, then upstairs the same way. Check the gauge every few radiators and top up before carrying on if it falls low.
One drop after a full bleed is normal. A pressure that keeps falling means water is escaping, through a weeping valve, a hidden leak, the safety valve or a failing expansion vessel. Stop topping up and have an engineer find it.
No. Work on the gas side, the burner, the flue and the inside of the case is restricted to Gas Safe registered engineers under the gas safety regulations in Great Britain and Northern Ireland. Bleeding radiators and using the existing filling link are the householder's part.
Locufy's UK price guide for radiator installation runs at £150–£400 per radiator, typically £250 — the same three figures shown in the strip at the top of this page. Bleeding radiators yourself costs the price of a key.
MORE GUIDES
More DIY guides
Sources — checked October 2026
- Worcester Bosch — system filling guide: the 1 to 1.5 bar band and the three filling links
- HSE — domestic gas safety: frequently asked questions (gas leaks, the emergency number)
- HSE — carbon monoxide awareness: symptoms and warning signs
- Gas Safety (Installation and Use) Regulations 1998, regulation 3
- Gas Safety (Installation and Use) Regulations (Northern Ireland) 2004, regulation 3
- gov.uk — Approved Document G: sanitation, hot water safety and water efficiency (England)
- Water Supply (Water Fittings) Regulations 1999, regulation 5
- Water Supply (Water Fittings) (Scotland) Byelaws Extension Order 2024
- Water Supply (Water Fittings) Regulations (Northern Ireland) 2009
- gov.uk — recycling collections: find your council (England and Wales)
- mygov.scot — find your council's recycling centre (Scotland)
- nidirect — recycling and reusing (Northern Ireland)
- Screwfix — price check: radiator keys