Summary
Bleeding radiators is the single most common heating-related fix homeowners try themselves — and the single most common thing they do wrong. The procedure looks simple: open a valve, let air out, close the valve. But every part of it has tripwires. Bleed a hot radiator and you can scald yourself. Bleed with the pump running and you'll pull air in, not push it out. Bleed the wrong radiator first on a vented system and you'll suck air down through the feed pipe. Forget to re-pressurise a sealed system and the boiler will lock out on low pressure within hours.
For a tradesperson, bleeding is rarely a stand-alone job — it's the last step after a power flush, a repair, or commissioning a new radiator. But it's also the thing a homeowner has often "had a go at" before you arrive, and you'll need to understand what they did wrong before you can diagnose the persistent cold-spots, the kettling, or the locked-out boiler in front of you.
This article covers the correct procedure for both sealed (combi/system) and vented (gravity-fed) heating systems, the order to do radiators in, what the bleed water should look and smell like, and what it means when it doesn't behave as expected.
Key Facts
- System pressure (sealed) — cold pressure 1.0–1.5 bar; hot pressure should not exceed 2.5 bar. Below 0.5 bar the boiler will lock out
- Bleed key size — standard square is 5mm across flats (also called "1/4" square); modern designer radiators sometimes need a flat-blade screwdriver or 8mm hex
- Order on sealed systems — upstairs first, working downstairs and away from the boiler
- Order on vented systems — downstairs first, working up and away from the boiler (vented systems use the upper feed-and-expansion (F&E) tank to displace air upwards)
- Boiler/pump must be OFF during bleeding — running the pump while a valve is open will suck air INTO the system
- System must be COOL — bleed water can be 60–80°C if recently run. Wait at least 30 minutes after switch-off
- What the water should look like — clear, slight brown tint acceptable. Black, sludgy, or smelly water means inhibitor has failed
- What the water should smell like — slight metallic. Rotten-egg (hydrogen sulphide) smell means microbial corrosion / hydrogen generation
- Re-pressurise sealed systems to manufacturer's spec (usually 1.0–1.5 bar cold) via the filling loop
- Top up vented systems via the F&E tank — the ball valve should refill automatically; if not, check the float
- Recurring air — air returning within weeks means a leak, a failing auto air vent, or hydrogen generation from corrosion. Not a bleeding problem — a system problem
- Building Regulations Part L1B — applies to existing dwellings; balancing and commissioning is implicit in efficient heating performance
Quick Reference Table
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Try squote free →| System Type | Bleed Order | Pump State | Refill Method | Target Pressure |
|---|---|---|---|---|
| Combi (sealed) | Upstairs first, work down/away from boiler | OFF | Filling loop | 1.0–1.5 bar cold |
| System boiler (sealed) | Upstairs first, work down/away from boiler | OFF | Filling loop | 1.0–1.5 bar cold |
| Conventional / open-vented | Downstairs first, work up/away from boiler | OFF | F&E tank auto-refill | F&E tank float-set |
| Underfloor (manifold-fed) | Manifold bleed valves only — never radiator-style on UFH circuits | OFF | System filling loop | 1.5–2.0 bar cold (typical) |
| Designer / vertical column | Top-vent screws — may need flat blade or 8mm hex | OFF | As host system | As host system |
Detailed Guidance
Step-by-step: bleeding a single radiator
Turn the heating off. On a combi, switch the boiler to "off" or "summer mode". On a system or conventional boiler, switch the programmer off. Leave the system to cool for at least 30 minutes — ideally a full hour. Hot water under pressure will scald.
Have a small container and a cloth ready. A 250ml takeaway pot works. Position it directly under the bleed valve. Old radiators can squirt rather than dribble.
Locate the bleed valve. It's the small square-headed brass nipple at the top of one end of the radiator (occasionally hidden under a plastic cap on modern designer panels).
Insert the bleed key. Standard square key is 5mm. Newer designer rads may use a flat slot or 8mm allen — check before you start.
Turn the key anti-clockwise about a quarter turn. You should hear a hiss as air escapes. Do not open further than half a turn — you need control to close it quickly when water arrives.
Hold the cloth around the valve as water starts to come through. Once a steady, air-free stream is flowing, close the valve clockwise — finger-tight is usually enough. Do not over-tighten or you'll round the brass square.
Wipe the radiator and floor. Even a few drops of system water will mark carpet over time (inhibitor stains).
Move to the next radiator in the correct order.
Why the order matters
On a sealed system (combi or system boiler), the boiler is usually at low level (kitchen or utility). Air rises to the highest point. Bleeding upstairs first means you remove the bulk of the air from the high points before working back toward the boiler. Working in the wrong direction can push air ahead of you and trap it under a radiator.
On a vented system with an F&E tank in the loft, the feed point is high. Air can be displaced UP through the feed pipe into the tank. Working downstairs first lets you push the air upward through the system. If you bleed upstairs first on a vented system, you can pull air DOWN through the feed pipe and into the rest of the system, making it worse.
Bleeding after a major job
After a power flush, full system drain, or radiator change, you'll often need to bleed every radiator twice — once after initial refill, then again 24–48 hours later as remaining dissolved air comes out of solution. Always re-pressurise to the cold-target after bleeding a sealed system, then run the heating up to temperature and check pressure stays within the manufacturer's hot-cold band (typically a 0.5–1.0 bar rise is normal).
What to do if no water comes out
If you open the valve and get neither air nor water:
- On a sealed system, the system pressure is too low (already below 0.5 bar). Re-pressurise from the filling loop first.
- On a vented system, the F&E tank may be empty or the ball valve has stuck. Check the loft tank.
- The bleed valve itself may be blocked with sludge. Close it, do not force.
What to do if water keeps coming and won't stop
Don't panic. Close the valve firmly. If it's been over-loosened and the brass square is stripped, you'll need to drain that radiator from the lockshield to replace the valve. Common on cheap radiator valves — always use a quality bleed key, never pliers.
Diagnosing what air means
Air found in radiator?
├── First bleed after install/drain → Normal. Bleed, top up, re-check 24h later
├── Returns within days → LEAK somewhere in system (check joints, towel rails, valves)
├── Returns within weeks → Failing AAV, or hydrogen generation (corrosion)
├── Smelly water (rotten egg) → Microbial / hydrogen — needs inhibitor test and possible power flush
└── Black/sludgy water → Inhibitor depleted, magnetite present — needs power flush + new inhibitor
Frequently Asked Questions
Do I need to turn the water off to bleed a radiator?
No. The mains water supply is unrelated. You need the heating system (the boiler/pump) off, not the cold mains. The radiator is part of a closed heating circuit, not the mains water plumbing.
How much water comes out when bleeding?
On a sealed system that's been bled before, expect a tablespoon to half a cup per radiator. After a refill or repair, expect significantly more. The volume itself isn't the issue — the indicator is going from hissing air, to spluttering air-and-water, to clean steady water.
Why does my pressure drop every time I bleed?
This is normal on a sealed system — you've removed water (along with air) from the closed loop. Re-pressurise via the filling loop after bleeding. If pressure drops without bleeding, you have a leak (check the boiler PRV outside, radiator joints, and the boiler heat exchanger).
Should I bleed radiators every year?
Only if there's a symptom — cold tops, noisy operation, or the boiler is short-cycling. Routine bleeding "just in case" is unnecessary and can pull air into a healthy sealed system if the pressure has been allowed to drift low. A well-inhibited sealed system can run for years without needing a bleed.
My radiator is cold at the bottom, not the top — do I bleed it?
No. Cold at the bottom is sludge (magnetite settling). Bleeding will not help. See radiator cold at top or bottom for diagnosis — bottom-cold radiators need a power flush or magnetic filter intervention, not bleeding.
Regulations & Standards
Building Regulations Part L1B — Conservation of fuel and power in existing dwellings. Implicit duty to commission heating systems for efficient operation, which includes bleeding/balancing.
BS 7593:2019+A1:2022 — Code of practice for preparation, commissioning and maintenance of domestic central heating and cooling water systems. Specifies water quality, inhibitor dosing, and commissioning procedure.
BS EN 12828:2012+A1:2014 — Heating systems in buildings. Design for water-based heating systems. Includes provisions for air separation and venting.
CIPHE / Heating Industry guidance — System commissioning checklist includes radiator balancing and air removal as final steps.
Gas Safe Register technical bulletins — Pressurisation and re-fill procedures for sealed systems (Gas Safe registered engineer needed for any work on the boiler/gas side; bleeding radiators alone is not gas work).
Energy Saving Trust: bleeding radiators — homeowner guidance
Gas Safe Register technical resources — Gas Safe
HHIC commissioning guidance — Heating and Hotwater Industry Council
balancing radiators step by step — after bleeding, balance the system for even heat
radiator cold at top or bottom — diagnose what cold-spots mean
magnetic filters and system protection — preventing the corrosion that causes recurrent air
boiler pressure too high — what to do when bleeding has dropped your pressure