Every autumn, as the heating goes back on after months of sitting idle, we get a wave of calls about radiators that aren't heating up properly. In a good number of cases, the fix is straightforward: the radiator needs bleeding. It takes about five minutes, costs nothing, and is something any homeowner can do safely themselves.
This guide covers exactly how to do it, what to watch out for, and — importantly — when bleeding a radiator won't actually fix the problem.
Why Do Radiators Need Bleeding?
Over time, small amounts of air work their way into your central heating system. This air rises to the highest point in each radiator and gets trapped there, stopping hot water from fully circulating through the radiator. The result is a radiator that's warm at the bottom but cold across the top — sometimes just at one corner, sometimes across the entire upper half.
Air can enter the system in various ways: through the expansion vessel, from micro-leaks in the pipework, or simply through the water itself releasing dissolved gases as it heats up. It's a normal part of how central heating systems work, and bleeding radiators once a year — typically at the start of the heating season — is good maintenance practice.
What You'll Need
- A radiator bleed key (available from any DIY or hardware shop for a pound or two — your radiators may have come with one)
- An old cloth or small bowl to catch any drips
- A paper towel or tissue
How to Bleed a Radiator — Step by Step
Step 1 — Turn the heating on
Switch your heating on and let it run for 10 to 15 minutes so all the radiators are up to temperature. This makes it easier to identify which ones need bleeding — feel across the top of each one and note any that are cold at the top but warm at the bottom. Those are the ones to bleed.
Step 2 — Turn the heating off and let it cool slightly
Before you start bleeding, turn the heating off and wait about 10 minutes. You don't want the system to be running while you bleed, as the pump can draw more air in. You also don't want scalding water spraying out — letting it cool slightly reduces that risk.
Step 3 — Find the bleed valve
The bleed valve is the small square fitting at the top of the radiator, usually at one end. It has a square slot in the centre where the bleed key fits. Place your cloth or bowl underneath it to catch any water.
Step 4 — Open the valve slowly
Insert the bleed key and turn it slowly anti-clockwise — just a quarter to half a turn is usually enough. You should hear a hissing sound as the trapped air escapes. Hold your cloth ready.
Step 5 — Wait for water to appear
Keep the valve open until the hissing stops and a steady trickle of water appears. Once water flows consistently without any air spluttering out, the radiator is fully bled. Close the valve by turning the key clockwise — hand tight is sufficient, don't overtighten.
Step 6 — Check your boiler pressure
Bleeding radiators releases a small amount of water from the system, which can cause the boiler pressure to drop. After bleeding, check the pressure gauge on your boiler — it should read between 1 and 1.5 bar when cold. If it's dropped below 1 bar, you'll need to repressurise the system. See our guide on how to repressurise your boiler for step-by-step instructions.
Step 7 — Turn the heating back on and check
Switch the heating back on and let the system run for 15 to 20 minutes. The radiators you bled should now heat up evenly from top to bottom.
When Bleeding Won't Fix the Problem
Bleeding is the right fix for a radiator that's cold at the top due to trapped air. But there are other reasons a radiator might not heat properly, and bleeding won't help with any of them.
Cold at the bottom — sludge
If your radiator is warm at the top but cold or lukewarm at the bottom, trapped air isn't the issue. This pattern is almost always caused by a build-up of magnetite sludge — the black iron oxide that settles at the bottom of radiators in older systems. Bleeding will do nothing for this. The radiator usually needs flushing, and depending on how widespread the problem is, the system may need a full power flush and inhibitor treatment. We cover this in detail in our article on cold radiators and magnetite sludge.
Completely cold — valve or flow issue
A radiator that's completely cold when others in the house are working is usually a valve problem rather than an air problem. The thermostatic radiator valve (TRV) may be stuck — this is particularly common on radiators in rooms that stay naturally warm, like south-facing kitchens, where the TRV hasn't been turned on for months. Try turning the TRV to a higher setting, or removing the TRV head and checking the pin underneath isn't seized down. If that doesn't help, the lockshield valve at the other end may be closed, or the system may need rebalancing.
Multiple radiators affected — system issue
If several radiators across the house are cold or heating unevenly, it's unlikely that air alone is the cause. This points to a more systemic issue — sludge build-up affecting flow, a circulation pump that's losing efficiency, or an imbalanced system where some radiators are getting most of the flow and others very little. This needs a proper diagnosis rather than a DIY fix.
You keep needing to bleed the same radiator
If a particular radiator keeps filling with air and needs bleeding every few weeks, that's a sign air is actively getting into the system somewhere. Common causes include a leaking auto air vent, a micro-leak drawing air in, or a failing expansion vessel. It's worth getting it investigated — repeatedly bleeding without addressing the root cause isn't a long-term solution.
When to Call Us
If bleeding your radiators doesn't fix the problem, or if you're finding yourself doing it repeatedly, give us a call. We cover Stourbridge and the surrounding area for all radiator work — from diagnosing and flushing individual radiators to full system power flushes and magnetic filter installations. Call us on 07961 169 681 or message us on WhatsApp and we'll advise on the best course of action.