How to bleed a radiator
A radiator that’s warm at the bottom but cold at the top has air trapped in it. Bleeding it takes a few minutes and a radiator key, and it’s the first thing to try before calling anyone out.
Guide · The Honest Fix
One of the easiest jobs in the house. Just remember the boiler pressure afterwards.
Tools and materials
- Radiator bleed key (a few pounds from any DIY shop), or a flat screwdriver for some newer valves
- An old cloth or kitchen roll
- A small tray or container
- Gloves, as the water can be dirty
What bleeding does and when you need it
Air builds up in a heating system over time and collects at the top of radiators. It stops hot water filling the radiator, so the top stays cold. You may also hear gurgling or bubbling.
Bleeding lets the trapped air out through a small valve at the top corner of the radiator. It’s worth doing at the start of the heating season, or whenever a radiator isn’t heating properly at the top.
Is it air? Check where it’s cold
- Cold at the top, warm at the bottom: trapped air. Bleed it.
- Cold at the bottom or in the middle, warm at the top: usually sludge (corrosion debris) collecting inside. Bleeding won’t fix it.
- Completely cold: check the valves at both ends are open and the thermostatic valve isn’t stuck. If it’s still cold, it may be a balancing or valve problem.
Step by step
- Turn the heating on and let it run for a while, then feel each radiator (carefully) to find which are cold at the top.
- Turn the heating off and let the radiators cool. Bleeding a hot radiator can spray scalding water, and with the pump running it’s harder to get all the air out.
- Find the bleed valve. It’s a small square or slotted fitting at one top corner of the radiator.
- Hold a cloth under the valve, fit the key and turn it anticlockwise about a quarter to half a turn. Don’t open it further. The screw can come out.
- Let the air hiss out. When water starts to come out in a steady stream, rather than spitting, close the valve clockwise. Firm, not forced.
- Repeat for the other cold radiators. On a two-storey house, start with the ground floor and then do upstairs.
- Check the boiler pressure. On a combi or system boiler, bleeding releases water and drops the pressure. Top it back up to the normal level. Our boiler pressure guide explains how.
- Turn the heating back on and check the radiators heat fully once they’re up to temperature.
Safety
- Let radiators cool first. The water inside can be hot enough to scald.
- Have a cloth ready. Heating water is often black and stains carpets and walls.
- In older or poorly maintained systems, the “air” can be hydrogen given off by corrosion inside the radiators. It’s flammable, so ventilate the room and keep naked flames away while bleeding.
- Keep water away from sockets and electrical fittings near the radiator.
- Never open the boiler casing to adjust pressure. Only a Gas Safe registered engineer should work inside a gas boiler.
Stop here: call a pro if…
- The bleed valve is seized, snaps, or won’t close properly.
- Water keeps leaking from the valve, or from the valves at the bottom of the radiator.
- The same radiators fill with air again within a few weeks.
- Radiators are cold at the bottom or middle, or the water that comes out is black or sludgy.
- You need to top up the boiler pressure regularly after bleeding.
- A radiator stays completely cold even after bleeding and checking the valves.
DIY or Hire?
DIY. There’s no good reason to pay someone to bleed radiators. It costs the price of a key and takes a few minutes.
Get a heating engineer or plumber in when bleeding doesn’t fix it. Radiators that keep needing bleeding, or that are cold at the bottom, point to sludge, a pump problem or air being drawn in somewhere. Those usually need a system clean, a magnetic filter, inhibitor or a closer look. Treating the cause is far cheaper than replacing radiators or a boiler heat exchanger later.
Common mistakes
- Bleeding radiators with the heating on.
- Forgetting to top up the boiler pressure afterwards, so the boiler locks out.
- Opening the bleed screw too far and losing it.
- Not having a cloth ready and staining the floor.
- Bleeding a radiator that’s cold at the bottom, when the problem is sludge.
- Overtightening the valve and damaging it.
FAQs
How often should I bleed my radiators?
Usually only when one isn’t heating properly. Checking once a year before winter is plenty. If you need to bleed more often than that, get the cause looked at.
Which radiator should I bleed first?
Start on the ground floor and work up, as air rises through the system. Bleed any radiator that’s cold at the top.
Do I need to top up the pressure if I have a tank in the loft?
No. Open-vented systems refill automatically from the feed and expansion tank. Topping up only applies to sealed systems with a pressure gauge.