Radiator Not Heating? How to Bleed It and Fix It
Cold at the top, bottom or all over? What each pattern means, how to bleed a radiator safely, and when to call a heating engineer.
A cold radiator is one of the most common home heating problems. The pattern of hot and cold tells you a lot about the cause, and the first fix takes about five minutes.
What the cold patch tells you
| Symptom | Likely cause | What to do |
| Cold at the top, warm at the bottom | Trapped air | Bleed the radiator |
| Warm at the top, cold at the bottom | Sludge build-up | Flush the system, by a heating engineer |
| Cold all over, others are warm | Stuck valve or closed valve | Check both valves and the thermostatic valve pin |
| Upstairs radiators cold | Air, low pressure or pump issue | Bleed, check pressure, then call an engineer |
| All radiators cold | Boiler or system fault | Check pressure and error codes, then call a Gas Safe engineer |
How to bleed a radiator
You need a radiator key, an old cloth and a small container.
- Turn the heating off and let the radiators cool, so you do not get scalded
- Hold the cloth under the bleed valve at the top corner of the radiator
- Turn the key a quarter turn anticlockwise and listen for the hiss of air
- Close the valve as soon as water starts to appear
- Repeat for other radiators, starting downstairs and working up
- Check boiler pressure against the manufacturer's instructions; only repressurise if you understand the procedure for your exact system
- Turn the heating on and check the radiator warms evenly
Checking boiler pressure
Bleeding radiators can lower the pressure in sealed systems. Many combi boilers sit at around 1 to 1.5 bar when cold, but check your boiler manual for the correct range and how to top it up using the filling loop. If the pressure keeps dropping, there may be a leak, so call an engineer.
A stuck thermostatic valve
If one radiator stays cold, take off the thermostatic valve head and check the pin underneath. If it is stuck down, gently tap or wiggle it to free it. Also make sure the valve at the other end, the lockshield, is open.
When to call a professional
- All radiators are cold or the boiler shows a fault code
- The pressure keeps dropping
- Radiators are cold at the bottom, which suggests sludge
- You smell gas: leave the property, do not use switches, and call the National Gas Emergency number on 0800 111 999
Boiler and gas work must be done by a Gas Safe registered engineer. Never open or take apart a gas boiler yourself.
Frequently asked questions
How often should I bleed my radiators?
When you notice cold spots at the top or gurgling, and as a quick check before winter.
Do I bleed radiators with the heating on or off?
Off, and once the radiators have cooled. This is safer and lets air collect at the top.
Why does my radiator need bleeding again so soon?
Repeated air can point to a leak, a pump problem or corrosion in the system. A heating engineer should take a look.
Related guides
- Plumbing and gas work
- How to choose a reliable handyman
- How to fix a dripping tap
- How to unblock a sink, shower or toilet
We help with valve handles, making good after pipe repairs and decorating, and arrange qualified engineers for boiler and gas work. Call 07444 262522.