The short answer
Where a radiator is cold tells you the cause. Cold at the top and hot at the bottom means trapped air, so bleed it. Cold at the bottom and hot at the top means sludge, which needs a flush. Cold all over on one radiator usually means a stuck valve pin. Several cold radiators upstairs means poor balancing or a weak pump. Bleeding and freeing a valve pin cost nothing. Flushes and pump repairs cost £150 to £420.
What the law requires
- Match your radiator to the cause
- Cold at the top: how to bleed a radiator
- Cold at the bottom: sludge
- Whole radiator cold: the stuck valve pin
- Upstairs radiators cold: balancing and air
- Everything lukewarm or cold with the boiler running
- How much does it cost in London?
- When to call someone
- Radiators not heating, quick answers
The diagnosis is written on the radiator
Switch the heating on, wait twenty minutes, then feel each radiator with the back of your hand.
What you feel | Most likely cause | What to do |
Hot at the bottom, cold at the top | Trapped air | Bleed the radiator |
Hot at the top, cold at the bottom | Sludge settled in the base | Flush, or remove and clean |
Cold in the middle, hot at the edges | Sludge in the centre | Flush |
Whole radiator cold, pipes also cold | Stuck valve pin or closed valve | Free the pin, open the valve |
Whole radiator cold, pipes hot | Blocked radiator | Remove and flush |
Only upstairs radiators cold | Air or poor balancing or weak pump | Bleed, then balance |
Every radiator lukewarm | Boiler flow temperature, pump or system sludge | Engineer visit |
Every radiator cold, boiler running | No circulation: pump, diverter valve or airlock | Engineer visit |
Cold at the top: how to bleed a radiator
You need a radiator key (about £2) and a cloth.
Cold at the top: how to bleed a radiator
Find the small bleed valve at the top corner of the radiator.
Hold the cloth under it and turn the key anti-clockwise a quarter turn. You will hear a hiss.
When water starts to spit out, close the valve firmly.
Check the boiler pressure gauge. Bleeding drops the pressure, so top up to between 1 and 1.5 bar if needed.
Work from the radiator nearest the boiler to the farthest, then upstairs.
If a radiator needs bleeding every week, there is a source of air. Look for a failing auto air vent, a small leak letting air in, or corrosion producing gas inside the system.
Cold at the bottom: sludge
Black magnetite sludge settles at the bottom of radiators and restricts flow. Signs include cold bottoms, hot tops, a radiator that takes hours to warm, brown or black water when bled, and a boiler that kettles. Our guide on boiler banging noises explains the link.
The fix is a chemical flush or a power flush, followed by a magnetic filter and fresh inhibitor. Without the filter and inhibitor, the sludge returns.
Half the house cold? Describe which radiators and where they are cold, and we will usually name the fix on the phone.
Whole radiator cold: the stuck valve pin
Thermostatic radiator valves have a small pin in the valve body. In summer it can seize in the closed position. Signs: the radiator is stone cold, the valve is set to maximum and the pipes either side are cold or cold on one side.
- Turn the heating off and let it cool.
- Unscrew the plastic head of the valve by turning it anti-clockwise.
- Under it you will see a small metal pin. Tap it gently with a small spanner or press and release it with pliers a few times. It should spring back.
- Refit the head and turn the valve to maximum.
If the pin is stuck, or water weeps from the valve, a plumber should replace the valve. It is a short job.
Upstairs radiators cold: balancing and air
Radiators nearest the boiler hog the hot water. Balancing means adjusting the lockshield valves on each radiator so the flow is shared fairly. An engineer measures the temperature drop across each radiator and adjusts until all heat evenly. It takes one to two hours for an average house and costs £90 to £180.
Check air first. Bleed the upstairs radiators, and bleed the highest one last. If a feed and expansion tank sits in the loft, check it is not overflowing or empty.
Everything lukewarm or cold with the boiler running
When every radiator is cold or only lukewarm, the problem is the circulation, not the radiators.
- The pump is stuck or running at too low a speed. Signs: no hum, or a hum with no flow.
- The diverter valve is stuck on hot water, which sends everything to the cylinder or the combi plate heat exchanger.
- An airlock after work on the system.
- Boiler flow temperature set too low. Condensing boilers are often turned down to 50 or 55 degrees to save gas, which can leave radiators lukewarm in a cold spell. Try 65 to 70 degrees for a day.
- A heavily sludged system.
These are engineer jobs. The engineer will check pump operation, flow and return temperatures and the diverter valve.
How much does it cost in London?
Fixed price quoted after diagnosis.
- Bleeding and top-up: free to do yourself, or £90 to £130 as a call-out.
- TRV valve replacement: £80 to £150 per valve.
- System balancing: £90 to £180.
- Chemical flush and inhibitor: £150 to £260.
- Power flush: from £350.
- Magnetic filter fitted: £130 to £220.
- Pump replacement: £220 to £420.
- Diverter valve replacement: £180 to £320.
Do not buy a power flush until the cause is confirmed. If the pump has failed, a flush will not fix it.
When to call someone
Book an engineer if bleeding does not help, if every radiator is cold with the boiler firing, if the boiler shows a pressure or overheat fault, or if you find water leaking at a valve. The earlier, the better in winter.
Radiators not heating, quick answers
Why is my radiator cold at the top?
Why is my radiator cold at the top?
Why is my radiator cold at the bottom?
Sludge. It needs a flush.
Why is only one radiator cold?
Usually a stuck valve pin or a blocked radiator.
Why are my upstairs radiators cold?
Air, poor balancing or a weak pump.
Does a power flush always work?
Only if sludge is the cause. Get a diagnosis first.
How often should I bleed radiators?
Once a year before winter is normal. More often means there is a source of air.