There are many reasons why one or more central heating radiators in a house are too cold or too hot, use the information here to identify the areas for investigation.

Always check the simple reasons first before looking for expert assistance, often the cause is simply due to radiator valves being turned off or incorrectly set.

The information here makes the fundamental assumption that the heating system does not work properly.

Generally downstairs radiators will be cooler as the rooms are insulated by any heated rooms above It is worth noting that radiators are likely to be slightly hotter at the top than the bottom, how much will vary from system to system, room to room, the design of the radiator and the airflow around it.

Firstly establish whether the problem affects one radiator or several to help narrow down the choices.

We have tried to put the faults in each section in order of how likely they are to be the cause.

One Radiator Is Cold

  • Thermostatic radiator valve ( TRV ) turned off - turn to open
  • Thermostatic radiator valve ( TRV ) cut off because room temperature higher than setting
  • Seized thermostatic radiator valve ( TRV ) caused by scale build up - tapping the control pin may fix it
  • The thermostat in the thermostatic radiator ( TRV ) valve head assembly has failed
  • Flow side into the radiator is hot but the lock shield side out is cold- check the lock shield valve is not turned off
  • Airlock in pipe work - often where the radiator is below the level of feeding pipes
  • Blocked thermostatic valve - caused by scale and sludge in the system- flushing system may fix
  • Blocked flow (lock shield) valve - caused by scale and sludge in the system- flushing system may fix
  • Blocked pipe feeding the radiator - caused by scale and sludge in the system- flushing system may fix
  • Blocked radiator - caused by scale and sludge in the system

One Radiator Is Too Hot

  • Thermostatic valve ( TRV ) turned fully on - turn to close
  • The thermostat valve (TRV) head assembly is loose on the thermostatic valve body
  • Seized thermostatic valve ( TRV) - caused by scale build up - tapping the control pin may fix it
  • The thermostat in the thermostatic valve (TRV) head assembly has failed

One Radiator Doesn't Get Hot Enough

  • Thermostatic valve cut off because room temperature higher than setting - turn to increase temperature
  • Airflow around the thermostatic radiator valve (TRV) is blocked by hanging clothing, furniture or a radiator cover
  • Partially blocked thermostatic radiator valve  (TRV) - caused by scale and sludge in the system
  • Partially seized thermostatic radiator valve (TRV) - caused by scale build up - tapping the control pin may fix it.
  • Partially blocked flow (lock shield) valve - caused by scale and sludge in the system- flushing system may fix
  • Partially blocked pipe feeding the radiator - caused by scale and sludge in the system- flushing system may fix
  • Partially blocked radiator - caused by scale and sludge in the system- flushing system may fix

One Radiator Is Cold At the Top

  • Air in the radiator - bleed the radiator
  • Low water level in the radiator - usually highest radiator in the house - caused by low water level in system

One Radiator Is Cold At The Bottom

  • Build up of sludge in the bottom of the radiator - usually in radiators fed from above - flushing the system may fix

One Radiator Only Is Hot

  • Central heating off, hot water only setting on the boiler is on
  • All thermostatic valves on the other radiators turned off
  • Circulation pump failed

All Radiators Are Cold

  • Controller/Time switch not switching on at the correct time
  • Room thermostat set too low
  • Boiler not working
  • Circulation pump failed
  • Controller/Time switch has failed

Several Radiators are Cold

  • Controller/Time switch not switching zone on at the correct time
  • Zone control valve switched off at the controller
  • Zone control valve failed
  • Airlock in pipe work - often where radiators are below the level of feeding pipes
  • Controller/Time switch has failed

Downstairs Radiators Are Cold, Upstairs Radiators Are Hot

  • Controller/Time switch not switching zone on at the correct time
  • Zone control valve switched off at the controller
  • Zone control thermostat set too low
  • Pump switched off
  • Zone control valve failed
  • Pump failed
  • Controller/Time switch has failed

Downstairs Radiators Are Hot, Upstairs Radiators Are Cold

  • Controller/Time switch not switching zone on at the correct time
  • Zone control valve switched off at the controller
  • Zone control thermostat set too low
  • Zone control valve failed
  • Low water level in the system - could be a stuck cistern valve
  • Controller/Time switch has failed

Radiators Furthest From The Boiler Are Cold

  • Pump speed set too low
  • System needs setting up and balancing
  • Pump has partially failed and running slow

All Radiators Don't Get Hot Enough

  • Room thermostat set too low
  • Boiler thermostat set too low
  • System pressure too low ( sealed systems only )
  • Pump switched off
  • Pump failed
  • Boiler thermostat has failed
  • Build up of sludge in the system - flushing system may fix

All Radiators Are Cold

  • Pump switched off
  • Pump failed

Some Radiators Are Warm When Water Heating Only Is In Use

  • A design fault rather than a fault as such. The pump isn't running but some radiators get warm - only option is to turn off radiators
  • Failure of a zone control valve
  • Failure of the main heating control valve

Noises From One Or Several Radiators

  • Air in the radiator or adjacent pipes - usually easy to solve by bleeding the radiator
  • Thermostatic valve(s) fitted wrong way round - usually happens when the system stabilises - opening one valve fixes temporarily
  • Boiler thermostat set too high - try turning it down (not the room thermostat)
  • Low water level caused by lack of water in the system - could be a stuck cistern valve
  • Low water pressure in closed systems
  • Scale and sludge in the radiators and pipes- flushing system may fix
  • Failed pump bearing sometimes manifests itself as noise from the radiators

One Or More Radiators Need Bleeding Regularly

  • Low water level in the system - could be a stuck cistern valve
  • Bleed process incomplete or large amount of air trapped in the pipes
  • Air leak in system
  • Hydrogen or other gases in the system (caused by corrosion due to inhibitor not working or not present)
  • Hydrogen sulphide in system (bad eggs smell) caused by bacteriological contamination - system needs decontamination
  • Pump speed set to high causing cavitation which releases air in the water