How to deal with faulty radiators

If your radiators aren't giving out enough heat, check them all and make a note of which are cool and which are hot. Some may be cooler at the top or bottom.


The different options…

Radiator cool at the top

  • Air is trapped at the top of the radiator. Turn off the central heating. Then use a radiator bleed key to open the air vent at one end of the radiator. Air should start to hiss out. When water appears, close the vent. Hold a rag under the vent to catch the water escaping from it. Turn the heating on again.
  • If radiators need bleeding more than once a year, air is entering the system and this can cause corrosion. There may be a serious fault that needs expert attention.
  • Some systems have one or more extra bleed points on the pipes either upstairs or in the loft. Manual bleed points are opened with a screwdriver.
  • On an automatic air valve the small, red plastic cap must be loose in order for air to escape. If it is tight, unscrew it.

radiator valve

Radiator cool at the bottom and hot at the top

  • Sludge (black iron oxide) produced by internal corrosion can build up at the bottom of a radiator and stop the circulation. Remove the radiator, take it outside and flush it through with a hose.
  • Alternatively, add sludge removal liquid to the feed-and-expansion cistern. 2 days later, drain and refill the system.

Top-floor radiators cold

  • Cold radiators upstairs only, often indicate that the feed-and-expansion cistern is empty. The ballvalve may be faulty.
  • Refill the feed-and-expansion cistern so that there is just enough water to float the ball when the water in the system is cold. The extra space accommodates expansion of the water in the system as it heats up.

Top-floor radiators hot, lower radiators cold

  • This is almost certainly due to pump failure.

Cold radiators throughout the house

  • Deposits of sludge caused by internal corrosion can result in poor water circulation and radiators being cooler than they should be. The system needs to be chemically cleaned out.

Radiators farthest from the boiler are cool

  • The system is not properly balanced.

Top radiators heat up when hot water only is selected on programmer

  • Hot water naturally rises above cooler water. On a gravity driven system, hot water for the hot water cylinder is prevented from creeping into upstairs radiators when the heating is switched off by a mechanical valve, called the gravity-check valve. It is situated on the flow pipe to the upstairs radiators.
  • If the gravity-check valve is stuck in the open position, the pipe on either side of the valve will be warm. Call a central heating engineer to replace it.