There are unfortunately numerous common problems that can occur with radiators, although the majority are fairly simple to solve. It is a good idea to regularly feel your radiators to ensure that they are all getting equally warm with an even spread throughout each individual radiator. If your radiators are malfunctioning you will not get best out of your central heating, using more energy to try and keep warm, giving yourself a bigger heating bill and wasting energy. If you are experiencing radiator problems, here are a few suggestions that may help.

Cool top, warm lower:

Bleeding RadiatorsAs you feel the radiator, it gets gradually colder from bottom to top. This is an indication that there is trapped air, and it will be necessary to bleed the radiator.

!Bleeding a sealed central heating system reduces the pressure, so it will need correction. This is usually done by turning on the cold water feed. If you are unsure about re-pressurising, contact a plumber of heating engineer.

Bleeding - Make sure the heating is off and the radiator cool, then use the radiator bleed key to open the air vent on one side at the top of the radiator. Insert it and give it a ½ turn anti-clockwise: do not open it fully. Hold a cloth to the vent to collect any dribbles of water. The vent will hiss first as the air escapes. Once the water starts, close the vent back its ½ turn. Do not turn it too tight.

If you have bled your sealed heating system, check the pressure and top it up if necessary to 1 bar, or as the instructions state.

One or all radiators requiring frequent bleeding:

Problem radiators mean that air is entering the system somewhere. This could be caused by or is causing corrosion and may need a heating engineer's attention. It may be that black iron oxide sludge has built up in the bottom of a radiator and preventing the water's movement. To resolve this, add a sludge removal liquid to the feed/expansion header tank, then drain and refill the system a couple of days later.

If this does not solve the problem, it could be caused by leaks, faulty design, or lack of water. Serious problems will not be affected by radiator bleeding. Try examining the pump and the flow regulator:
  • Note the setting of the flow regulator, and then turn it on and off rapidly several times using a screwdriver. If that fails...
  • Bleed the pump via the bleed valve on the casing. Open and close rapidly. Any hissing means you have found the fault.
Downstairs cool, upstairs warm:

Colder lower radiators are a problem that can usually be traced to the pump. Start by turning of the central heating and allow time for the pump to cool. The pump is located in or near the boiler; it whirrs and vibrates as the motor works to push water through the impellers. If you need to manually start the pump, unscrew the centre and turn the impeller.

Another trick to try is gently tapping the pump a few times with a mallet, or it can be removed and flushed though with a hosepipe of clean water. If it still not working a new pump will be required.

Upstairs cool, downstairs warm:

Radiator ValvesCheck that the feed/expansion tank is not empty. If it is, refill it with enough water so that the ball just floats. The water will expand when heated so allow extra space for this. Don't forget to check the ballcock and valve for leaks or corrosion.

Upstairs cool, except when programme is on hot water only:

In a gravity-based heating system the gravity check valve can switch off the heating, meaning that hot water from the cylinder will not reach the upstairs radiators. When the heating is on, hot water rises above cooler water. The gravity check valve is attached to the flow pipe toward the upstairs radiators. If it stuck open, the pipe running through it will be warm. This will require the attention of a heating engineer.

Of course, do not forget to eliminate the obvious causes first: are the thermostatic valves stuck low, or are the caps for the thermostatic valve bearing no relation to altering the temperature? You may need replacement thermostats.

Cool radiators furthest from boiler:

If the radiators get cooler at a greater distance from the boiler, this indicates that your central heating system is not properly balanced. This occurs on feed and return systems whereby the pipe flowing to every radiator is from the main flow pipe. At the furthest radiator, the main flow pipe turns back toward the boiler and becomes the return pipe, into which pipes from every radiator feed. The water does not flow in and out of each radiator in turn, but comes from the main pipe that feeds them all, and goes into the main pipe returning from them all. If the system is not balanced, you will not get the right feed to the furthest radiator.

When the radiators are first installed they are given a lockshield valve which allows for balancing adjustment. Once installed and balanced, they should not require further altering unless there are changes to the radiators or the pipework.

Balancing Your Central Heating - Before you begin, turn off the central heating and give it plenty of time to cool down.

  1. Locate the lockshield valve at one end of the radiator under a push on cover. Using an adjustable spanner, open the lockshield and the control valve (at the other end of the radiator) on all the radiators.
  2. Attach radiator thermometers to the inlet and outlet pipes of the radiator nearest to the boiler, not to the main flow and return pipes.
  3. Now switch on the central heating system.
  4. Partially close the lockshield valve on the nearest radiator. As the temperature rises, slowly open the valve until the temperature difference between the two thermometers is about 20°F (12°F).
  5. Attach the thermometers to the next radiator along from the boiler. Repeat the process as above.
  6. Continue along all the radiators until they have been balanced.

The effectiveness of balancing the radiators can be affected by external temperature. If the external temperature is higher than the system design value, the heat disseminated by each radiator will be less than the design intends. Consequently, the temperature drop across each radiator will be less than 20°F. If the balancing occurs on a summer's day, adjust the lockshield valves to get a lower temperature difference.