So you've got radiators that are cooler than the rest or not coming on at all? Well balancing the heat distribution may be the answer to your problems. This should have been done when the system was installed but if the system was altered or it wasn't done properly then it will need to be sorted. "But how do I do this?" I hear you cry, well by following this simple guide below.
The Balancing Act
Isolate the boiler (at controls) - this gives the system chance to cool down and should be done well before you want to undertake this task.
Close all the locksheild valves on all the radiators - this is the valve that has a top that you can't turn. You will have to pull off this cap to turn close the valve.
Now turn the heating (not the hot water, isolate on the hot water circuit if necessary) back on.
Place your thermometers( be that analogue, digital or infra red) on both the flow and return pipes on the first radiator - needless to say these are the pipes that directly connect to the radiator at either end.
Slowly open the locksheild valve (would recommend you turn no more than a ¼ turn at a time).
Adjust the valve so that the flow and return pipe differ by only about 1 degree Celsius ( or Fahreheit what ever your happy with using).
Repeat above three steps on all radiators.
Once your happy with the radiators check the hot water(turning it back on if you turned it off), and turning it down if necessary.
Don't use an analogue strap-on thermometer these take longer to use and don't give you a very accurate reading.
Check that the boiler flow and return pipes have the correct difference of 11 degrees Celsius. This optimum difference temperature and shows that the boiler flow rate is correct (heating - not hot water flow rate). If your reading is below this then it is a simple case of the pump speed being too high and/or the radiators being undersized for your requirements. Or it could even be a case of your boiler being oversized for your requirements.
You will need to balance your system every time you alter it - for example adding a new radiator.
Allow system too cool before undertaking this task.
You may need to turn your room thermostat up to full to keep the heating on while you do this exercise. But remember to turn it back to what you set it to.
Remember you turn it clockwise to close and anti-clockwise to open.
Make a note of which is the flow and which is the retun on the radiators. The flow will be the first to get warm.
Also start in the order the boiler serves the radiators. By this we mean which radiators get hot the first (the radiators closest to the boiler).