After installation and commissioning, a new central heating system should be balanced.

The aim of balancing is to achieve the correct temperature drop across the boiler flow & return pipes (11 deg. C). The design operating temperature of the boiler is typically 80 deg. C.

The lock-shield valves on each radiator in a normal fully pumped two pipe system allow for closing to reduce the flow. Thus the more 'closed' the valve the greater the cool off of the water in the loop: to reduce the water temperature at the outlet pipe, the lock-shield is closed to increase the temperature the valve is opened up.

Obtain 2 clamp-on thermometers: mine are large 4" diameter dials which have springs which hook around the inlet and outlet pipes of a radiator to clamp them on (bought from ECS).

Make sure timers and room stats, Thermostatic Radiator Valves etc. are set so that the system will not switch off during adjustment period.

Attach the clamp on thermometers to the boiler flow and return pipes, both equidistant from the boiler and far enough away that the thermometers are not upset by the radiant heat of the boiler.

Prepare for balancing by fully opening all control valves and all lock-shield valves.

Balancing is done using a differential setting: that is, the absolute temperature is unimportant what you are setting is the difference between the flow and return. The usual setting is 11 degrees C (approx 20 degrees F).

Allow the house to get to its typical room temp, then adjust the flow rate by altering the circulating pump speed until you get this 11 degrees difference between flow and return. If the pump is not powerful enough, or in the case of a variable head pump it is set at too low a speed, even with all the lock-shields fully open there will still not be enough flow to keep the temperature gradient shallow enough. With the speed on maximum, if the drop is greater than 11C, the pump will have to be replaced with a larger model.

Remember to allow sufficient stabilising time after each adjustment.

Once this is correct, you then measure the temperature drop across each radiator. You will find that one has a much higher drop across it than the rest, and this is known as the INDEX radiator (or index loop, for a single pipe system). This will nearly always be the radiator furthest away from the pump, and its lock-shield valve should be left fully open.

The other radiators should now be adjusted to achieve 11 degrees drop across each, if possible working backwards towards the boiler. This will take time and patience, because of the stabilising period (at least 15 minutes) between adjustment and measurement, and because the adjustments will be interdependent. But you should soon get quite good at estimating the effect of a certain degree of adjustment. Most lock-shield valves need to be opened and closed about 50% from current position to have any effect on the temperature.

When the radiator temperature differences are getting somewhere near the correct values, and before any final fine adjustments, the temperature drop across the index radiator should be considered. If this is considerably different from the design figure ( 11 deg. C ) then the pump needs to be adjusted. If the temperature drop is greater than the design figure the pump setting needs to be increased in speed, and the converse for too little a temperature drop. If it is not possible to decrease the pump speed any lower, its isolating valves can be used to restrict the flow further.

Having done this the lock-shields should then have their final fine adjustments.