Balancing a Central Heating System

To ensure an even distribution of the hot water to the radiators in a system you’ll need to do what’s known as balancing the system. Because the water is pumped via a pipe, and branches to feed each radiator, the ones at the beginning of the ‘run’ tend to get more than their fair share.

The way to do this is by using the lock shield valve on each to regulate the flow of water to the radiator. By partial closing of these valves the radiators nearest the pump can be restricted more than those further away.

The best way to get this right is by using radiator thermometers, which can simply be clipped to the flow and return pipe at either end, and measuring the difference. You won't often see a plumber using these though - experience enables them to adjust the valves without measuring the temperature.

Remember, the pipe to each radiator is a branch from the main flow pipe. Near the furthest radiator, the main flow pipe turns back towards the boiler and becomes the return pipe. The pipe from each radiator then feeds back into this.

The water does not go into the first radiator then out and on to the next etc. It actually flows from one main pipe feeding all of them, and then flows back out into the return coming back from all of them.

Turn the heating off well in advance of wishing to balance the system, so that the water has a chance to cool right down. Each radiator has a control valve – the one you use to turn it on or off, and a lockshield valve – the one with a cover which you don’t normally turn.  To open or close this lockshield valve, you need to remove the cover and use a pair of pliers. Open both valves on all the radiators.

Now, turn the heating back on and go to the first radiator. You are going to adjust them in the order in which they are served, so if you don’t know this, make a note of the order in which they heat up.

Fit the radiator thermometers to the flow and return. Turn off the lockshield valve then open it gradually again until the difference between the two thermometers is about 50 degrees Farenheight. 

Move on to the second radiator on the system and do the same. If you repeat this for all of them in order, you should have a perfectly balanced system so that all radiators heat up efficiently. You may find the last one will need the lockshield valve fully open.

A new system should always be balanced. The aim being to achieve a 11% drop between radiator flow and return pipes.

On a normal fully pumped two pipe system each radiator has two valves called lock shield valves.  These are used to open and close the flow of water through the radiator. This means that the more the valves are closed the greater the cooling off factor. To reduce the water temperature at the outlet pipe, the lock shield is closed; to increase the temperature the valve is opened up.  

You will need 2 clamp-on thermometers which have springs which hook around the inlet and outlet pipes of a radiator to clamp them on (they can be purchased from plumbers merchants or d.i.y stores).

Ensure all timers and room stats, trv's, (thermostatic radiator valves) are all on so the system will not switch off during balancing period.

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

Open 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 stabilizing 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 stabilizing 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.

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