Balancing Central Heating Radiators

By Phil Addison

This is a topic covered in the uk.d-i-y FAQ, but I offer this article as an alternative slant on the subject. I should say I have no professional knowledge, and this is just what I've figured out for myself. I hope it's helpful to other DIYers. Comments and questions welcome.

EQUIPMENT

To do the job properly you will need a means of measuring a change of 1 degree C or better on a temperature of about 80C. I actually use the Fahrenheit scale on my digital thermometer because 1 degree F is about 1/2 a degree C and hence twice as sensitive as the Centigrade scale (my meter will not display decimal points). I use a thermocouple digital thermometer* (e.g. from Maplins) with the sensor bead pressed onto the chrome valve body with a soft clean dry cloth to get good thermal contact and to insulate the probe from the air temperature.

* Since writing the above I have obtained a non-contact (infra-red) thermometer and this makes the task VERY much easier and quicker. As the reading from this type of instrument depends on the emissivity of the  surface being measured, the reading will vary slightly depending on whether it is pointed at a dark copper pipe, chrome fittings or white radiator paint. I found the easiest way to overcome this was to fix a piece of black PVC tape on each of the points I wanted to measure. That gave me consistent results which agreed well with my old thermo-couple thermometer. The instrument I have is a model IR-88 Pocket InfraRed Thermometer obtained from CPC for £35, part number IN02293.

I have doubts that the job can be achieved with clamp-on dial thermometers because of (a) the usual lack of a suitable location to fix them to the radiator connections, (b) the poor thermal contact and large radiation area gives inaccurate low readings, (c) the slow thermal response, and finally (d) the sheer inconvenience of attaching and detaching them at each measurement.

PRINCIPLES

The general idea is to reduce the flow to the hotter radiators so that more water flow is available for the cooler ones. The system is "balanced" when all radiators get a good flow through them and have the same temperature drop across them. In practice it does not matter if some have a smaller drop (hence a higher flow) so long as there is enough flow to get them all good and hot. However, a low temperature drop indicates that you may be able to reduce the pump speed (which will increase the drop). The important thing is that the return water coming out of each radiator is good and hot. If it's only warm you need to open its Lock Shield Valve (LSV) some more, or if it's already wide open you have to close the LSVs on the hottest radiators instead. The LSVs are the ones with a cover preventing you from adjusting them until the cover is removed (hence the term).

As you close down the LSVs on the hot radiators you might find the boiler starts cutting out. This could mean the pump is not powerful enough (assuming the pipes are not clogged up) so check if it has an adjustable speed and try a faster setting. Radiators are designed to give their rated output with a mean water temperature of about 80 deg C. It doesn't matter whether you have that value or not when you balance as it can be adjusted by the boiler thermostat according to the heat you actually need. Similarly the oft quoted 11 deg C drop across the radiators is really nothing to do with the radiators! It is the temperature gradient the *boiler* is designed to produce when it is going flat out *and* the pump is producing the specified flow rate. So if you have less than 11C drop it just means that either you have too high a water circulation rate or/and the radiators are not large enough to extract the full rating from the boiler. This need not be a problem; it may be that the boiler is oversized for the house, and the converse of course follows.

PROCEDURE

1. Make a chart similar to the following to suit the location/number of your radiators, to record the flow and return temperatures and LSV settings over several rounds of adjustment. This is a tedious, time consuming and back-breaking job (up and down to your knees many times) and you won't remember where you are without writing it down - trust me, I'm a know-all.

The chart needs to be long enough for several trial runs, say five or six.

   

LSV Knob Position/Temperature

Run

 

Kitchen

Hall

Lounge

B1

B2

B3

Bathrm

Boiler

Max

Min

Diff

1

LSV

                     
 

Flow

                     
 

Return

                     
 

Drop

                     

...

                       

6

Etc

                     

2. Decide on some way to record LSV positions. I use "0" for fully closed and count in 1/4 turns when opening. Some valves have several turns and some open fully in one turn or less.

3. Record initial LSV positions in case you want to return the system to what it was.

4. Open all radiator Control Valves fully. The Control Valves are the ones with adjustable knobs. Usually a knob taken off a control valve will fit the LSV. Take off the LSV covers and open all those as well.

5. Turn off the domestic hot water heating circuit, turn off the boiler, and let the system cool.

6. When cooled, turn on the boiler, if necessary turning up the room stat to make it fire up.

7. While the system is warming up, go round all the radiators and feel the flow and return connections.

8. Establish which is flow and return (flow gets warm first).

9. Find which radiators are heating up quickest and which are hardly getting warm. Check at both flow and return connections. The flow gets hot first, then the radiator body and finally the return pipe.

10. While still warming up, try to turn down the LSVs of the hottest radiators so that the cooler ones "catch up". This will give a rough balance (and may be all that was done when the system was installed, if even that).

11. Go round with a thermometer and fill out a 'Run' of the table.

12. Work out the temperature drop of each radiator and boiler.

13. Go round again adjusting the LSVs, closing a little those on the radiators with the smallest drop and opening the coolest a little. The coolest, or biggest temperature drop, radiator LSV should be left wide open otherwise the pump will finish up working harder than needed.

14. Make sure you have written down the data.

15. Repeat 11 - 14 until you are satisfied that all the radiators are fully hot all over and the differentials are as close as you can get them.

16. Restore covers to LSVs.

17. If necessary, adjust control valves to reduce room temperatures. This should only be necessary if a radiator is oversized and is thus over-heating that room, and does not apply if a thermostatic valve is fitted.

18. Reset any thermostatic valves and the room thermostat to give desired room temperatures.