1. Most radiators have two valves, generally one on the bottom of each end. There are other connection options, but they are rare in domestic systems in the UK.
2. One of these valves is a lock shield valve. To adjust them you generally have to remove a non-rotating cap and use a spanner, key or Allen key to open or close the valve.
3. The other will be one of two types:
3.1 A Thermostatic Radiator Valve (TRV). This automatically closes down the valve as the room (not the radiator) reaches a temperature pre-set on the valve handle. The handle is relatively broad, and has numbers on it which equate to different temperatures. Where the numbers are 1 to 5, 1 is roughly 15 degrees, 2 is 16.5, 3 is 18, 4 is 21 and 5 about 23 (all roughly). If you set a TRV to 4, it will allow water to flow into it or out of it until the room reaches 21 degrees, when the valve will be fully shut. Most (but not all) modern TRVs can be fitted to either the flow or return side of the radiator, as they are not sensitive to the direction of the flow of water through them.
3.2 A wheel head valve. This is a manually adjustable valve, very similar to a lock-shield valve, which varies the flow rate of water into or out of the radiator. They normally have handle / knob smaller in size than a TRV and generally made of white beige or grey plastic. So if you want to cease heating a room, you fully close this valve, if you want the room very hot you open it fully. Or put it somewhere in between. These valve are quite prone to leaking at the spindle as frequent use wears the seal between the
4. It generally doesn't matter whether the lock-shield valve is on the flow (into) or the return (from) the radiator. Its purpose is to restrict the flow of water through the radiator with the wheel head / TRV fully open. In this way it determines the maximum amount of heat a particular radiator can take from the heating system.
5. The control valve (wheel head or TRV) can vary the heat taken from the system between zero and the maximum allowed by the lock-shield.
6. The purpose of balancing is to ensure that no particular radiator is taking too much heat from the system, leaving insufficient heat for other radiators.
7. Radiators heat space by losing heat from the hot water circulating in the central heating system. Because they lose heat, one end (the return end) must be cooler than the other (flow) end. It used to be that this difference was adjusted to be 11 degrees Centigrade. In more modern systems with condensing boilers, it is more efficient in terms of fuel usage for the temperature of the water returning to the boiler be at or below 55 degrees. To achieve this while still heating rooms at a reasonable rate, the flow temperature needs to be higher. In consequence the temperature difference between flow and return points on the radiator should be about 20 degrees.
8. To balance the radiators you need either two radiator clip on thermostats or an infra red thermostat. Then:
8.1 Open all the control valves.
8.2 Open all the lock-shield valves (but see 9. below).
8.3 Turn on the boiler, for heating.
8.4 Start with the radiator furthest from the boiler. For preference this is the one hydraulically furthest from the boiler, but try your best.
8.5 Adjust the lock-shield valve to get the required temperature differential. This takes time and you need to allow 10 minutes between each adjustment for the adjustment to be reflected in the temperature differential.
8.6 Repeat 8.5 for each radiator.
8.7 When you have finished, go round again, checking the differential, and making minor adjustments.
9. You can often get a good approximation by setting the radiators nearest the boiler (downstairs radiators?) about 1/2 turn open, and those furthest away 1 to 1 1/2 turns open, at least as a starting point.
10. Particularly in summer you can get problems as the system turns off as the rooms reach temperature. All you can do is keep windows open and be prepared to stop and let things cool down.
11. If you have a system with TRVs on most radiators, the TRVs sort of balance the system. If a room is taking more than its fair share of heat, it will warm up quickly, and the TRV will shut the water flow to its radiator. There is thus more hot water available to the system. The next room will heat, and its TRV shut, and so on. However, it is highly likely that the property as a whole will take longer to reach temperature, with some rooms heating up before others. This can be avoided with proper balancing via the lock shields.
12. The above is a long and boring process. If you do it, it is worth going round each radiator and:
12.1 Close the lock-shield completely, making a not of the number of 1/8 turns from the balance position to fully closed.
12.2 Open the lock-shield again by the same number of 1/8 turns.
12.3 You now know the balance point for each radiator, so if any are removed for decoration / maintenance they can be replaced and balance restored.
13. Note that it is not the temperature of the radiator which counts, it is the temperature of the room.
