System drain-down and refill
Most work on radiators or their valves starts with a drain-down of the system, and at the end of the job the system needs to be refilled, all radiators bled of air, and the heating run to check that all radiators heat up okay. For work involving only upstairs radiators or valves, only a partial drain-down and refill is necessary, taking perhaps 1 hour. Work on downstairs radiators requires a full drain-down and refill which may take about 2 hours, or sometimes longer in a large home. The extent to which a full drain-down is necessary also depends whether pipework needs soldering, which can only be done if water is fully removed from the pipework. If soldering is not required, valve replacement may be done with a less thorough and less time-consuming drain-down. A 500ml bottle of corrosion inhibitor needs to be added after a partial drain-down (£8), or a 1 litre bottle after a full drain-down (£15).

Radiator Valve replacement
Each pair of radiator valves typically takes about an hour or slightly over, plus the system drain-down & refill time (see above). Consequently, replacing valves on 6 radiators is typically a full day's work. A pair of radiator valves costs about £10 to £30 depending on brand and model. While a radiator is having new valves fitted, it is often a good idea to take the radiator outside, connect it via a hose to the garden tap and flush out accumulated sludge using mains water. Allow an extra 1/2 hour per radiator, and expect about 4 or 5 radiators to take a day's work.

Radiator replacement
Each radiator takes about 2 to 2.5 hours to replace with like-for-like, plus the system drain-down and refill time (see above). Consequently replacing 3 radiators is typically a full day's work.

Moving a radiator
The time taken to move a radiator depends on how far it is to be moved, and on whether the new pipework is to be surface mounted, or routed under floorboards or behind skirting boards. Taking boards up and moving a radiator across to the other side of a room can often take the best part of a day's work.

A radiator gets hot most of the way up but not at the top
This is caused by air that has collected in the top of the radiator. The radiator needs bleeding. There is a bleed valve at the top of a radiator which you turn using a bleed key. If the system is pressurised, as with a combi boiler, bleeding radiators will cause the pressure to drop, so you then need to top it up at the boiler using the fillip loop. The filling loop may be a braided flexible hose with black valves located under the boiler. Or sometimes it is an integral part on the underside of the boiler. When the radiators are cold it should be set to 1 bar pressure. When the radiators get hot the water expands, so the pressure increases.

A radiator gets hot, but the lower middle section stays cold or just gets slightly warm.
This is caused by sludge that has collected in the lower half of the radiator, inhibiting flow to that section. Radiators are made of steel and sludge is an accumulation of dissolved rust. The system needs flushing (see below) and then inhibitor needs to be added to prevent the radiators rusting internally.

A leaking radiator valve
If a radiator valve develops a slight leak it will need replacing. Meanwhile, you can usually stop the leak by closing both valves to that radiator (turn fully clockwise). To close the lockshield valve you first need to remove the plastic cap and you may need a small adjustable spanner or pair of pliers to turn the top of the valve. If it is still leaking, place a bowl under the valve or next to the pipe, then tie a J-cloth tightly round the valve and direct the end of the cloth into the bowl. That way, the leak won't run down the pipe and into the floor. Then call me and book an appointment to change the valve.

A radiator is not working
If most radiators are working but one stays cold, there are several possible solutions:
1) Sometimes it is because the thermostatic valve has been closed and then sticks in the closed position even after you open it. If you turn the valve to maximum heat, then undo the securing collar (turn anticlockwise) and remove the thermostatic head, you will see a metal pin that sticks out of the chrome part of the valve. If you press the pin firmly (you might first want to cover it with a coin), you should be able to press it down (which closes the valve) and when you let go it should spring back up (which opens the valve). But if the pin has jammed, try gripping it with a pair of pliers and pull it upwards. Press the thermostatic head back onto the valve (it first needs to be set to maximum) and tighten the securing collar. Then run the heating and test to see if the radiator heats up.
2) If it still doesn't work, check that the other valve (the lockshield valve) is fully open. You usually need to remove the plastic cap and then turn the valve using a pair of pliers or small adjustable spanner. Check it is turned anticlockwise as far as it goes. (Note: if you turn lockshield valves that are more than about 5 years old, there is a risk of them starting to leak slightly, in which case the valve will need replacing).
3) Try closing the thermostatic valves on the other radiators and run the heating. This will boost pressure through the one that isn't working. See if it now heats up. If so, let it run until it's really hot and then re-open the thermostatic valves on the other radiators. If it stops working again, then the system needs balancing (see below).
4) If the radiator pipework comes down from the ceiling above, rather than up from the floor or out of the wall, it is possible that an airlock is preventing flow. This could be fixed by adding auto-vent valves to the highest point of the pipework.
5) If none of the above work, the next step would be to close both valves, drain and remove the radiator, and try flushing it through with mains water pressure by connecting it via a hose to a garden tap. Check to see if the water comes out black and sludgy to begin with. If so, it might start working after you refit it, but it would be best to have the whole system flushed (see below). If one radiator has been blocked up with sludge then the system as a whole will have a lot of sludge in it, and if that isn't dealt with it will eventually damage your boiler and cause it to break down.

Radiator Flushing
Radiators are made of steel and, unless sufficient inhibitor is added to the system, they rust internally. That rust accumulates at the bottom of the radiators as sludge. The sludge reduces flow through the radiators so that they work less efficiently, and will also eventually damage your boiler and central heating pump.
I use the Adey Magnacleanse Rapid Flush method of flushing radiators. This process involves the following steps:
1) Drain the system and fit an Adey Magnaclean filter to the return pipe where the radiator pipework goes back into the boiler. A Magnaclean uses a strong magnet to catch any rust particles as they flow round the system. After the flushing process has been completed this filter will remain in place to maintain the cleanliness of the system.
2) The Magnaclean is then temporarily removed and its mounting brackets are instead used to attach a much larger Magnacleanse Rapid Flush filter. 3) Part refill the system so that radiators are half-full and add Adey MC5 cleaning chemical to the system. The heating is then run for about half an hour until all radiators are hot.
4) The valves are then closed on all but one radiator and the heating is run through just the one radiator for 15 minutes. The radiator is also agitated using a hammer drill with a special head bit, which helps to loosen accumulated sludge while the flushing is in process.
5) The next radiator is opened and the first one is closed. In this way, each radiator is individually flushed in turn until all have been completed. After every few radiators the process is stopped and the Magnacleanse filter is wiped clean to remove the sludge that has stuck to it.
6) The system is then flushed with mains water to flush out the dirty water and cleaning chemical, each radiator being flushed in turn for 5 minutes.
7) Remove the Magnacleanse Rapid Flush filter and replace the Magnaclean maintenance filter. Bleed all radiators until they are full, and add Adey MC1 inhibitor to maintain system cleanliness. The Magnaclean filter should then be cleaned each time your boiler has its annual service.
Including the cost of the Magnaclean filter (about £120) which will then maintain system cleanliness, together with the chemicals (about £40), and labour, it costs approximately £520 to flush a central heating system in a typical house with about 9 radiators. However, a pre-requisite of this flushing process is that your radiator valves are in good working order such that each radiator can be flushed in turn while the others are closed. If any of your valves are in poor condition they should be replaced first, adding additional cost (see above 'Radiator Valve Replacement').

Radiator Balancing
If your system has been properly balanced, then all your radiators warm up simultaneously at about the same rate. Otherwise, smaller radiators, or those located closest to the boiler, will tend to heat up more quickly. Larger ones, or those located further from your boiler, have a higher resistance to flow and will tend to heat up more slowly (or even hardly at all). Each radiator (except the one closest to your room thermostat) should have a thermostatic valve and a lockshield valve. The thermostatic valve allows you to adjust the desired temperature of each room. The lockshield valve allows you to adjust the resistance of each radiator and is for balancing the system. Balancing the system involves a bit of trial and error over a period of time and is something you can try yourself. However, please note that balancing radiators is best done when the radiator valves are fairly new. If the valves are more than about 5 years old there is a risk that some of the lockshield valves will start leaking slightly after you turn them, and if so they will need replacing. That said, the steps to balancing radiators are as follows:
1) Turn all the thermostatic valves to maximum. Remove the plastic caps from the lockshield valves and using a small adjustable spanner or pair of pliers, make sure each of the lockshield valves is fully open (turned fully anticlockwise).
2) Turn on the heating and note how quickly each of the radiators heats up. Try to grade them as fast / medium / slow, or rank them in order from fast to slow.
3) On the radiators that heat the fastest, close the lockshield valves (turn fully clockwise), then place your ear next to the valve and slowly open it but only until you can hear water flowing, and then stop. This adds resistance to this radiator. On radiators that heat up at a medium rate, close the lockshields slightly. On those that heat up the slowest, leave the lockshields fully open.
4) Allow the system time to cool and then repeat steps 2 to 3. If any of the radiators that you adjusted now heat up too slowly, open the lockshield slightly more, perhaps an extra quarter turn. Once again, allow the system time to cool and repeat the process. Tweak the lockshields until, by trial and error, you eventually get them all heating up at the same rate.
5) Refit the plastic caps on the lockshield valves. Then adjust the thermostatic valves on each radiator according to the temperature you desire in each room.

Meanwhile, if any of the valves have developed a leak, see above 'A leaking radiator valve'.