First, you should switch off the central heating to stop more air from being sucked into the system.  You also need a receptacle such as a bowl to place under the valve to collect any liquid that may be released.

If you do not have a radiator key, buy one at your local DIY shop.  The radiator has a valve on top with a tiny square shaft in the centre.  Using the radiator key turn the shaft carefully until you hear that air is being released.

If you place your hand on the top of the radiator as the air is being released, you will feel it start to heat up.  As soon as water starts to leak out of the radiator, use the key to tighten the shaft in the valve.  If you have a sealed system, be sure to check the pressure after bleeding, and top up the pressure if needed.

If the square shaft in the valve has a slot cut into it, it will usually be possible to bleed your radiator with a small screwdriver instead of a key.

Water is supplied to a house by the local water authority, and reaches it via an underground service pipe. The usual entry point for this is in the kitchen, where it is termed a rising main. A stop valve fitted into the rising main allows for the water supply to be cut off in an emergency. From this point the system may be one of two types, indirect-feed or direct-feed.

The most common system is the indirect-feed. With this type, the rising main feeds a storage cistern which in its turn provides a feed powered by gravity to the taps, hot water cylinder and the WC cistern. Water from a storage cistern, however, is not fit for human consumption and so the cold tap in the kitchen is connected directly to the rising main.

In a direct-feed system; the toilet cistern and all the taps are supplied from the main.  However, a storage cistern in such a system is still needed to supply the hot water cylinder.

Hot Water

The most common method of supplying hot water for domestic use is by incorporating a storage cylinder into the system that feeds the hot taps under gravity. A boiler is often used to heat the water, either directly or indirectly.

With the indirect system; a closed primary circuit is run from the boiler and then through a coiled heat exchanger in the cylinder and back again to the boiler, and water is drawn from the top of the heated water in the cylinder. This means that the water in the primary circuit is always being re-circulated, which reduces boiler corrosion.

Water in a direct system flows from cylinder to boiler and then returns to the top of the cylinder to be drawn off from there.

To bleed a radiator it first must be left to go cold. Using a radiator key, loosen the air bleed valve that is located at one end of the radiator at the top. Having an old rag or towel close by is a good idea, as water will also leak from the valve. Once loosened there will be a hissing sound as the air escapes.

Once water starts to escape, use the key again to close the bleed valve. If the radiator is not left to go cold the water that escapes can be very hot and there is a risk of scalding. This is a process that should be needed quite infrequently. If a radiator needs to be bled on a regular basis there may be a problem with the central heating system and an engineer will be required.

Radiators on a pressurised system cannot be bled in this way and expert advice will be required.

Location is vital when it comes to underfloor heating systems.  This not only means the specific room in which floor heating is to be installed, but also the country in which the home is located.  Since its introduction, underfloor heating has become extremely popular in Asian and European countries.  In these locations, such systems will be cheaper than they would be in, say, the United States, where this technology is not as prevalent.

However, underfloor heating is gaining popularity throughout the world.  As nations push for cleaner, more sustainable energy sources, underfloor heating systems are being looked to as energy savers.  As traditional compression and combustion cooling and heating get phased out, underfloor heating will rise up as the new system of choice.

Underfloor heating is also popular in community based systems.  These systems have to deal with temperature differentials between generating plants and the buildings being heated.  The principles behind community based systems are easily adapted to underfloor heating systems, allowing them to be used in places like multi-story buildings.

Energy efficient buildings are also looking to switch over to underfloor heating.  The system lends itself well to renewable energy sources like solar and geothermal power.  Additionally, underfloor heating systems can heat small spaces efficiently and without using as much power as a conventional system.

Here is where the energy source comes in.  Hydronic heating systems require some source of energy to help circulate the mixture.  Coincidentally, this will also help manage energy costs. 

This energy source can come in a number of different forms and combinations.  It can be a single source of energy or a combination of energy sources that all work together.

One of the options for a floor heating energy source is a boiler.  This is a common option.  However, boilers are not one size fits all.  They too come in a number of varieties.  Some boilers heat via electricity.  Others will use natural gas, coal, oil or waste oil to produce energy.  On the cutting edge of boiler and heater technology are those powered by alternative fuels.  Today, there are even boilers powered using solar energy and bio-fuels.  Back on the more traditional end of things are wood-burning heaters and boilers.

The other options are heat pumps or chillers.  These can be powered by electricity or natural gas.  A heat pump is different from a boiler in that it literally pumps the heat through the system.

If you wish to reuse the valves from your previous radiator, use an adjustable spanner to remove the tailpieces by turning them anti-clockwise.  Clean the tailpieces and air valve and place PTFE tape around them.  The tailpieces need to be screwed into the gaps at the bottom of your new radiator.  Fix the old air valve to your new radiator in the same way and create a tight seal with a radiator spanner. 

Please note, your new radiator might have an open tapping at the opposite end.  You will need to purchase a new plug to fit tightly into the hole.  These plugs can be purchased at most DIY stores.

Fitting the new Radiator

First, check to see if the brackets are in the right position for your new radiator.  If they are not, you will need to re-fit them.  Lift the radiator onto the brackets and follow by connecting the valve union nuts to the pieces that can be found on both ends of the radiator.  Next, open the lock shield valve completely in order to let the radiator fill up.  Release the air valve located on top of the radiator.  As soon as water starts to come out of the air valve, close it off tightly.  

Finally, make sure there are no leaks around the control valve, air valve and gate shield valve, as well as around the union nuts.  If any of these are leaking, simply tighten them.

First, switch off the gas and electricity supplies and allow the boiler to cool down. Next, unscrew and lift off the casing of the boiler so that you can get an unimpeded view of the heat exchanger. This should be cleaned thoroughly using a stiff-bristled flexible wire brush. When doing this, if the gas valve and thermostat are situated directly beneath the heat exchanger you need to protect them from soot and dirt with a rag.

Remove the flue cover next and use a wire brush again to clean inside the flue. You can check the condition of the gasket whilst doing this; it lies between the boiler and the flue and this might need to be replaced if it has become corroded or worn. Use a nozzle attachment on a household vacuum cleaner to clear loose debris from all the boiler surfaces, including the inner surface of the boiler casing.  Finally, take the burner unit out to brush it down and clean the gauze filter if it has one fitted.

Most balanced-flue gas boilers have essentially the same layout internally, and the burners are usually located directly below the heat exchanger.  To clean the burners you will have to turn off the gas supply first and remove the cover plate at the base. The burners will slide out so you can thoroughly clean them.

Turn off the boiler to begin with, and the immersion heater if you have one fitted in a direct system. Remember to allow a solid fuel boiler time to cool down before draining. The next step is to stop water from entering the header tank by turning off the stop tap that feeds it. You could try tying up the ball valve arm instead.  Either way, you then attach a hose to the lowest drain cock and feed it to a drain outside the house.  Next, undo the drain cock, and as the water level drops, open the radiator vents. When refilling the system later on you simply reverse the whole process.

The stop tap will be on a branch from the rising main that feeds the header tank, and if it is stuck, you can use penetrating oil to loosen it. If there is no stop tap, which is often the case, you can tie up the ball valve arm in the tank instead using a batten and string laid across the top of the tank. Do this before turning off the boiler and draining the system.

It is a straightforward matter to bleed a radiator.  When the radiators were fitted you will have been supplied with keys for them, which are used for the bleeding.  A brass radiator key is used on most modern radiators, whereas on older ones you can use a screwdriver with a flat head.  Use either of these to open the bleed valve which will be situated at one end of the radiator, at the top.  Switch off the central heating before bleeding any radiators.

Although there is nothing to it, bleeding a radiator can result in some splashing of water, so it is sensible to have a cloth or towel ready to hand to catch any spillage.  Insert the key or screwdriver into the valve and start turning it anticlockwise until you hear a hissing sound, indicating that air is escaping.  Keep watching the valve and the second that water starts coming out catch it with the cloth and turn the valve clockwise to close it again.

Repeat this process for all your radiators to maximise their efficiency.  If your central heating is a closed system, then rather than releasing air from each radiator you will have to top up the pressure from a central hub.