It pays to know which Central Heating System you have and these are the basic varieties.
Single Pipe System:
As it says this system operates through a single pipe which comes from the boiler running along at floor level to each radiator. A pipe is branched off the feed pipe to take hot water to (usually) the top of the radiator where it passes through the radiator coming out, a little cooler, to mix with the hot water in the main feed pipe again.
The single pipe system is a very inefficient system which allows the first radiator to be very hot, while the last in the system, having mixed with a great deal of cool water, is usually cold.
Double Pipe System:
With a double pipe system, two separate pipes go to each radiator, one feeding the radiator (flow), and one taking water back to the boiler (return). Much more efficient than the single pipe system this system can be balanced properly.
A pressure relief valve is fitted between the feed and return pipes allowing the pump simply to circulate the water if the radiators are closed.
In the image below the one pipe central heating system is shown at the top and the two pipe system shown at the bottom.
Large pipes (up to 28mm) are fed with hot water from the boiler. A manifold is connected to this pipe. There is a separate manifold for both the feed and return paths. From this manifold the radiators are serviced by (usually) 8mm flexible copper pipes. The distance the water travels in the smaller pipes is kept to a maximum of 5m wherever possible for maximum efficiency.
A fairly major drawback to the micro bore system is that its not practical to add another radiator to the system should you plan an extension etc as, unless there is a spare flow and return port at the manifold, a new manifold is required.
If all radiators are closed a similar by-pass valve allows the water to circulate.
Programmers and timers, together with motorised valves and pumps, send the water to the areas that need it, when they need it, and efficiency is the name of the game, but because there are so many “moving parts” things can, and often do, go wrong.
For more information regarding central heating please see all of our central heating projects especially our project on Radiator Problems