Single Pipe Central Heating System
A single pipe central heating system operates through a main single feed hot water supply pipe which comes from the boiler supplying hot water to each radiator.

Each radiator has a smaller hot water supply pipe branched off the main feed pipe to supply the radiator, the water passes through the radiator coming out the other side a little cooler and then mixed back in with the hot water in the main single feed hot water supply pipe.

The single pipe system is a very inefficient system and must be balanced properly at the end of the installation because if it is not balanced / commissioned correctly the first radiator will be very hot, while the last radiator in the system will be much colder having been supplied with a great deal of mixed cooler water from the other radiators.

Two Pipe Central Heating SystemWith a two pipe central heating system you will have two separate pipes going to each radiator, one feeding the radiator (flow), and one taking the water away from the radiator back to the boiler (return).
The two pipe system is much more efficient than the single pipe system this system but still needs to be balanced properly at the end of the installation.