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 is then mixed back in with the hot water in the main single feed hot water supply pipe.
Single pipe central heating system
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 or 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.
With a two pipe central heating system, two separate pipes go to each radiator, one feeding the radiator (flow), and one taking the water away from the radiator back to the boiler (return).
Two pipe central heating system: flow and return pipes.
The two pipe system is much more efficient than the single pipe system but still needs to be balanced properly at the end of the installation.