The basic central heating system for UK homes combines the boiler, the radiators, and the pipework that connects these elements. The water is heated and fed from the boiler, usually circulated by a pump mechanism across the pipes and back again. In the same way that you can introduce different boilers and methods of heating, it is also possible to implement different pipework.
Piping is usually fitted below the radiator under the floorboards. It is either run between the joists or over them in cut-out ridges. Unless it is part of a micro bore system, the pipework will need additional support so that it won't have to support heavy weight on the floorboards. Pipework below suspended floors will not usually require insulation; although any high level feed pipes that are installed in loft space must have adequate insulation.
There are three main layouts for central heating pipes:
- Single pipe loop
- Feed and return pipes
- Micro bore pipes
Single Pipe Loop
Aptly named, this system has a single pipe that runs from the boiler, to all the radiators, and then returns back to the boiler. Both radiator connections are made into the same pipe, with the hot water rising through natural convection into one side of the radiator, and the displaced cooler water falling back into the same pipe at the other end of radiator.
Single pipe systems were in frequent supply about 30 years and so are still around in properties today. They are now however considered to be the least effective and as such are generally no longer reinstalled.
The loop can be large and can accommodate any number of radiators. For this reason it is still found in many old industrial buildings.
Most of the heat is absorbed by the first few radiators, so the further from the boiler you get, the cooler the radiators are. Therefore, while you can fit an unlimited number of radiators to the pipe loop, it will not necessarily offer great heat efficiency.
Feed and Return Pipes
This is a two pipe system of ‘feed' and ‘return'. The hot water from the boiler enters via convection into one end of the radiator through the feed pipe, and when it cools exits at the other end out of the return pipe. Each radiator therefore has its own pipe circuit.
Greater efficiency than a single pipe loop as the hot water that enters each radiator is roughly the same temperature; every room heats and cools to the same temperature at the same time. There is a pressure relief valve attached between the feed and return pipes, so that if all the radiators are shut off, the valve still allows the pump to circulate the water.
The amount of radiators that can be used is limited by the size of the circulating pump, and by the fact that each radiator is fitted in parallel. Conventional domestic pumps will serve up to 12 radiators, with each only receiving about 8% of the water flow.
Micro Bore Pipes
The micro bore system uses feed and return pipes, but the feed from the boiler reaches a manifold first, and passes through the manifold on the return before re-entering the boiler.
Stemming from the manifold are small pipes of about 8mm circumference which lead off to the different radiators. The distance between the manifolds and the radiators is usually less than 5 metres, and both the feed and return pipes stem from the same side of the radiator.
The boiler is again protected by the pressure valve in case all the radiators are turned off, and the use of small pipes mean that less water is used and less heat is lost through the pipes. As with the normal feed and return system, the radiators receive about the same amount of heat and as such warm up and cool down at the same time. Installation is simplified also, as the pipework can be bent to fit and does not need as many joints as standard designs.
As the pipes are very small they are more likely to come under pressure from the increased resistance caused by the boiler trying to circulate the water. Blockage and build up can occur in such narrow pipes, particularly in hard water areas, which in turn cause more resistance in the pipe. If you are in a hard water area, adding water softener to the micro bore system is essential.