Gravity Heating System
This was one of the earliest types of heating system and is referred to as a gravity heating system. In these systems there was a single flow and return, and the pipes are set to a fall to help with the circulation of the water. The principal behind the full gravity system is that the difference in density between hot water and cold water causes a flow through the pipe work. Hot water rises and cold water drops. This results in a circulation of the water from the boiler to the radiators and then back to the boiler again. The pipes used are larger than those for modern systems and this type of system can easily be recognised by these.
Semi Pumped Heating Systems
The gravity type of heating system has obvious limitations and a pump added in the flow of the heating circuit greatly improves things. A semi pumped system is so called because the heating circuit is pumped and the hot water circuit still relies on gravity for circulation. Some older semi pumped systems, referred to as one pipe, have one pipe going round the heating circuit and returning to the boiler. The radiators are connected to this pipe by very short branches. Again, control is very limited and radiators furthest along the pipe work would naturally get less heat than the ones at the beginning, because each radiator in turn would lose some of the initial heat going round the system. The need for short tails to be used to connect each radiator means that the main pipe will need to run close by each one in turn. Adding a radiator to a one pipe heating system will therefore involve considerably more upheaval to do this. You cannot simply branch off at a convenient point in the existing pipe work – you will need to reroute so that the main circuit runs past the new radiator and then connect with short tails.
Two pipe systems overcome this restriction and limitation of control. The pumped flow feeds all of the radiators and a separate return pipe takes the cooled water back to the boiler. The result is that the water is actually pumped through the radiators unlike the one pipe system where the pump only acts on the water travelling round the main pipe. With the two pipe system radiators can have their flow restricted to balance the amount of hot water going to each which cannot be done with the single pipe system.
Identifying whether your type of heating system is single pipe or two pipe is straightforward. Look at the pipe at each end of the radiator and see if they both connect to the same pipe or separate ones. Connection to the same pipe means it is a single pipe system. Connection to separate pipes means it is a two pipe system.
Pumped Heating Systems
This type of system is easily identifiable by the pump being mounted on the flow from the boiler before it branches to serve the heating and hot water circuits. The pump therefore pushes the water around both and the hot water side no longer relies on gravity to circulate the water.
Obviously separate control of the heating and hot water are required so a method of isolating the two is needed. Older control is by means of a motorised valve called a three port diverter valve. This has two positions available and can therefore open the flow to the heating circuit or the hot water circuit but not both. They have a resting position and an activated position and, as hot water is usually the most important of the two, this circuit is open by default.
A better method of controlling the two circuits is by means of a mid-point three port valve. This type of motorised valve, as the name suggests, can be set to one of three positions – hot water only, heating only, or both. There are other methods of controlling the two circuits including the use of separate two port motorised valves on each of the circuits after they branch off from the pumped flow from the boiler. In addition, further control can be achieved by having more than one heating circuit and adding a two port motorised valve for each of these. This creates separate zones for the central heating allowing each to be operated independently.
Vented System and Sealed System Identification
Vented, and sealed or unvented types of central heating systems are very different. Identification of each is straightforward though. On a vented system you will have a cistern, usually in the loft, called a feed and expansion cistern. This is a smaller cistern than the cold water storage one and its function is to supply water for filling the system as well as allow for expansion as the water is heated. So, if you have a feed and expansion cistern, yours is a vented central heating system. If there is no feed and expansion cistern, you have an unvented or sealed central heating system.