Traditionally all heating systems were Open Vented, meaning the water was fed from a feed and expansion tank and the boiler was vented via the flow pipe and vent pipe. Pressures in such systems are typically low. Today sealed systems are becoming more common.
Open vented systems are filled and vented to a Feed & Expansion (F&E) tank. This is usually a small plastic tank in the roof space which is fitted with a float valve to fill automatically from the mains supply. The outlet pipe is the F&E pipe and the vent pipe inverts (upside down U shape) over this tank.
Open vented systems must have a vent pipe running from the top of the boiler, rising continually, to terminate over the F&E tank in an inverted U shape. The vent pipe allows air to escape from the system but more critically it is an escape route for steam in the event of the boiler overheating (e.g. failed boiler thermostat). Part of the vent pipe is usually combined with the flow pipe, but this section of pipe must not have any valves on it that might block the vent (e.g. the pump).
The circulation to and from the radiators (rads) is always powered by a pump or circulator. The pipes from the boiler to the rads are flow pipes and those from the rads back to the boiler the return pipes.
Gravity circulation was often employed on the DHW (primary) circuit, but has now been superseded by pumped circulation. Gravity circulation relied on the weight difference between hot and cold water to drive the circulation and required large (28mm/1”) pipes to allow this.
The circulation from the boiler to the cylinder coil heat exchanger was usually known as the primary circulation. Traditionally this would be gravity circulation which would take place in large pipes (28mm/1”) powered only by the relative lightness of hot water compared to cold. Modern systems use pumped primaries to achieve faster heat transfer using smaller pipes.
Fully pumped systems use the power of the pump to drive the circulation to both the DHW cylinder and the radiator circuits. This system is taking over form the older gravity circulation formerly used for the DHW circuit.
Central heating pumps are very reliable and long lasting. They consist of an electric motor which turns an impeller which accelerates the water flow. CH pumps typically generate a maximum pressure or head of 5 or 6 metres. Pumps are often built into the boiler casing.
Sealed systems do not require a feed and expansion tank or vent pipe. The feed water is supplied manually using a filling loop. Expansion of the system water is accommodated by an expansion vessel. A Pressure Release Valve (PRV) is required as a safety device to release excess pressure. The boiler will require an overheat cutout to protect against thermostat failure.