The diagrams below show various distribution methods for large heating systems.
For clarity some pipes and fittings are omitted such as; cold feed, isolating valves, non-return valves, safety devices, sensors, air vents and drains.
The heat emitters may not necessarily be radiators, other heat emitters may require a 3-port diverting valve instead of a mixing valve.
U Shaped Header
The system below has a U shaped header pipe for both flow and return water.
The main reason for this header is to provide a position for immersion sensors to measure average flow and average return water temperatures.
Also a U shape takes on less space than a long header.
Low Loss Header
The drawing below shows one method of distribution.
The Low Loss or low pressure drop Header is sized so that the Primary pumps do not interfere with the Secondary pumps.
Also the pumps are sized accurately so that interference does not occur.
The Header has a large enough volume of water so that it acts like a vessel to which hot water is supplied from the boilers and extracted by the Secondary pumps.
The above system is divided between the primary and the secondary circuits.
The primary circuits are for circulation around the boilers.
The secondary circuits are for water circulation to the heat emitters.
It is a good idea to have a run on time control for the primary pumps so that they continue to run some time after the boiler is switched off, this dissipates heat away from the boiler and reduces the risk of tripping the boiler high limit thermostat.
Fast Heat-up Systems
The drawing below shows a method of heating up the boilers quickly from cold.
The motorised two-port valves on the bypass pipe open when the boilers are starting from cold and the motorised valves on the flow close.
This enables the boilers to heat up quickly, thus preventing differential expansion and uneven heating of the boiler over its length.
A similar system could be operated with a 3-port motorised valve instead of 2 No. 2-port valves.
the boilers have been heated up, then the
2-port motorised valves operate in the reverse mode to allow hot water into the
The drawing below shows an Injection Circuit for a constant volume variable temperature (CVVT) heating system.
The Secondary circuit operates with a variable water temperature.
The primary circuit may operate with a higher water temperature thus making the primary pipes smaller.
The regulating valve is used to balance the circuits so that the design flow rate of water flows in the secondary circuit.