One advantage of hydronic heating is the ability to easily from separate zones for heating; different rooms of a building can have their own pipe section. Earlier designs would use T junctions to both separate various room sections and their piping, and to later reintroduce the water back into the system for reheating.
The introduction of new hydronic controls and condensing boilers into hydronic heating meant that other aspects of the system’s design had to be altered. It has become normal to introduce primary and secondary heating piping loop that work independently of each other. Closely spaces T’s parallel and series piping allowing boilers to maintain required, consistent flow rates.
Heated water is piped through a loop that runs under the floor, heating the room before it is returned to the boiler to be reheated. The return of the pipe loop should be close to the supply, again no more than 4 pipe diameters away. If the return water flow is less than the supply there will be some hot supply water mixed in with the cooler return water. Ideally the return and supply should have the same flow, as this will prevent any mixing of the outgoing and returning water. If the return has greater pressure it will cause some of the cooler return water to be mixed with the supply.
Parallel and series pipes must take many things into considerations. If all rooms are to be heated equally there must not be lower temperature water from a return loop reintroduced later into the system. Constant flow must be maintained to prevent this scenario. Apart from the uneven heating of rooms a poorly designed system would have abrupt temperature changes occurring whenever a section was connected or disconnected from the water flow.
Ideally, a parallel and series hydronic design is set up so each rooms supply receives water of the same temperature. The return water should have the same flow, preventing any supply and return mixture, allowing the system to connect and disconnect independent sections without affecting other sections. It is more expensive to incorporate the hydronic controls and valves to achieve this arrangement, but the result is a better performing system.