There are two basic categories of pipe heating systems: one-pipe and two-pipe systems. The type of system installed will have an influence on dimensioning radiators and piping. A general characteristic of a two-pipe system is that the radiators are dimensioned for the same flow temperature and the same temperature difference.
Two-pipe system with direct return
In a two-pipe direct return system, the total pipe length from the pump to and from each radiator is shorter for the radiators closer to the pump and longer for the more distant radiators. For this reason, the differential pressure can be significantly higher at the closest radiator than at the most distant radiator. This must be taken into consideration when designing the system. The advantage of the direct return system is that pipe routing is more straightforward compared to the reverse return system.
Two-pipe system with reverse return (Tichelmann system)
In a two-pipe reverse return system the total pipe length from the pump to and from each radiator is the same for all radiators on the same story. This gives a favorable water distribution.
Two-pipe system with overhead piping
The distribution pipe is located in the suspended ceiling, and air vents are installed in central positions. This type of system is common in larger buildings as it is relatively easy to balance and regulate the system. It is also easy to extend the system.
Two-pipe system with underfloor piping
The system is very common in houses and in buildings where the piping cannot be fitted in the available ceiling space. The distribution pipes are placed under the floor. In multi-story buildings air vent screws are needed on the radiators.
Grundfos circulator pumps and in-line single stage pumps are typically used for two-pipe heating systems in domestic and commercial heating applications.