When 1/2-inch tubing is used, circuit lengths should generally be limited to 300 ft. With 3/8-inch tubing, the circuit length should not exceed 250 ft. In larger buildings, these circuit-length requirements result in several tubing circuits spread out across the floor framing.
Instead of a traditional centrally located manifold station with closely spaced tubing connections, install two long copper headers perpendicular to the joists over the full width of the floor. Tap each floor circuit into these headers as they pass by. This almost eliminates “leaders” from where the circuit heats the floor to where it connects to the manifold. A typical arrangement is shown in Figure 4. A photo of an installed extended manifold serving an underfloor tube-and-plate system is shown in Figure 5. A close-up of a typical circuit take-off with ball valve is shown in Figure 6.