Hydronic Pipe Sizing: Method

Sizing Criteria

Hydronic pipe is sized between two limits. Too small and the friction rate and velocity climb: the pump head, the pump and the energy grow with them, and small pipe above about 4 ft/s (1.2 m/s) carries flow noise into occupied rooms. Too large and the pipe, insulation, hangers and the water volume cost more than they need to; below about 1 ft of water per 100 ft (100 Pa/m) with a low velocity the pipe is oversized, and very low velocities fail to carry air to the vents. The customary band is 1 to 4 ft of water per 100 ft of pipe, with velocity held under 4 ft/s in 2 in. and smaller and under 10 ft/s (3 m/s) in larger pipe against erosion of the elbows.

The loss is Darcy-Weisbach: h = (f L/D + ΣK) V²/(2g), head of water at the pipe's temperature. The friction factor f comes from the Colebrook equation at the segment's Reynolds number and relative roughness, 0.00015 ft (0.046 mm) for commercial steel, 0.000005 ft for copper and plastic; the tool iterates it to convergence. Fittings and valves are entered as a sum of loss coefficients K referred to the segment's velocity pressure.

The pump sees the series path to the farthest (critical) load: the sum of its segment losses plus the equipment head of that circuit, the coil or terminal, its control valve, the strainer and the chiller or boiler. Parallel branches that are not on the critical path do not add to the pump head; they are balanced to it. The pump flow is the first segment's flow. Hydraulic power is ρ g Q H; the shaft and motor powers follow from the efficiencies.

The friction chart draws every Schedule 40 size between 1 and 12 ft/s and marks the segments on it, which is the quickest way to see which segments to move a size up or down. Water properties are taken at the entered temperature: hot water is less viscous, so a heating system runs at a slightly lower friction factor than the same pipe on chilled water.