Free Primary-Secondary Pumping & Tees Tool

1. The Necessity of Hydraulic Decoupling

In multi-zone hydronic systems, connecting multiple zone circulator pumps directly in series with a modern high-efficiency modulating-condensing (mod-con) boiler creates severe hydraulic conflicts. High-head boiler heat exchangers starve zone loops of flow, and opening one zone circulator changes flow rates across all other active zones.

Primary-Secondary Pumping decouples the boiler loop (primary) from the building heating zones (secondary) using a short stretch of interconnecting pipe called the common pipe.

2. The 4-Pipe Diameter Rule ($S le 4D$)

The two tees connecting the secondary loop to the primary loop must be installed as close together as physically possible: $$S le 4 cdot D_{ ext{pipe}}$$ When $S le 4D$, the length of the common pipe is so short that its friction loss is practically immeasurable ($Delta P < 0.01 ext{ ft of head}$). Because there is no pressure drop across the common pipe, operation of the secondary circulator creates zero pressure differential across the primary loop, and vice versa. The two circulators operate completely independently.

3. Blended Secondary Supply Temperatures

The temperature delivered to the secondary zones depends on the flow ratio between loops:

• $Q_{pri} ge Q_{sec}$: The boiler supplies more water than the zone demands. Excess hot water flows forward through the common pipe back to the boiler. Secondary supply temperature is full boiler temp: $T_{sec} = T_{pri}$.
• $Q_{sec} > Q_{pri}$ (Common in Radiant Heating): The secondary loop demands more flow than the boiler delivers. The deficit must be drawn backwards through the common pipe from the secondary return: $$T_{sec} = rac{(Q_{pri} cdot T_{pri}) + (Q_{sec} - Q_{pri}) cdot T_{ret}}{Q_{sec}}$$ This natural blending allows high-flow, low-temperature radiant floor loops ($110^circ ext{F}$) to be fed safely from higher-temperature boilers without complex mixing valves.