| Zone Name | Heat Load (kW) | Zone ΔT (°C) | Zone Type | |
|---|---|---|---|---|
| Total Load: 0 kW | Total Secondary Flow: - | |||
Per-Zone Secondary Flow
Full Piping Breakdown
| Parameter | Value | Notes |
|---|
How to Use the Primary-Secondary Piping Calculator
Enter the boiler's supply and return temperature for the primary loop. Primary return is typically a flow-weighted blend of all the secondary zone returns, so it often sits between the coolest and warmest zone return temperatures. Use the boiler sizing calculator if you haven't confirmed the boiler's operating parameters yet.
Add one row per zone - each zone typically corresponds to a distinct area or emitter type with its own circulator. Enter the zone's heat load and its own delta T, which may differ from other zones (a radiant floor zone might use 8°C delta T while a baseboard zone uses 14°C). Use the delta-T heating calculator to help choose appropriate values per zone.
Each zone's flow rate is calculated independently from its own load and delta T using the standard flow equation. These flow rates size each zone's own circulator pump and secondary piping, separate from the primary loop sizing.
The primary loop flow rate is calculated from the total combined load across all zones and the primary supply-return delta T (which may differ from any individual zone's delta T). This flow rate, combined with your target velocity, determines the recommended primary pipe diameter.
The diagram shows the standard closely-spaced tee arrangement - install each secondary zone's supply and return tees within 4 pipe diameters (or 12 inches, whichever is less) of each other on the primary loop, in that order, to maintain proper hydraulic separation. Confirm this spacing during installation regardless of the calculated pipe size.
Primary-Secondary Piping for Multi-Zone Hydronic Systems
Primary-secondary piping is the standard approach for hydronic systems serving multiple zones with different flow requirements, different temperatures, or different control needs, without the zone circulators interfering with each other or with the boiler's own flow requirements. Understanding how to size both the primary and secondary sides correctly avoids callback-generating problems like uneven heating, boiler short-cycling, and zone circulator conflicts.
Why Hydraulic Separation Matters
In a simple single-loop hydronic system, one pump moves water through the boiler and all the heat emitters in series or a single parallel arrangement. This works fine for small, simple systems, but breaks down once you need multiple independently-controlled zones, since a single pump can't serve zones with different flow needs while allowing each to turn on and off independently without disrupting the others. Primary-secondary piping solves this by hydraulically isolating a primary loop (which the boiler sees) from one or more secondary loops (each zone's own circuit), so each side can operate largely independently.
How Closely-Spaced Tees Work
The key detail that makes primary-secondary piping function correctly is the closely-spaced tee pair: two tees on the primary loop, one for the secondary supply connection and one for the secondary return connection, placed close enough together (typically within 4 pipe diameters, and never more than about 300mm / 12 inches apart) that the pressure drop between them is negligible. This near-zero pressure differential means the secondary zone's circulator, when running, draws exactly the flow it needs without fighting the primary pump, and when the secondary circulator is off, primary flow simply bypasses across the closely-spaced tees without diverting through the idle zone. Tees spaced too far apart create meaningful pressure drop between them, which causes the two pumps to interact unpredictably - sometimes starving a zone of flow, sometimes causing flow to reverse through an idle zone.
Primary Loop Flow and Temperature
The primary loop's flow rate is determined by the total combined heat load across all connected zones and the primary loop's own delta T - which is often, but not always, different from any individual zone's delta T. If the primary loop runs 70°C supply and 55°C return (15°C delta T) while individual zones use anywhere from 8°C to 14°C delta T, the primary flow calculation uses the primary loop's own 15°C figure applied to the combined total load, not a simple sum of the individual zone flows. This is an important distinction: primary flow and the sum of secondary flows are generally not equal, because different delta Ts across the primary and secondary sides mean the same heat quantity moves at different flow rates on each side.
Sizing Individual Secondary Zones
Each secondary zone is sized independently using its own load and its own chosen delta T - see the delta-T heating calculator for guidance on selecting appropriate zone delta Ts. A radiant floor zone typically uses a narrower delta T (around 8-11°C) to maintain even floor temperature, while a baseboard or panel radiator zone can use a wider delta T (12-20°C) for reduced flow and smaller pipe. Each zone gets its own circulator sized to its own calculated flow rate and head loss - use the system curve calculator for each zone's pump selection.
When to Use a Low-Loss Header Instead
For systems with many zones (roughly five or more) or multiple boilers, a low-loss header often simplifies piping compared to a long primary run threaded with multiple closely-spaced tee pairs. A low-loss header is a larger-diameter vessel that accomplishes the same hydraulic separation goal through its own low internal velocity and cross-sectional area, with all zones and boilers connecting directly to the header rather than tapping off a shared primary pipe. See the low-loss header calculator for sizing guidance when a header makes more sense than sequential closely-spaced tees.
Common Installation Mistakes
The most frequent primary-secondary piping error is placing the tees too far apart, defeating the hydraulic separation the design relies on. Another common mistake is installing the supply and return tees in the wrong order relative to flow direction, which can cause a zone to receive cooled return water instead of hot supply water under some flow conditions. A third issue is undersizing the primary loop pipe for the combined total load of all zones, causing excessive primary-side pressure drop and forcing the boiler's own circulator to work harder than necessary. Careful attention to tee spacing, connection order, and primary pipe sizing - verified against this calculator's output - avoids these problems.
Frequently Asked Questions
Closely-spaced tees - typically within 4 pipe diameters, and never more than about 300mm (12 inches) apart - create near-zero pressure differential between the primary loop and each secondary connection. This ensures the secondary zone circulator sees minimal interference from the primary pump, and vice versa. Without close spacing, or with tees too far apart, the two circulators can fight each other, causing flow reversal, reduced flow to a zone, or unpredictable flow splitting. Correct tee spacing is what makes the hydraulic separation actually work - see the piping diagram above for the standard arrangement.
Both achieve hydraulic separation between a primary loop and secondary zones, but differently. Primary-secondary piping uses closely-spaced tees directly on the primary pipe, with each zone tapping off its own tee pair. A low-loss header uses a larger vessel that the primary loop feeds into and secondary loops draw from, achieving separation through low internal velocity rather than tee spacing. Low-loss headers are often preferred for systems with many zones or multiple boilers since they simplify piping layout. See the low-loss header calculator for sizing when a header is the better fit for your project.