Primary vs Secondary Chilled-Water Loops: Smart Design & Best Practice

Mastering chilled water piping and pumping is essential for energy savings, proper control, and long-term reliability. In this post, we’ll explain primary vs secondary loops, how they work, what ASHRAE says, and how to design them right.

Introduction

Chilled water systems are the heart of large HVAC installations: central plant chillers, pumps, AHUs/FCUs, coils, etc. Many designers struggle with the choice between different piping & pumping configurations. One of the most common and effective is the primary / secondary (P-/-S) loop arrangement (sometimes with variable flow, tertiary loops, etc.).

In line with your provided info, combined with ASHRAE guidelines and hydronic design best practices, this article explains:

• What is a primary loop vs a secondary (distribution) loop
• Why use them
• How to control them
• Key design rules, trade-offs & pitfalls
• Sizing, pump head, components, and compliance with standards

This will be useful for mechanical engineers, designers, energy managers, and anyone interested in optimizing chilled water systems.

What is Primary‐Secondary Chilled Water Loop Configuration?

A primary-secondary chilled water piping arrangement splits the chilled water system into two (or more) hydraulically separated loops with a short bridge or decoupler:

1. Primary (plant) loop – this loop is dedicated to the chillers. It delivers constant flow (through each chiller evaporator) so that requirements like minimum flow, stable ΔT, chiller control, etc., are met.
2. Secondary (distribution) loop – this loop distributes the chilled water to building loads (AHUs, FCUs, coils) in response to demands. Flow is variable depending on load; control valves modulate flow (two-way valves), etc.

Between them is a decoupler (also called “bridge” or “neutral bridge”) whose function is to hydraulically separate the loops, so that the flows and pressure drops of each loop don’t interact improperly.

Why Use Primary‐Secondary vs Other Configurations

Here are the motivations and advantages:

• Stable chiller operation & minimum flow: Chillers need a certain minimum evaporator flow; primary loop ensures that regardless of how much load the building is demanding, chillers are not starved.
• Better control of ΔT (delta-temperature): Variable flow in the secondary loop plus possibly ΔT reset helps avoid “low ΔT syndrome” (where return water gets too warm, coils under-perform, energy waste).
• Energy savings: Secondary pumps (and valves) adapt to building load; pumps don’t have to always run at full constant flow. Less wasted pump power.
• Flexibility & modularity: You can stage chillers and pumps; you can add / reduce load more easily.
• Reduced mechanical stress: Pumps and chillers don’t have to handle extremes of flow/pressure when the building load is small.

ASHRAE and other design guides all support primary-secondary schemes for large chilled water systems.

Basic Components & Their Roles (From Your Data + ASHRAE)

Component	Role in Primary Loop	Role in Secondary Loop
Pumps	Constant speed, one per running chiller; ensures evaporator design flow. Possibly variable primary in some advanced plants, but must guarantee minimum flow.	Variable-speed (VFD) pumps, controlled via differential pressure sensors or coil/valve feedback, modulates to meet building load.
Decoupler / Bridge	Short pipe connecting primary and secondary; must have low pressure drop. Separates loops hydraulically.	Same – interface point. Ensures that flows in secondary do not “push back” into primary improperly.
Control Valves	Generally two-way valves in secondary coils; in primary loop, bypass or minimum flow controls to ensure chiller requirements.	Two-way valves in the coils; control the flow to meet cooling demand. Also valves upstream/downstream for balancing.
Differential Pressure & Sensors	Across evaporator, decoupler, etc. To ensure the chiller sees correct flow and differential pressure. ASHRAE suggests DP measurements and controls.	Remote DP sensors in secondary loop; measure at hydraulically critical or remote coil. Used for pump speed control or DP reset.
Pumps & Impellers	Consider head, efficiency, NPSH, motor RPM etc. Your data: horizontal split-case for large flows, end suction or vertical inline for smaller; bronze or stainless impellers per specs.	Similarly, secondary pumps are sized for head + flow of the distribution network. Use VFDs, etc.

ASHRAE Guidelines & Standards to Follow

While ASHRAE doesn’t provide one fixed recipe for every system, several standards/guidelines are particularly relevant:

• ASHRAE Guideline 36 (sequence of operation / control): includes recommendations for pump staging, DP sensor control, minimum flow, etc.
• ASHRAE/ANSI standards on energy efficiency (e.g. 90.1) for pumping power limits, pump & motor efficiency. Ensuring pumping power is minimized.
• Publications like HVAC Chilled Water Distribution Schemes (e.g. by Bhatia) and Chilled Water Plant Design Guides that outline good design practice.

Key ASHRAE-oriented design constraints include:

• Ensuring minimum flow through chillers (evaporator) at all times.
• Using differential pressure sensors and DP reset in the secondary loop.
• Using variable speed pumps where possible in the secondary (and in some cases primary) loop to handle varying loads efficiently.
• Avoiding excessive pump head, over-sizing, valve throttling losses.
• Ensuring proper sizing and minimal pressure drop in decouplers.

Detailed Design & Sizing Rules

Here are practical design “rules of thumb” (from your notes + ASHRAE/other sources) plus important checks.

Flows & ΔT (Temperature Difference)

• For many systems, ΔT (entering vs leaving chilled water) is around 5.5 °C (~10 °F) design (sometimes more in actual plants). Using that:
  Q = Load/Cp.ΔT
  Also a common conversion is ≈ 2.4 gal/min per ton (≈ 0.151 L/s per refrigeration ton) at ΔT ≈ 10 °F. Your data uses approx.
• Primary flow per running chiller = its evaporator design flow. Secondary flow = what building currently demands (which can be less or more in some cases, depending on load & decoupler behaviour).

Pump Head & Power

• The pump power equation:
  P ≈ ρ⋅g⋅Q⋅H/η
  Where;
  ρ = density of water
  g = gravity
  Q = flow
  H = head
  η = pump (and motor) efficiency.
  
  
• Typical head (pressure rise) requirements: Primary loop: lower head, because it only overcomes chiller evaporator, headers, strainers, decoupler losses. Typical values ~ 60-120 kPa. Secondary loop: larger head because you have to overcome riser + branch friction, coil DPs, control valves etc. Typical ~ 120-250 kPa.

Pump Type, Hardware & Other Details

• Large flows: horizontal split-case pumps are efficient and maintainable.
• Smaller or more compact installations: end suction or vertical inline pumps.
• Motor speed: 4-pole motors (e.g. 1500/1800 rpm, depending on region/frequency) are preferred for quieter operation and better net positive suction head (NPSH) margin.
• Impellers: bronze is common for corrosion resistance; stainless when stronger chemical / environmental demands; follow project specs.

Control Logic

• Primary pumps: constant speed usually; one pump per running chiller; interlocked with flow switch. In modern designs, you might allow variable primary but must guarantee minimum evaporator flow (via bypass(es) or other control logic).
• Secondary pumps: variable speed drives with DP sensors located at hydraulically remote / critical points (or DP reset done via valve position).
• Valves: use two-way modulating valves at coils. Also manage ΔT reset to avoid low ΔT problems.

Common Pitfalls & “Golden Checks”

To ensure your system works as intended, avoid these problems and perform these checks:

1. Decoupler / common pipe sizing and differential pressure across it If the secondary flow exceeds primary flow, the decoupler “borrows” water, and supply temp & return temp mix in undesirable ways. This can degrade cooling performance. ASHRAE design guides warn about “low ΔT syndrome”.
2. Locating DP Sensors correctly The DP sensor(s) in the secondary loop should be near the hydraulically remote (farthest) coil or critical zone, not only at the base of the riser. If not, pump speed control won’t properly respond to far-end demand.
3. Valve authority, coil DP, strainers etc must be included in head calculations Don’t guess these. Size valves, compute friction losses through coils, valves, fittings. Include strainers.
4. Maintain compliance with ASHRAE 90.1 (or your local code) This impacts allowable pumping power, motor efficiency, minimum flow conditions, etc.
5. Avoid low ΔT syndrome When ΔT is too small, water returns warm, and you lose cooling capacity.
   Solutions: ensure coils operate properly (valves close when reduced load), ensure secondary flow control works, ensure chiller bypass and minimum flow strategies are active, use ΔT reset where applicable.
   
   

6. Pump staging (and chiller staging) Make sure the number of primary pumps matches the operating chillers. Stage pumps / chillers so that primary flow ≥ secondary flow to avoid reverse flow or undesired mixing.

7. Hardware selection Proper impellers, proper materials. Motor efficiency. Avoid oversizing pumps or excessive speed.

Example: Applying the Rules

Here’s how your “mini example” can be used to illustrate the design:

• Plant capacity: 1,200 TR (≈ 4.2 MW) with design ΔT ≈ 5.5 °C
• Primary flow = 2.4 × 1200 = 2880 gpm (~182 L/s)
• Primary head ~70 kPa (~7 m) yields pump hydraulic power ≈ 12.7 kW; with ~75% efficiency, motor ~17 kW.
• Secondary at full load: head ~180 kPa (~18 m) at similar flow → hydraulic power ≈ 32.8 kW; motor perhaps ~44 kW.

This shows that the secondary loop tends to “do more head work” (i.e. higher pressure drop, more pumping energy per unit flow) because it distributes water throughout the building with all the fittings, valves, coils etc.

Variations / Alternatives (and When to Use Them)

Besides the classic primary-secondary, there are some variations, trade-offs, and other configurations to consider:

• Constant Primary / Constant Secondary: both loops have constant flow. Simple, but inefficient at part load, often leads to excess flow in the system. ASHRAE/other guides tend to move away from this for larger/modern systems.
• Constant Primary / Variable Secondary: pretty common. Secondary pumps respond to load. More efficient.
• Variable Primary Flow (VPF): in some systems, eliminate secondary pump(s), allow primary loop flow to vary. More complex controls; need bypass strategies; must ensure chiller minimum flow etc. ASHRAE provides guidelines for this.
• Primary-Secondary-Tertiary / Distributed / Zone Pumping: for very large installations or campuses, sometimes you have tertiary loops or distributed secondary/tertiary pumps near the loads for remote buildings or zones. Saves pumping energy in big systems with long pipe runs.

Putting It All Together: What To Do When Designing

Here is a checklist / design flow you (or your design team) can follow to ensure the chilled water piping & pumping system is well designed, ASHRAE-compliant, efficient, and reliable.

1. Estimate load & ΔT Determine cooling load (tonnage or kW) and choose a design ΔT (°C or °F). This gives design flow.
2. Select configuration Choose primary vs secondary vs tertiary, constant vs variable flow, etc., based on building size, range of load, budget, control capability.
3. Size primary pumps Match number of chillers, ensure minimum flow through each evaporator. Compute head (evaporator + headers + strainer + decoupler etc).
4. Size secondary pumps Determine building distribution layout: risers, branches, valves, coils. Compute pressure drop through these and design for the most hydraulically remote/critical coil. Use DP sensors to enable variable speed control.
5. Design decoupler / bridge / common pipe Keep it short; tee spacing limited (often small multiples of pipe diameters) so pressure drop is negligible. Size common pipe properly, ensuring that at no operating condition does the secondary flow significantly exceed primary flow without acceptable mixing or risk. Possibly include a check valve in the common leg if needed.
6. Select hardware & materials Pump types, impellers, motors, valves. Materials per corrosion, water quality.
7. Control strategy Interlock primary pumps with chillers. Use differential pressure sensor(s) in the secondary loop. Two-way modulating valves at coils.ΔT reset (if applicable). Minimum flow bypass or other safety to protect chillers.
8. Compliance & standards check ASHRAE 90.1 for energy efficiency. ASHRAE Guideline 36 / local equivalents for sequences. Manufacturer’s data (AHRI 550/590 for chillers, coil datasheets, etc.).
9. Commissioning & testing Measure ΔP across decoupler. Verify flows & ΔT.Check valve operation where applicable. Confirm pump speeds, sensor locations. Ensure valves have correct flow authority.

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Conclusion

A properly designed primary/secondary chilled water system can yield excellent control, energy efficiency, and reliability. It ensures chillers always have required flow, while letting secondary systems respond to building loads, avoiding waste. By following ASHRAE guidelines, sizing carefully, choosing good control schemes, and doing the “golden checks” at design and commissioning, you can avoid common pitfalls like low ΔT syndrome, excessive pumping energy, and control instability.