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What is a bypass valve in hydronic HVAC systems?

bypass valve hydrolic system mixing valve diverting valve

A bypass valve in a hydronic system is a control valve that allows water to flow around part of the system instead of through it. It is used to maintain proper flow, pressure, and temperature stability when system demand changes.

Think of it as: A controlled “alternate water path” that keeps the system stable when coils or zones close.

Bypass Valve Common Uses

What is a bypass valve used to do?

At a high level, a bypass valve helps maintain flow, stabilize pressure, and protect equipment when system conditions change.

A bypass valve is commonly used to:
  • Maintain minimum flow through boilers or chillers
  • Prevent pump dead-heading when control valves close
  • Control differential pressure in variable flow piping systems
  • Reduce noise and valve hunting at terminal units
  • Stabilize system temperature by blending supply and return water
  • Protect equipment from low-flow or high-pressure conditions
Bypass valves are commonly found in:
  • Chilled water systems
  • Hot water heating systems
  • Boiler loops
  • Primary/secondary pumping systems
  • Older constant-flow systems with 2-way control valves
Typical scenario: As zone or coil valves close, system flow drops. The bypass valve opens to maintain safe minimum flow and stabilize pressure.
Common Applications

3-Way Control Valve Bypass

Two common configurations are used when talking about 3-way control valve bypass applications: mixing and diverting.
Mixing Valve

Blends supply water with return water. This configuration is commonly used in heating systems where temperature control is the main goal.

Diverting Valve

Sends water either through the coil or around it through a bypass path. This is common in older constant-flow systems and applications requiring minimum flow through a circuit.

Used in: Older constant-flow systems, bypass arrangements, and applications where maintaining minimum circuit flow is important.

Special note on valves we carry: All Belimo 3-way CCV ball valves and their current generation 3-way globe valves (2" and below) can be applied in both mixing and diverting configurations for easier selection.
Configuration takeaway: If the valve is used before the coil, it is a diverting configuration. If it is used after the coil, it is a mixing configuration.
Common Configurations

Some Other Common Bypass Valve Configurations in Hydronic HVAC

Beyond mixing and diverting valve arrangements, there are several other bypass approaches commonly used in hydronic HVAC systems depending on the control strategy, load profile, and equipment protection requirements.
Fixed (Manual) Bypass

Configuration: A simple manual balancing valve or globe valve set during commissioning and left in a fixed position.

Used in: Small systems or stable-load applications where operating conditions do not change significantly.

Where It Fits

Manual bypass arrangements are often used where simplicity is preferred and the system does not require automatic adjustment as flow conditions change.

Limitations: A fixed bypass does not automatically respond to changing loads and can waste energy if set incorrectly.
Differential Pressure Bypass Valve (DPBV): Automatically modulates based on the pressure difference between supply and return. It opens as system pressure rises due to closing control valves.
Temperature-Controlled Bypass Valve: Opens or modulates based on water temperature to help maintain proper temperature conditions through the circuit.
Primary/Secondary Loop Bypass (Decoupler Pipe): A pipe connecting supply and return between primary and secondary loops. It is not a valve, but it functions as a hydraulic bypass path.
Modern Systems

How Are Bypass Valves Used in Modern Systems?

In modern hydronic design, bypass valves play a much smaller role than they once did—especially in systems built around smarter flow control strategies.
Common Modern Features

Modern systems often use VFD-driven pumps and pressure-independent control valves (PICVs) to dynamically control flow based on actual system demand.

What That Means for Bypass Valves

In many cases, bypass valves are reduced in size or eliminated entirely, and are only used as a safety measure or minimum-flow backup.

Why this matters: Modern systems control flow dynamically instead of diverting excess water through a bypass path, which improves efficiency and reduces the need for traditional bypass arrangements.
Key Takeaways

A bypass valve in hydronic HVAC is:

A valve that maintains safe flow and pressure by allowing water to circulate around part of the system when demand decreases.

Common configuration take away :If it is used before the coil, it is a diverting construction, if it is after the coil, it is a mixing construction

If you have any further questions contact us and we will be happy to assist. 

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