Taco SR502-2 2-Zone Circulator Relay with Priority, Old Part #SR502-4



Overall, the Taco SR502-2 provides a compact and efficient control solution for managing 2 zones and a circulator pump in hydronic heating systems. Its priority function, diagnostic lights, compatibility with various valve types, built-in transformer, and circulator relay make it a versatile choice for small hydronic systems with zoned temperature control.


  • Manages water flow to 2 zones in a hydronic heating system, enabling efficient zone-based temperature control.
  • Activates and deactivates a circulator pump to maintain water circulation throughout the system when needed, optimizing performance.
  • Offers a priority function, allowing one zone to take precedence over the other when activated, ensuring immediate comfort in critical areas.

Key Features:

  • 2-zone capacity: Suitable for small hydronic systems with 2 distinct zones.
  • Switchable priority: Prioritizes one zone for faster heating when needed.
  • External diagnostic lights: Facilitate troubleshooting by indicating system status and zone operation.
  • Compact design: Optimizes space utilization in control panels.
  • Compatible with various valve types: Works with 2-, 3-, or 4-wire zone valves for flexibility in system design.
  • Built-in circulator relay: Eliminates the need for a separate relay to control the circulator pump, simplifying wiring and installation.
  • Field-replaceable transformer: Facilitates easy maintenance if needed.
  • Universal thermostat compatibility: Works with a wide range of thermostats.
  • Fuse-protected: Ensures electrical safety.


  • Hydronic heating systems (radiant, baseboard, fan coil)
  • Closed cooling circuits (zoned air conditioning)
  • Hydronic heating and cooling systems (combined heating and cooling zones)

Things to Consider:

  • System compatibility: Verify compatibility with your system’s voltage, control signals, and valve types.
  • Installation complexity: While DIY installation is possible, consulting a qualified professional is recommended for optimal performance and safety.
  • Expansion: If you need more than 2 zones, the SR506-2(4) model offers 6 zones and expandability up to 120 zones.


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