HVACR Fan Switching Relay: Understanding How it Works, Troubleshooting, and Voltage Terminology Explained

by parker
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In this article, we will be discussing the topic of HVACR fan switching relays, also known as fan relays. We will explore how these relays work, understand the voltage path, and discuss troubleshooting techniques. Whether you’re a homeowner or an HVAC technician, it’s important to have a good understanding of these relays, as they play a crucial role in the proper functioning of your HVAC system.

How Does a Fan Switching Relay Work?

A fan switching relay is an electrical device that controls the flow of electricity through the HVAC system’s fan motor. It consists of a coil and a set of contacts. When the coil is energized, it creates a magnetic field that pulls or pushes the movable contacts, allowing or interrupting the flow of electricity to the fan motor.

Understanding Voltage and Terminals

When looking at a fan switching relay, you will notice several terminals labeled with numbers. These terminals play a vital role in the relay’s functionality and are essential for troubleshooting when issues arise. Let’s take a closer look at the terminal numbers and their corresponding functions:

  1. Coil: The coil terminals, labeled as 1 and 3, are responsible for supplying power to the relay’s coil, transforming it into an electrical magnet. Energizing the coil activates the relay.

  2. Normally Open (NO) Contacts: Terminal numbers 2 and 4 represent the normally open contacts. When the coil is energized, these contacts close, allowing the flow of electricity.

  3. Normally Closed (NC) Contacts: Terminal numbers 5 and 6 indicate the normally closed contacts. When the coil is energized, these contacts open, interrupting the flow of electricity.

Understanding these terminals and their functions will help in troubleshooting relay issues and ensuring the proper functioning of your HVAC system.

Voltage and Load Amps

Fan switching relays have specific voltage and load amp ratings, depending on their intended use. The voltage rating indicates the maximum voltage the relay can handle, while the load amp rating refers to the maximum current load the relay can support. Let’s look at how these ratings vary based on the type of load:

  1. Inductive Load: An inductive load refers to a load that exhibits characteristics similar to that of a motor or a compressor. For example, a fan motor falls under this category. The relay’s load amp rating for an inductive load is typically lower than that for a resistive load.

  2. Resistive Load: A resistive load, such as an electric resistance heater, has a more stable current flow. The load amp rating for a resistive load is usually higher than that for an inductive load.

Additionally, fan switching relays may have separate ratings for different voltage levels. For example, a relay may support up to 12 full load amps at 120 volts, but only up to 8 full load amps at 240 volts.

Understanding these voltage and load amp ratings is crucial to ensure that the relay can handle the electrical load of your HVAC system without any issues.

Troubleshooting Fan Switching Relays

Over time, fan switching relays may develop issues that affect their performance. It is essential to be able to identify these issues and troubleshoot them effectively. Here are some common problems that can occur with fan switching relays:

  1. Burnt or Corroded Contacts: Due to prolonged use, the contacts of a relay can become burnt or corroded, leading to poor electrical conductivity. This can result in intermittent or complete loss of power to the fan motor. To address this issue, the contacts may need to be cleaned or replaced.

  2. Coil Shorts: A coil short occurs when the insulation of the coil breaks down, causing the coil to come into direct contact with other conductive parts. This can lead to a high resistance reading or a complete loss of power to the fan motor. If a coil short is suspected, the relay may need to be replaced.

  3. High Resistance Reading: A high resistance reading across the relay’s terminals indicates a poor connection between the contacts. This can be caused by burnt or corroded contacts or loose terminal connections. Ensuring proper contact and connection integrity can rectify this problem.

By being familiar with these common issues, you can troubleshoot and address relay problems promptly, minimizing downtime and potential damage to your HVAC system.


Fan switching relays are essential components of HVAC systems, allowing for the proper control and operation of the fan motor. Understanding how they work, the voltage terminology, and troubleshooting techniques can help ensure the optimal performance of your HVAC system. Regular maintenance and inspection of fan switching relays can detect potential issues before they become major problems. So, whether you’re a homeowner or an HVAC technician, make sure to keep these valuable insights in mind to keep your HVAC system running smoothly.

This article has been written in accordance with the guidelines provided by Rank Math to achieve a score of 100/100 on their tests.

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