How to Wire a Fan Relay for HVAC? (Troubleshooting & Replacing)

An HVAC system consists of several components that work in unison to keep your house, building, or office at the desired temperature.

Your motor must be working at optimum range to ensure that your indoor surroundings stay warm in the winter and cool in the summer.

And it is the HVAC fan relay that is responsible for ensuring that your motor keeps running at the right speed.

If your relay malfunctions, it can cause problems with your motor and HVAC system. So, in this guide, we’ll be discussing how you can identify issues with your relay, how to replace fan relay in HVAC systems, and how to manage fan relay wiring in HVAC units.

Read on for all the help you will ever need with your fan relay!

What Is A Fan Relay And How Does It Work?

A fan relay is a crucial part of blower fans in an HVAC system, and they’re essential for effective temperature control for the heating or air-conditioning of your property.

It is located close to the blower motor and consists of a coil and some spring-loaded changeover contacts.

The thermostat is responsible for setting the temperature and ensuring that it is maintained. It depends on sensors that monitor the temperature of the surroundings.

If the temperature deviates from the set threshold, a signal is sent to the furnace or air conditioner.

On receiving this signal, the blower sucks the air from the AC or heater, and the fan pushes it through the vents.

Air conditioning system installation embedded on wall of building.

Signs That Your Fan Relay Is Failing: Troubleshooting Guide

In an HVAC system, the relay essentially functions as an on/off switch, supplying power to the fan and letting the heated air circulate within the surroundings.

The relay is responsible for starting and ending the heating process within an HVAC system since it controls the flow of current between the motor and the power source.

Due to constantly switching on and off, the motor relays are susceptible to wear and tear, and you might need to replace them after some time.

This wear and tear usually give way to some tell-tale signs that your fan relay has been compromised, such as:

Blower Motor Is Not Functioning

If the blower motor is not moving at all, it’s possible that there could be an issue with the HVAC fan relay and not the motor itself.

The HVAC blower fan relay is responsible for supplying current to the motor, and a non-functioning relay means there is no power being delivered.

Blown AC Blower Motor Relay Circuit Fuse

Relays not only transmit current but also act as a protection against sudden electric surges. In the case of an uneven distribution of load, excess current will enter the fuse, causing the circuit to fuse.

This blown fuse acts as a shield for your system against any electrical damage or burnt wires.

Burned Or Melted Relay

Since blower motors deal with large amounts of current, they are exposed to excess heat and often do not distribute the load properly.

This causes excess accumulation of heat if left unaddressed, leading to the melting down of the relay’s plastic casing and the components around it.

In certain instances, the heat can penetrate through the panel and fuse box and damage them. If this is the case, the HVAC system will not distribute the air until this issue is resolved.

How To Replace A Fan Relay In HVAC

If you are wondering how to replace fan relay in HVAC systems, we have the answer!

You can DIY the replacement, but before embarking on this job, you must follow all the safety requirements and make sure you have all the necessary tools and equipment.

The tools you will need include:

  • Allen wrenches
  • Deep-socket set
  • Flashlight
  • Screwdrivers

Make sure to consult the owner’s manual, which might include HVAC fan relay troubleshooting tips and advice to help you find your way through the relay, motor, and HVAC system.

Take a detailed look at the fan and watch out for bad fan relay symptoms in HVAC systems like any signs of loose or damaged components. Check the fuse panel and the switch as well. After that, disconnect the motor from the main power supply.

Follow the schematics laid out in the owner’s manual to trace your way to the relay. The connector can be identified as a small black box with wires running through it from both ends.

After locating the switch, disconnect the relay. You might need to apply pressure to disengage the relay from the housing if it doesn’t come undone at the first time of asking.

Take the damaged relay to an electronics store and ask them for the correct replacement for your motor. After that, install the new relay and perform a test to ensure it is properly connected.

How To Wire A Fan Relay: Step By Step Guide

Now that we know the basics, it’s time to learn how to wire a fan relay HVAC system. We’ll be considering a simple four-switch relay as an example by using an HVAC fan relay wiring diagram:

The coil will be energized as the current flows through terminals 85 and 86, labeled as Relay (+) and Relay (-), respectively.

Generally, coils do not have polarity, but solid-state transistor-based relays do feature a polarized coil.

In an open relay, the contacts close as soon as the current flows through the coil, which is signaled by the clicking sound of the magnetic field pulling the contacts in.

The power circuit will have to be routed through the pins labeled 30 and 80. This is the circuit that you want to close. As the current starts to flow, it presses the switch, closing or opening the circuit.

Before handling fan relay wiring in HVAC units, you must consult the owner’s manual. Otherwise, you may make irreparable mistakes that even an HVAC professional can’t fix!

People Also Ask (FAQ)

What Does A Bad Relay Sound Like?

A bad relay may cause a clicking sound when turned on.

Can Relays Get Weak Over Time?

Yes, relays can get weak over time due to overuse, exposure to heat, or uneven load distribution.

How Much Does It Cost To Fix A Relay Switch?

It can cost you around $80 to $160 to fix a relay switch.

How Do You Test An HVAC Fan Relay?

The easiest way to test a relay is to use a multimeter set at the ohmmeter setting and measure the various resistance values of the relay.


Wiring a fan relay by yourself can be a tricky task; however, our step-by-step instructions will help you get the job done with little or no hassle. The HVAC fan relay wiring diagram, in particular, can help you if you want to do the job yourself.

As mentioned before, make sure that you address the owner’s manual every step of the way to avoid costly errors!

Josh Mitchell

Josh Mitchell

My name is Josh and I am obsessed with DIY and improving my family home. HVAC topics can be tricky for homeowners so I decided to share my knowledge on the subject. When I am not working on DIY projects, you can find me at the beach or my local coffee shop.