What Are HVAC Relays and How Do They Work?

What Are HVAC Relays and How Do They Work?

, by Thrive Agency, 5 min reading time

Is your heating, ventilation and air conditioning (HVAC) system not turning on, even if you’ve verified that it’s getting power from your circuit breaker? Or is it cycling on and off, leading to inconsistent cooling? If so, it might be due to a faulty relay. 

An HVAC relay, also known as a control relay, is a small yet essential part of your HVAC system. It is crucial in regulating electricity between control panels and various system components. 

In this guide, we delve deeper into what HVAC relays are, how they work, potential failure scenarios and signs that indicate a need for replacement. 

What Are Relays?

HVAC relays are electromechanical switches that control the flow of electricity to different components of your HVAC system. They utilize an electromagnet to control physical contacts, functioning as an intermediary between low-voltage control circuits and high-voltage load circuits.

Control circuits handle low-voltage signals that trigger the relays. Meanwhile, load circuits carry the higher voltage that powers HVAC components. By regulating these circuits, relays control compressors, fans, and heating elements, ensuring proper temperature management and overall system functionality.

You'll typically find relays installed in key areas, such as the blower motor and the compressor in the condenser unit.

Blower motors and compressors rely on electricity to function correctly. Relays act as gatekeepers, allowing or disrupting the flow of electricity to these components as needed. For instance, when your AC system kicks on, relays enable electricity to flow to the blower motor and compressor to cool your home. Once the desired temperature is reached, relays interrupt the flow of electricity to these parts, conserving energy and maintaining comfort.

How Relays Work

Relays operate automatically in response to electrical signals from your HVAC system. Electricity is sent to the relays when your system initiates a heating or cooling cycle. 

As electricity flows through the relay's coil, it generates a magnetic field. This magnetic field pulls down (or attracts, depending on the relay design) a metal armature within the relay. The movement of the armature either opens or closes contacts within the relay. 

These contacts are responsible for completing (or interrupting) the flow of electricity in a separate circuit that powers the HVAC component (like a compressor or fan). When the electrical signal ceases, the magnetic field weakens, and a spring or gravity returns the armature to its original position, opening or closing the contacts and stopping the flow of electricity to the component.

Types of Relays

HVAC relays come in various configurations to accommodate different control scenarios:

    • Single Pole Single Throw (SPST): Basic on/off switches suitable for simple control applications
    • Double Pole Single Throw (DPST): Provides independent control over two separate circuits
    • Single Pole Double Throw (SPDT): Switches between two circuits, offering versatility in control options
    • Double Pole Double Throw (DPDT): Features two sets of contacts and two switching positions, ideal for complex control scenarios

What Happens When HVAC Relays Fail

When a relay malfunctions, it can disrupt the flow of electricity to HVAC components, leading to several consequences for your HVAC system. These may include the following:

  • Disrupted operation: A failed relay can disrupt the control signals to essential components like the compressor or blower motor. This can lead to system malfunctions.
  • Inefficient heating/cooling: Without proper control, a failed relay can prevent the system from reaching or maintaining desired temperatures.
  • Increased energy consumption: An inefficiently running HVAC system due to relay issues will likely consume more energy.
  • Overheating: A malfunctioning relay could sometimes generate excessive heat due to internal friction or electrical faults. This heat, while unlikely, could potentially damage surrounding components or in rare cases, pose a fire hazard.

Additionally, malfunctioning temperature control due to a faulty relay can lead to uncomfortable conditions for the occupants of a home or building.

Signs of a Bad Relay

To help you or your HVAC technician determine whether a faulty relay is to blame for your HVAC’s issues, here are some signs to look out for: 

  • Weak Cooling:  If the compressor relay is faulty, your AC system may fail to turn on. While the fan may still operate, you'll likely experience room-temperature air instead of cool air from the vents.
  • Blower Motor Issues: A malfunctioning blower motor relay may result in the HVAC system's fan failing to turn on, leading to inadequate airflow.
  • Audible Clicking or Buzzing: Unusual sounds from the HVAC system, such as clicking or buzzing noises, may indicate relay malfunction.
  • Frequent Cycling On and Off: If the HVAC system repeatedly cycles on and off rapidly, it could signal relay issues disrupting temperature control and system stability.
  • Inconsistent Temperature Control: Rooms experiencing temperature fluctuations or failing to reach the desired temperature may indicate relay problems affecting the operation of heating or cooling components
  • Air Conditioner Not Turning On: If the relay fails completely and gets stuck in the "off" position, it can prevent power from reaching the compressor. This would prevent the AC from turning on at all, resulting in the fan blowing warm air.

Explore Value Controls’ Selection of HVAC Relays

While HVAC relays may be small, their significance in maintaining HVAC system functionality cannot be overstated. By understanding how relays work, recognizing potential failure scenarios, and identifying signs that indicate the need for replacement, homeowners and HVAC professionals can ensure optimal system performance and occupant comfort. 

If you need high-quality HVAC relays and other HVAC parts, turn to Value Controls. We offer a comprehensive range of high-quality products to meet your needs. Browse our selection, and place your order today!


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