HVAC Relays 101 3D

Relays are electrically operated switches that generally allow a lower-voltage control circuit to control higher-current circuits to help turn things on and off safely. The main component of a relay is a coil or electromagnet, and there is also a series of electrical contacts. When current passes through the coil, it generates a magnetic field that moves a metal armature, opening or closing the attached contacts (also known as switches or contact points). These contacts allow or disrupt the flow of electricity, depending on whether they’re closed or open, respectively. These contacts may be normally open (NO) and close when the coil is energized or normally closed (NC), which open when energized. Relays provide isolation between the control circuit and the circuit being controlled. That is why we can control 240v components with much safer 24v controls in residential HVAC systems.

We must pay attention to the coil or control voltage rating, contact voltage, and amperage ratings to make sure we have the right relay for the application.

Two common relay types we see in HVAC include 90-340 and ice cube relays. 90-340 relays are double-pole, double-throw (DPDT) relays with two sets of contacts (one NO and one NC in isolated rows, making them “dry contacts”) and eight terminals; they are multi-purpose relays that can control components that draw less than 15 amps, like fan motors. The coil pulls the switch contacts to change the states of the contacts. Ice cube relays have a clear casing, a coil, an armature, and contacts.

Some controls that function like a relay include contactors, which have contact points and can handle current up to 40 amps and often control compressors; their contacts are normally open and close upon energizing. Motor starters are like large contactors that come with an overload and are designed for starting and stopping motors, providing control and overload protection.

MOSFETs also work similarly to relays, but they have different components and switch electronically rather than physically, and their three components are the gate, drain, and source. Stack sequencers use physical components to control sequences in electrical heaters and fans; they consist of a heater and a bi-metallic disc for on/off delays.

You can test many of these controls for continuity with a multimeter.

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