Fan relay (cube style)

For homeowners

A fan relay is a small switch inside your indoor air handler that controls when the blower fan runs and at what speed. Your thermostat puts out a low-voltage 24V signal that’s too weak to directly start a 120V or 240V blower motor. The relay is the in-between — the thermostat energizes the relay’s coil, which closes a heavier-duty contact that can carry the full line voltage and amperage of the blower.

Most older systems use a “cube relay” — a small clear plastic block that plugs into a socket. Newer systems often have the relay soldered directly to the control board. Either way, the job is the same. Signs of a failing fan relay include the blower running constantly when it shouldn’t, the blower failing to come on when the system calls for cooling or heating, or the blower running at the wrong speed (cooling speed during heat, or vice versa). Cube relays are cheap (under $25) and easy to replace if you can identify them and match the pin configuration. If the relay is integrated into a control board, you replace the whole board — which is significantly more expensive.

---

For technicians

The classic plug-in cube relay — eight pins, octal base, clear plastic housing — appears on most older residential air handlers and on a lot of commercial equipment still in service. Modern equipment has moved toward integrated control boards where the relay is soldered directly onto a circuit board, but the working principle is identical and the diagnostic approach is the same.

The job: switch the indoor blower between two roles — running at heating speed during a heat call, running at cooling speed during a cooling call, and on some configurations running independently of either call when the thermostat is set to “fan on” mode. The relay handles the switching because the thermostat’s 24V control signal can’t directly carry the amperage of a blower motor; you need a relay between them just like you need a contactor between the thermostat and the outdoor compressor.

How the contacts work. A relay contact pair has two states: normally open (NO) and normally closed (NC). “Normally” means “when the coil is de-energized.” A normally open contact is open at rest and closes when the coil pulls in. A normally closed contact is closed at rest and opens when the coil pulls in. A single-pole, double-throw (SPDT) contact stack has both — one common terminal that swings between an NC terminal and an NO terminal depending on the coil state.

The fan relay above is double-pole, double-throw (DPDT) — two independent SPDT stacks driven by the same armature, the same coil. Two completely separate circuits switched simultaneously, electrically isolated from each other. This is why a single fan relay can handle both “switch from low speed to high speed” and “energize the fan circuit at all” — the two stacks do different jobs but operate together.

The cooling/heating speed switching uses one of the stacks. Common terminal on that stack ties to the line side of the blower motor (or the appropriate motor speed tap). NC terminal ties to the heating-speed motor tap (the slower one — heat doesn’t need as much airflow as cooling and a slower fan gives the air more time in contact with the heat exchanger). NO terminal ties to the cooling-speed motor tap (faster — cooling needs more airflow because the temperature drop across the coil is bigger and you want to move more air to extract more total BTUs). When the relay is at rest, the blower runs at heating speed if it runs at all. When the cooling call energizes the coil, the contact flips and the blower runs at cooling speed.

The “fan on” function uses a different control wire from the thermostat. The thermostat has a terminal labeled “G” — the fan-only call. When you flip the fan switch from “auto” to “on” at the thermostat, 24V appears on G regardless of whether you’re calling for heating or cooling. G connects to one side of the relay coil. The other side of the coil connects to the common 24V return. Coil energizes, fan runs.

Physical operation. 24V across the coil terminals creates a magnetic field in the iron core inside the coil. The magnetic field pulls the armature down against the spring’s resistance. The armature carries both movable contact arms, so when it pulls down, the common terminal on each stack swings from the NC position to the NO position. Both stacks switch together because they’re driven by the same physical motion. Remove the 24V and the spring pushes the armature back up, contacts return to NC.

Pull-in voltage versus drop-out voltage. A relay coil designed for 24V will reliably pull in somewhere around 18 to 20 volts and reliably drop out somewhere around 8 to 12 volts. Below pull-in voltage but above drop-out, the relay is in an unstable region — it might be in the energized position, or it might not, and a small disturbance can move it either way. Brownouts and undersized control transformers can put the system into this unstable region. Symptom: the fan chatters or hums or randomly cycles. The cure is checking 24V at the coil under load and tracing back to whatever is dragging the voltage down.

Failure modes:

Pitted or burnt contacts. Every time the contacts open under load, there’s a tiny arc as the inductive load tries to keep current flowing. The arc vaporizes a microscopic amount of silver-alloy contact material each time. Multiply by tens of thousands of cycles and the contacts pit, oxide builds up, contact resistance increases, voltage drop across the contacts grows, the blower motor downstream gets reduced voltage. Symptom: blower runs slower than it should, motor draws more current trying to compensate, sometimes the motor’s internal overload starts cycling.

Welded contacts. The arc vaporizes enough metal in a single event that the two contact faces fuse. The relay is stuck in one position even when the coil drops out. The blower runs continuously, regardless of thermostat call. Customer complaint: “the fan won’t turn off.” Welded contacts are the dangerous failure mode because they don’t respond to control inputs — you have to physically remove power to stop the fan.

Open coil. The magnet wire inside the coil has broken, no continuity, no magnetic field. The relay never pulls in even when energized. Test with an ohmmeter across the coil terminals — should read somewhere around 50 to 200 ohms for a 24V coil. Infinite resistance means open coil.

Shorted coil. Insulation between turns has broken down, the coil draws too much current at 24V, the control transformer either overheats or trips its overload. Resistance check shows very low resistance, much lower than the 50-200 ohms expected.

Mechanical wear. Springs lose tension over years, armature pivot loosens, contacts no longer align properly. Less common than electrical failure but it happens. Symptom: relay clicks when it shouldn’t, doesn’t click when it should, fails intermittently.

Testing in the field. Pull the cover off the indoor unit. Identify the relay — usually labeled, but if not, follow the wires from the thermostat connections back to whatever they hit first. Power down the unit. Pull the relay out of its socket (cube relays unplug from their bases without tools). Inspect visually for burnt contacts or melted housing. Set the meter to ohms. Measure across the coil terminals (pins 2 and 7 in my diagram, but consult the relay’s actual pinout) — should read 50-200 ohms for a 24V coil. Measure across each NC pair — should read near zero ohms (closed contact). Measure across each NO pair — should read infinite resistance (open contact). Apply 24V across the coil terminals from a known-good source. The relay should click audibly and the NC/NO readings should reverse.

Replacement is a five-minute job. The new relay matches the old by pin count, voltage, contact rating, and physical envelope. Pin counts are 8 (octal) or 11 (undecal, more contacts) for cube relays. Voltage matches the control circuit — 24V for HVAC. Contact rating has to handle the load — 10-amp contacts are common, 15-amp are heavier duty for higher-current blowers. Pin compatibility is the most important — getting the pinout wrong by even one position means the relay doesn’t function or the coil sees line voltage instead of 24V.

Modern systems. Cube relays are increasingly being replaced by board-mounted relays — small black plastic relays soldered directly to a printed circuit board, often labeled K1, K2, K3 on the silkscreen. Functionally identical, mechanically the same, just packaged differently. When one of those board-mounted relays fails, you can’t pull it out and swap it — you replace the entire control board, which costs ten times what the relay would have. This is the trade-off the industry made for cheaper manufacturing and smaller control panels: easier to assemble, more expensive to service.