What it is

A relay is a coil-operated switch — energize a small control coil and it throws a set of contacts to switch a separate circuit. It's the same idea as a contactor but built for smaller loads and more varied switching jobs. In HVAC you meet two broad families:

  • Definite-purpose relays — purpose-built for HVAC loads like a blower motor or a condenser fan. The classic is the "fan relay," a quarter-style relay with spade terminals, often a SPDT or DPDT switch and a 24V coil. Rugged, simple, made to switch a PSC motor.
  • General-purpose relays — the "ice-cube" relays that plug into a socket. Square or cylindrical clear/black bodies, blade or octal pins, numbered terminals. Used on control boards, in panels, and anywhere you need a compact switch with a defined pin layout. Common configs are 8-pin (DPDT) and 11-pin (3PDT).

This article is about telling them apart and matching a replacement by coil voltage, contact arrangement, and current rating.

How it works

Send voltage to the coil, the coil's magnetic field pulls an armature, and the armature moves the contacts. The contact terms you need:

  • NO (normally open) — open when the coil is de-energized, closed when energized. Most "turn something on with a call" jobs use NO contacts.
  • NC (normally closed) — closed at rest, opens when energized. Used to break a circuit on a call, or for fail-safe logic.
  • Common (C / pole) — the shared terminal the NO and NC swing between.

Pole and throw describe how many circuits and positions:

  • SPST — single switch, on/off (one NO or one NC).
  • SPDT — one common that switches between an NO and an NC (single pole, double throw).
  • DPDT — two independent SPDT switches in one relay.
  • 3PDT — three of them (the 11-pin ice cube).

In the field

Identify the family. Spade terminals and a chunky body screwed to a bracket = a definite-purpose fan relay. Numbered pins that plug into a socket = a general-purpose ice-cube relay. The ice-cube usually has the pin diagram printed on the side.

Read the coil voltage. Printed on the relay — 24V AC is the workhorse, but you'll find 120V and 208/240V coils, and DC coils on some board-mounted relays. The coil voltage must match the control circuit that drives it.

Match the contact configuration. Count poles and throws. An 8-pin ice cube is DPDT; an 11-pin is 3PDT. A fan relay might be SPDT or DPDT. You can sometimes use a relay with more poles than you need (leave the extra unused), but never fewer.

Match the contact current rating. Relays are rated for resistive and inductive loads — often something like 10A or 15A at the line voltage, with a separate (lower) inductive/motor rating. The blower motor you're switching is inductive, so respect the inductive number, not the resistive one.

On ice-cube relays, follow the printed pin diagram. Don't guess — the numbering is standardized for a given base (octal vs blade), but coil pins and contact pins vary by configuration. The diagram on the relay and the socket markings tell you which pins are coil, which are common, and which are NO/NC.

Normal values & targets

  • Coil voltage: 24V AC most common; also 120V, 208/240V AC, and 12V/24V DC on board relays. Must match the driving circuit.
  • 24V AC coil resistance: roughly tens to a few hundred ohms depending on size (a small ice-cube coil reads higher than a beefy fan-relay coil). Compare to known-good; you want sensible resistance, not open or shorted.
  • Contact ratings: commonly 10A or 15A resistive on general-purpose ice cubes; definite-purpose fan relays often rated for the FLA of a typical PSC blower. Always check the inductive/motor rating for motor loads.
  • Standard pin counts: 8-pin = DPDT, 11-pin = 3PDT (square blade ice cubes). Octal (round) bases run 8 or 11 pins too.

Common faults & what they mean

  • Coil gets voltage, relay doesn't click → open coil or stuck armature.
  • Relay clicks but the load doesn't energize → burnt or pitted contacts not making; check voltage across the closed contacts under load.
  • Load won't turn off → welded contacts holding closed, same failure mode as a contactor.
  • Intermittent operation, especially when warm → marginal coil or contacts; thermal expansion opens a borderline connection. Replace it.
  • Ice-cube relay loose in its socket / corroded pins → high-resistance pin connection mimics a bad relay. Reseat or replace the socket before condemning the relay.

Tech tips & gotchas

The biggest field error is wiring an ice-cube relay by assumption instead of by the printed diagram. An 8-pin and an 11-pin look similar at a glance but have completely different pin maps. Read the label.

NO vs NC is the second trap. If a circuit energizes something that should turn off (or vice versa) after a relay swap, you probably landed on the wrong throw. Common-to-NO makes on a call; common-to-NC breaks on a call.

A definite-purpose fan relay is the right tool for switching a PSC blower in an air handler. Don't substitute a tiny general-purpose relay for a motor load it isn't rated to switch — it'll weld.

When an ice-cube relay "fails" repeatedly, suspect the socket. Corroded or spread socket clips create resistance and heat that kills relays. Replace the socket, not just the relay.

Keep a couple of common ice cubes (8-pin and 11-pin, 24V coil) and a universal fan relay on the truck. They cover a huge range of board and panel repairs.

Safety / code notes

De-energize before pulling a relay from a live socket — the line-voltage contact pins can be hot even when the coil side is dead. Match coil voltage and contact ratings (including the inductive/motor rating) to the circuit; the relay's listing depends on staying within its rated load. On board-mounted relays that are soldered in, the whole control board is usually the replacement part — don't field-solder a relay onto a furnace board unless the manufacturer supports it.