8-pin relay pinout reference

For homeowners

When replacing a cube-style relay, the most important thing to get right is matching the pin configuration. Not all 8-pin relays are wired the same way internally — there are multiple “standard” pinouts, and plugging in a replacement with the wrong configuration can damage the relay, the control board, or both. This page is a reference card.

For homeowners: this is a tech-level reference, not a self-service guide. If your fan relay needs replacing and you’re hiring it done, what matters is that the tech reads the actual relay’s printed pinout (not what they remember from the last job) and matches a replacement that has the same numbering convention. For DIY-inclined homeowners, do not guess on relay pinouts. Use the schematic printed on the side of the original relay.

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For technicians

A few things worth saying about the 8-pin octal pinout that the diagram alone doesn’t make obvious.

The pinout shown is a standard 8-pin octal that you’ll find on common HVAC fan relays — Honeywell R8222, ICM 89-series, White-Rodgers 90-340, others. But “standard octal” has variations. There are at least three common pin assignments for 8-pin cube relays, and the wrong assumption will cost you a working relay. Before you plug a replacement in, look at the actual pinout printed on the side of the relay or on the wiring diagram inside the air handler door. The relay’s own housing will show the schematic — the coil terminals identified, the contacts mapped to pins. Match that, not what you remember from the last one.

The bottom view convention exists because that’s how you wire the socket. The socket is installed in the panel with the pin holes facing you; when you look at the socket, you’re looking at the bottom of the relay that plugs into it. So the diagram is drawn from the socket’s perspective. If you flip the relay over and look at it from the top (the dome side), the numbering rotates mirror-image. This trips people up. Always confirm whether the diagram is showing “bottom view” or “top view” before counting pins.

The key notch matters. Octal relays have an asymmetric key — a tab or notch on the base that aligns to a matching slot in the socket — so the relay can only plug in one way. The numbering starts from that key and goes clockwise (looking at the bottom view). If a relay’s key is broken off, you can plug it in 180° rotated and the coil suddenly sees line voltage instead of 24V. That kills the relay immediately and may damage other components. Don’t reuse a relay with a damaged key.

Coil polarity. The coil terminals are AC-rated in most HVAC relays, which means they’re non-polarized — either coil pin can be hot, either can be common. Doesn’t matter which way you wire it. DC-coil relays exist (rare in residential HVAC) and they ARE polarized; reversing them might not damage the relay immediately but the coil pulls in weakly or not at all. If the relay has a ”+” marking next to one of the coil pins, it’s a DC coil and the marked pin is the positive supply.

Contact ratings. The contacts are voltage-rated and current-rated, and the two ratings are independent. A relay rated “10A at 240V AC, 24V coil” can switch up to 10 amps on the load side at up to 240 volts AC, controlled by 24V on the coil. Switching less than 10 amps is fine; switching more than 10 amps is not. Switching DC loads dramatically reduces the contact rating — DC doesn’t have the zero-crossings that AC has, so the arc doesn’t self-extinguish, and contacts pit much faster under DC inductive loads. Most HVAC applications are AC and this isn’t an issue, but if you’re using a cube relay to switch a DC-powered accessory (some condensate pumps, some zone dampers), check the DC contact rating, which is usually about a third of the AC rating.

Two poles, isolated. The two poles being electrically isolated means you can use them for completely separate jobs. One common HVAC use: pole 1 switches the cooling-speed blower tap, pole 2 simultaneously energizes a humidifier or condensate pump that should only run when the system is cooling. Or pole 1 switches the blower while pole 2 sends a signal to an outdoor unit’s lockout circuit. The two switches happen together but the circuits don’t talk to each other through the relay.

The NC contacts often go unused. In a lot of HVAC fan relay applications, only the COM and NO terminals get used — wire goes to COM, switched load goes to NO, NC pin is left floating. That’s fine. The NC contact is still there mechanically; it’s just not connected to anything. Some installers will jumper the NC pin to a known harmless point as belt-and-suspenders, but it’s not required. What is NOT fine is using both NC and NO simultaneously to switch the same circuit between two destinations without understanding that for a tiny window during the transition, both contacts are open at the same time (called “break before make”) — fine for most loads, but not for circuits that need continuous connection like certain microprocessor-controlled boards.

Sockets and bases. The relay plugs into a base, and the base is what’s actually wired to the system. Bases come in two main mounting styles: DIN-rail (a metal rail in the panel that the base clips onto) and surface-mount (screwed directly to the cabinet wall). The base’s pin holes correspond to the relay’s pin numbers — base pin 1 connects to relay pin 1, and so on. The base itself has terminals on its other side where the actual wires attach via screws. So the path is: wire from system → base terminal screw → base pin hole → relay pin → relay internal connection. When you replace a relay, you’re only swapping the cube — the base and all its wiring stays put. The wires don’t have to be re-landed unless the base itself is damaged.

Vibration kills cube relays. Air handlers shake when the blower starts and stops, and over years that vibration loosens the relay in its socket. A loose relay has high contact resistance at the pin-to-socket interface, which causes voltage drop, which causes the relay to operate marginally or intermittently. The fix is a hold-down clip — a small metal spring that clamps over the top of the relay and keeps it seated in the socket. Most professional installations include them; cheap installations skip them. A relay that “started acting weird” after years of working can sometimes be fixed by simply unplugging it and plugging it back in firmly, then adding a hold-down clip if one wasn’t there.