Relays, Contactors, Sequencers

A relay is an electrically-operated switch. A small current through the coil magnetically pulls an armature that opens or closes one or more sets of contacts, switching a larger current through the load. Every relay — from a $3 ice-cube control relay to a $40 industrial contactor to a $20 heat sequencer — is the same fundamental device: coil on one side, contacts on the other.

The three labels — relay, contactor, sequencer — describe the same mechanism applied to different scales and purposes. Learning to see them as one family, with different packaging, makes diagnosis much simpler.

The three labels

Relay. Low-current switching (typically under 10 A), general-purpose. “Ice-cube” plug-in relays, board-mounted relays, 24V control relays. Used when a small signal needs to switch a moderate load — a thermostat signal switching a blower motor, a pilot signal switching a circulator.

Contactor. High-current switching, purpose-built for motor loads. Contactors have larger contacts sized for motor inrush currents, arc-suppression features, and are rated in amperes for specific motor categories (AC-1 for resistive, AC-3 for motor switching). The outdoor AC condensing unit’s contactor is the classic example — 24V coil pulls in 30–40 A compressor and fan contacts.

Sequencer. A time-delayed relay, usually operated by a bimetal strip that heats up and bends when a heater element inside the sequencer is energized. Used in electric furnaces to stage multiple heating elements — the first element comes on immediately, the second 15–30 seconds later, the third another 15–30 seconds later. Staggering the startup prevents a huge inrush that would trip the main breaker.

Pole and throw configuration

The contact configuration of a relay is described by its number of poles (independent switch circuits) and throws (positions the contacts can be in).

For field work, the most important distinction is NO (normally open) vs NC (normally closed):

• Normally open contacts are open when the coil is de-energized, and close when the coil is energized. Most load-switching relay contacts are NO.
• Normally closed contacts are closed when the coil is de-energized, and open when the coil is energized. Used for interlocks, safety bypasses, and “de-energize to alarm” circuits.

A schematic will label contacts as NO or NC near each contact symbol. Reading these correctly is essential for tracing signal paths.

Testing coils

Power off. Disconnect the coil leads. Measure resistance.

• Healthy 24V coil: typically 40–120 Ω. Exact value depends on the coil but within spec is “not open, not shorted.”
• Open coil: OL (infinite). Coil has failed — replace the relay.
• Shorted coil: near 0 Ω or very low (a few Ω). Coil has failed, will draw huge current when energized, likely has already damaged the transformer or blown the fuse.
• 120V or 240V line-voltage coil: resistance is much higher, often 800–3000 Ω.

If coil resistance is in spec, coil is probably fine. Corroborate by energizing and watching for physical operation (armature pulls in, audible click).

Testing contacts

With relay de-energized:

• Normally open contacts: should read OL (infinite).
• Normally closed contacts: should read near 0 Ω (closed).

Energize the coil. Contacts should switch states:

• NO contacts: now close, read near 0 Ω.
• NC contacts: now open, read OL.

Welded contacts read near 0 Ω when they should read OL. This is a failure — contacts have fused and won’t open when the coil drops. Welded contacts on a compressor contactor are a classic serious fault because the compressor runs even when the thermostat is calling off. Replace immediately.

Pitted or burned contacts read higher than 0 Ω when closed, often 5–50 Ω or intermittent. This is developing failure. Resistance in the contact causes voltage drop, heating, and progressive deterioration. Replace before it welds.

Contactor chatter

A contactor that “chatters” — repeatedly closing and opening with an audible buzzing — is receiving inadequate coil voltage. The coil has just enough current to pull the armature partway but not hold it. The partial contact creates a voltage drop on the supply side, the armature drops back, the drop recovers, the coil pulls again, over and over.

Causes:

• Low 24V control voltage (often a weak transformer or a partial short elsewhere on the secondary).
• Partially-failed coil with increased resistance due to insulation degradation.
• Mechanical binding on the armature that requires more pull than the coil can deliver.

Chattering contactors destroy their contacts quickly due to repeated arc-welding and opening. Diagnose and fix within minutes, not hours.

Sequencers

Electric furnaces use sequencers to stage strip-heater elements on and off with deliberate delays. Typical 20 kW electric furnace:

• Stage 1 sequencer: contacts close 15–30 sec after energizing, energize 5 kW heating element.
• Stage 2 sequencer: contacts close 30–60 sec after Stage 1, energize second 5 kW element.
• Stage 3: another 15–30 sec, third element.
• Stage 4: final delay, fourth element.

Each sequencer is a bimetal heated by a small heater element in the coil. The warming bimetal bends over seconds and eventually pushes a contact closed. On de-energize, the bimetal cools and contacts reopen after another delay.

Failure modes:

• Doesn’t close at all: heater element in coil failed, or bimetal broken.
• Closes but doesn’t open: bimetal stuck, or contacts welded.
• Timing off: bimetal drifted. Sequencers have rated delay ranges; replace if outside spec.

From the field

Call on an electric furnace that tripped the main breaker every time all stages came on. The homeowner had replaced the breaker twice, each time to the same result. Measured the current draw — it was well within the 60A breaker rating on each individual stage, but on simultaneous startup inrush, all four stages pulled current together.

Two of the four sequencers had failed to “closed” mechanically — the bimetal was stuck in the closed position. Instead of staging, all four elements were connecting as soon as the contactor closed. The total startup inrush was tripping the breaker. Replaced the two failed sequencers, staging resumed, breaker held. The replacement breakers had been the right part and the wrong diagnosis.

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Check your understanding

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01A compressor contactor's coil reads 45 Ω. 24 VAC is present at the coil terminals. The armature pulls in and hums but the contacts never fully close. What's happening?

AThe coil is good but contacts are welded openBThe contactor is chattering due to marginal voltage or a partially-seized armature — coil isn't pulling in fullyCThe thermostat is faultyDThe transformer is oversized

02On an electric furnace, all four heating stages turn on simultaneously when the thermostat calls, tripping the main breaker. Sequencers and their contactor both check out electrically. What's the remaining possibility?

AThe thermostat is in emergency heat modeBOne or more sequencers has bimetal stuck in the closed position, defeating the staggered startupCThe transformer is oversizedDThe breaker is undersized for the equipment

03A DPDT relay is used on an older furnace fan center. When the coil is energized, what happens to the contacts?

AAll contacts closeBBoth NO contacts close and both NC contacts open simultaneouslyCOnly the first pole activatesDThe contacts reverse every half second

Relays, contactors, and sequencers are the most mechanical components in an HVAC control system. They wear in predictable ways and fail in diagnosable patterns. Testing is fast: meter the coil, cycle it, meter the contacts, done. Replacement is usually straightforward. The hard part is remembering to actually check them rather than assuming the control board is the problem — a bad relay can look exactly like a bad board, but costs a fifth as much.