What it is

A relay is the most basic building block of control logic: a coil that, when energized, mechanically throws a set of contacts. That's it. But chain a few relays together and you can build real decision-making — "run the compressor only if the fan is proving airflow AND the high-pressure switch is closed AND there's a call." That kind of conditional control built from relay contacts is relay logic, and the safety/permission conditions baked into it are interlocks.

Even on boards and DDC systems that have replaced discrete relays with software, the logic is the same and there are almost always still physical relays and interlocks in the chain (contactors, isolation relays, proving switches, safety strings). Reading relay logic is a core troubleshooting skill.

How it works

A relay has a coil (the input — energize it to operate the relay) and contacts (the output — they switch a separate circuit). The magic is that the coil and contacts are electrically isolated, so a small/low-voltage signal on the coil can switch a bigger or different circuit on the contacts.

Contacts come in two states:

  • Normally open (NO): open when the coil is de-energized, closes when energized. Use it to turn something on when a condition is met.
  • Normally closed (NC): closed when de-energized, opens when energized. Use it to turn something off (or break a circuit) when a condition is met.

"Normally" means the de-energized, resting state. From those two contact types you build logic:

  • AND logic = contacts in series. Put two NO contacts in series and the load only runs if both are closed (both conditions true). This is how a safety string works — every interlock in series must be satisfied or the circuit is broken.
  • OR logic = contacts in parallel. Put two contacts in parallel and the load runs if either one is closed.

An interlock (also called a permissive) is exactly this: a contact placed in series with the equipment's start circuit that must be satisfied before the equipment can run. Classic examples:

  • Fan/airflow proving: a sail switch or pressure switch proves the blower is actually moving air; its contact must close before the heat or compressor is allowed to energize. No airflow → no firing.
  • High/low-pressure switches: in series with the compressor circuit; if a pressure goes out of range the switch opens and drops the compressor.
  • Door/limit switches: a furnace blower-door switch breaks the 24V circuit when the door's off so the unit can't run.
  • Pump/flow proving on hydronics: the boiler won't fire unless flow is proven.

There are also relays whose only job is to separate circuits rather than make a decision:

  • Isolation/pilot relay: a small relay used so one device can switch another without tying their circuits together — e.g., isolating two transformers so you don't parallel them, or letting a low-current thermostat output drive a higher-current load through the relay's contacts instead of through the stat directly. The thermostat energizes the pilot relay's coil; the relay's contacts do the heavy switching.

In the field

  • Find the resting state first. Determine whether each contact is NO or NC and whether its coil is energized right now. Half of relay troubleshooting is knowing what "normal" looks like for that contact in the current condition.
  • Trace the series safety string. When equipment won't start, walk the interlock string in series: which permissive isn't made? An open sail switch, pressure switch, flow switch, or limit anywhere in that series chain breaks the whole circuit. Measure voltage across each contact — the open one drops the voltage.
  • Energize the coil, confirm the contacts move. Test a relay by confirming (a) the coil gets its rated voltage when it should, and (b) the contacts actually change state when the coil energizes. A coil that's powered but contacts that don't switch = failed relay (welded or burned contacts, broken mechanism).
  • Measure across contacts, not just to ground. A closed contact reads ~0V across it (it's a short); an open contact in a live circuit reads source voltage across it. That across-the-contact reading instantly tells you which interlock is open.
  • Check the coil voltage type. Relay coils come in 24V, 120V, 240V, etc. The wrong coil voltage won't pull in (too low) or burns out (too high). Confirm the coil rating matches the circuit driving it.
  • Use a pilot relay to offload a stat or isolate transformers. If a thermostat output is switching a load that draws too much, or you need two power sources kept separate, an isolation relay is the clean fix — coil on the control side, contacts doing the work on the load side.

Normal values & targets

  • Closed contact: ~0V across it (essentially a short); full load current passes through.
  • Open contact in a live circuit: reads the source voltage across the open gap (e.g., ~24V across an open 24V interlock).
  • Coil voltages: common HVAC relay coils are 24V AC (control circuits), 120V, or 240V (line-voltage relays/contactors). Match the coil to the driving circuit.
  • Contact arrangement shorthand: SPST (one circuit), SPDT (one common, NO + NC), DPDT (two circuits) — describes how many circuits the relay switches and whether it offers both NO and NC.
  • Interlock string: every permissive in series must be closed; one open contact breaks the whole start circuit.

Common faults & what they mean

  • Equipment won't start, "permissive not made": an interlock in the series safety string is open — sail/airflow switch, pressure switch, flow switch, door/limit switch. Measure across each in the string to find the open one.
  • Relay coil energized but nothing switches: failed relay — contacts welded open, burned, or mechanism broken. Confirm coil voltage is present, then condemn the relay.
  • Relay chatters or buzzes: marginal coil voltage (low control voltage, weak transformer, power-stealing stat), or worn relay. Verify coil voltage under load.
  • Load runs when it shouldn't (won't shut off): welded-closed contacts stuck shut, or a NC contact that should have opened didn't. The relay's contacts failed closed.
  • Coil burned out / relay dead: wrong coil voltage applied, or coil failure. Verify the coil rating matches the circuit.
  • Two transformers smoked / fuse pops: circuits that should have been isolated got paralleled — a missing isolation/pilot relay, or contacts wired so two power sources tie together. Isolate them with a relay.

Tech tips & gotchas

  • "Normally" = de-energized resting state. NO is open at rest, NC is closed at rest. Burn that in — every relay diagram depends on it.
  • Series = AND, parallel = OR. That single idea decodes most relay logic. A safety string is a bunch of ANDs in series; any one failing kills the whole circuit.
  • Measure across the contact to find the open one, and don't stop at "the coil has voltage" — confirm the contacts actually moved. In a live series string the open interlock reads source voltage across it while the rest read ~0V; a powered coil with contacts that never switch is a failed relay.
  • Use isolation/pilot relays to protect stats and separate power. Don't switch a heavy load directly through a thermostat or tie two transformers together — drop a pilot relay in and let it do the dirty work.
  • Interlocks exist for a reason — never jumper one out to "test." Bypassing a proving switch or pressure safety to get a unit running defeats a safety that's there to prevent firing without airflow or running at dangerous pressure. Find why it's open instead.

Safety / code notes

  • Interlocks and proving switches (airflow proving, pressure limits, flow switches, door/limit switches) are safety devices; they must remain functional and in the circuit — bypassing them to force operation can allow firing without airflow or operation at unsafe pressures, contrary to the equipment's listed safety requirements.
  • Line-voltage relays and contactors switch full voltage (often with higher current and 3-phase on commercial gear) — de-energize and verify zero energy before working on the power side.
  • Isolation between separate power sources (e.g., two transformers) must be maintained per the control design; improperly paralleled sources can backfeed and create a shock or equipment-damage hazard.