Monitoring Relays in HVAC and Building Services
What role does a monitoring relay play in HVAC and building services? A monitoring relay in a chiller plant, air handling unit (AHU), or pump room continuously measures a motor's supply conditions, a temperature sensor, or a liquid level input against an adjustable threshold and switches a change-over output contact per IEC 60947-5-1 when the value leaves the set window. Skip it and a compressor started on a reversed phase runs backwards within seconds, or a condensate pump left running dry burns its impeller before the building management system (BMS) shows anything wrong. This covers phase-sequence and voltage protection on compressor and fan motors, current-based dry-run and belt-break detection on pumps and fans, PTC/PT100 temperature protection on compressor and motor windings, conductive level sensing on condensate pans and cooling-tower basins, and how the relay's dry contact feeds the BMS alarm chain.
Phase and Voltage Protection on Chiller Compressors and AHU Fans
Chiller compressors and AHU supply fans are three-phase motors, usually started direct-on-line through a contactor or via a VFD. A phase-sequence relay wired ahead of the starter checks L1-L2-L3 rotation before the coil is allowed to energize; wrong rotation on a scroll or screw compressor drives it backwards and can damage the valve plate within one start. Devices such as the Schneider RM17TE or ABB CM-MPS combine phase sequence, phase loss, asymmetry, and over/undervoltage checks in one 17.5-22.5 mm DIN-rail module, and several need no separate auxiliary supply — they draw power straight from the measured three-phase feeder.
Formula: Voltage Asymmetry — Source: IEC 60034-26 (motor derating basis), used as the relay's asymmetry threshold
A% = (Vmax dev / Vavg) x 100
| Symbol | Description | Unit |
|---|---|---|
| A% | Voltage asymmetry (unbalance) | % |
| V max dev | Largest deviation of one phase voltage from the average of the three | V |
| V avg | Average of the three phase-to-phase voltages | V |
A typical relay is set to trip somewhere around 5-10% asymmetry. Below that, the motor still runs, but an unbalanced supply drives disproportionately higher current in one winding and shortens insulation life over years of chiller operation, not the single event a phase-loss relay is built to catch.
Current Monitoring for Pump and Fan Dry-Run and Belt-Break Detection
Chilled-water and condenser-water pumps, and belt-driven cooling-tower fans, share one failure mode: the motor keeps drawing current close to no-load while doing no useful work. An undercurrent relay on a built-in shunt or an external CT catches a snapped fan belt, a pump that has lost its water column, or a coupling that has sheared, all of which sit below the running current a standard thermal overload is set to react to. Overcurrent detection on the same device catches the opposite fault — an impeller jammed by debris.
What we see in the field: undercurrent thresholds set too close to the no-load current chatter on pump start-up ramp, so most panel builders add several seconds of trip delay rather than tightening hysteresis. That rides through the ramp without missing an actual dry-run event later in the run.
PTC and PT100 Temperature Protection on Compressors and Motors
Scroll and screw compressors, and larger AHU/fan motors, embed a PTC thermistor in the winding. A dedicated PTC relay, built to IEC 60947-8, trips when the embedded sensor's resistance rises past its reference point on winding overtemperature — independent of the current the motor is drawing, which matters because a partially blocked condenser can overheat a compressor that is not yet in overcurrent. PT100/PT1000 relays instead read an actual temperature value, which is what a chiller with a discharge-temperature interlock or a boiler flow sensor needs rather than a simple go/no-go trip.
Series such as the Schneider RM35ATR and ABB CM-TCS/CM-MSS read PTC or Pt100 inputs and switch a change-over output on the fault. Neither series is a substitute for the compressor's own internal protection module where the manufacturer supplies one — the external relay is the layer that gets the fault out to the panel and the BMS.
Level Monitoring for Condensate Pans, Cooling Towers, and Storage Tanks
Conductive level relays sense liquid between two or more probe electrodes and switch an output for pump up/down control or dry-run protection. Sensitivity, expressed in kOhm, is adjustable for the liquid's conductivity — this is the setting that trips up commissioning teams on HVAC work more than on any other application.
Condensate from an AHU coil is close to distilled water — its conductivity is far lower than tap water in a cooling-tower basin or a raw-water storage tank. Leave the factory-default sensitivity in place on a condensate pan relay and it can read "dry" with the pan full, so the sensitivity needs raising until it reliably reads the condensate itself, then a margin retained above the tower or tank setting. Devices such as the Schneider RM22LG/RM35LV or ABB CM-ENS/CM-ENE cover this range, and models with "positive safety" logic are built to fail to the safe (pump-off) state if a probe wire breaks.
Wiring Monitoring Relay Outputs into the BMS Alarm Chain
A monitoring relay does not interrupt the compressor, pump, or fan motor itself. Its SPDT or DPDT output contact drives something else — the starter contactor's control circuit, a PLC digital input, or a hardwired point back to the BMS front end. Fail-safe (normally-energized) wiring means the output also drops out, and the alarm fires, if the relay loses its own auxiliary supply, which is the behavior most specifications want for a plant room nobody is watching overnight.
Manual (latched) reset versus automatic reset is a site-specific call. Automatic reset on a nuisance-prone voltage dip lets the chiller restart itself once the supply recovers, which some operators want; manual reset on a compressor thermal trip forces someone to look at the unit before it runs again, which most specifications for critical cooling — data center chiller plants in particular — require.
Selecting and Fitting Monitoring Relays into HVAC Control Panels
HVAC control panels are usually tighter on DIN-rail space than a straight motor control center, especially in packaged AHU or rooftop unit enclosures. Module width matters here: 17.5 mm single-function relays fit where a 22.5 mm combined phase-voltage-current unit does not, but the combined unit removes two or three separate devices and their wiring from the same panel. Fault-memory (the relay retains the trip after a power cycle rather than auto-clearing) is worth specifying on compressor and fan-motor circuits where a nuisance trip masking a real fault is the worse outcome.
Both the Schneider Zelio Control (RM17/RM22/RM35) and ABB CM range cover the phase, voltage, current, level, and temperature functions an HVAC panel needs, and stock availability, not a small feature gap between the two, usually decides which one goes in a given panel. See the full monitoring relay engineering guide for how the ranges compare function by function, and the phase sequence and phase failure relay and PTC and PT100 temperature monitoring relay articles for the setting detail behind the two functions used most in chiller plants.
The monitoring relay is one layer in the motor circuit, not the whole of it. It still sits alongside a motor protection circuit breaker for short-circuit and overload duty and a contactor to actually switch the motor — the monitoring relay's contact is what tells that contactor's control circuit to drop out, or tells the BMS something is wrong before the breaker ever needs to trip. Stoklink stocks the monitoring and control relays covering all five functions above for chiller, AHU, pump, and cooling-tower panels.
Frequently Asked Questions
Does a monitoring relay stop the compressor by itself?
No. Its output contact is a signal, not a power path — it drives the starter contactor's control circuit, a PLC input, or a BMS alarm point. The contactor or breaker does the actual switching.
What's the difference between a monitoring relay and a protection relay in an HVAC panel?
A monitoring relay (IEC 60255 measuring class, IEC 60947-5-1 control-circuit class) checks one or a few parameters against a threshold. A protection relay is typically a numeric device with multiple protection functions and event logging, more common on medium-voltage or generator feeders than on a compressor or fan branch circuit.
Why does a condensate pan level relay need a different sensitivity setting than a cooling-tower relay?
Condensate is close to distilled water and conducts poorly compared to tap or tower water. A sensitivity setting tuned for a tower basin often fails to detect condensate at all, reading the pan as dry when it is actually full.
Should HVAC monitoring relay outputs be set to manual or automatic reset?
It depends on the fault and the criticality of the load. Automatic reset suits transient supply dips where unattended restart is acceptable; manual reset suits compressor thermal trips and any circuit where the specification requires someone to check the equipment before it runs again, common on critical cooling duty.
Can one relay cover phase sequence, phase loss, and voltage on the same chiller feeder?
Yes — three-phase supply relays such as the Schneider RM17TE and ABB CM-MPS combine phase sequence, phase loss, asymmetry, and over/undervoltage detection in one module, which is standard practice ahead of a compressor starter.
Do monitoring relays need a separate power supply?
Some do, some do not. Many three-phase supply monitors draw power directly from the measured lines and need no auxiliary supply; single-function voltage, current, level, and temperature relays more often need a separate 24V DC or 230V AC auxiliary feed — check the specific series before wiring the panel.
Conclusion
An HVAC control panel puts a monitoring relay to work in five places: phase and voltage ahead of the compressor and fan starters, current on pumps and belt-driven fans, PTC or PT100 on motor windings, conductive level on condensate and tower basins, and a dry contact feeding the BMS alarm chain in all four cases. None of it replaces the breaker or the contactor. It is the layer that catches a fault condition, reversed rotation, a dry pump, an overheating winding, an empty pan, before that condition turns into a burned motor or a chiller down for the day.
