6.3.1 — Capacitors & Potential Relays

Run capacitors maintain the phase shift needed for single-phase motor operation. A weak capacitor causes the motor to draw higher current, run hotter, and lose torque. Start capacitors provide a brief boost for high-inertia loads (compressors, large fans); a potential relay drops the start capacitor out of circuit once the motor reaches approximately 75 % of running speed.

Testing Capacitors

Measure capacitance with a capacitor-function multimeter or dedicated capacitor tester. Discharge the capacitor first by briefly shorting its terminals through a 20 kΩ resistor — never short them directly as the current spike can damage the meter.

Potential Relays

A potential relay coil opens its normally-closed contact when counter-EMF from the motor reaches the relay’s “pick-up” voltage. If the relay contact fails to open, the start capacitor remains in the circuit and overheats, eventually failing open or short. If the relay coil is open, the start capacitor never engages and the motor won’t start under load. Test the relay coil resistance (typically 8,000–20,000 Ω) and verify contact continuity (NC contact should read zero ohms at rest).

6.3.2 — Control Components: Contactors, Relays & Solenoid Coils

Contactors

The compressor contactor is a high-current switching device controlled by the 24 V thermostat circuit. Contactor problems that reduce efficiency:

• Pitted or burned contacts: Increased contact resistance raises heat and voltage drop; motor receives reduced voltage. Inspect contacts visually; replace if pitting depth exceeds 1/32 in. or if contacts are welded.
• Weak hold-in coil: Contacts chatter under load, causing arcing and further pitting. Check coil voltage and resistance.
• Welded contacts: Compressor runs continuously regardless of thermostat; replace immediately.

Measure voltage drop across closed contacts; more than 0.2 V drop on a 24 V control circuit or more than 2 V on a 240 V power circuit indicates excessive resistance.

Relays

Control relays switch lower-current loads (fans, solenoids, defrost heaters). The same inspection and testing principles apply as for contactors. Additionally, check that relay mounting is secure — vibration loosens relay bases and causes intermittent contact problems that are difficult to reproduce during service visits.

Solenoid Coils

Solenoid valves control liquid line, hot gas, and reversing valve functions. A solenoid coil that draws excessive current runs hot and fails prematurely; a coil with an open winding prevents the valve from operating at all.

1. Measure coil resistance with power off. Compare to manufacturer specification.
2. Apply rated voltage; verify the valve actuates (audible “click” or feel shaft movement).
3. Measure operating amperage; should match nameplate within 10 %.
4. Feel the coil body after 5 minutes of operation; it should be warm but not too hot to hold.

6.3.3 — Motors, Overloads & Phase Imbalance

Motor Operating Efficiency

Electric motors in HVAC systems include compressor motors, condenser fan motors, evaporator fan motors, and pump motors. Motor efficiency deteriorates through:

• Winding degradation: Heat cycles and moisture degrade insulation; measure winding resistance and megohm insulation resistance annually on critical motors.
• Bearing wear: Increased friction raises motor temperature and amp draw. Listen for grinding, rumbling, or squealing; replace bearings before failure causes winding damage.
• Contaminated windings: Grease or dust on motor windings reduces heat dissipation. Clean with approved electrical contact cleaner.

Overload Protectors

Internal thermal overloads protect motor windings from sustained overcurrent. An overload that trips repeatedly indicates an underlying problem — never bypass or defeat an overload to restore operation. Instead, investigate:

• Measure motor amp draw against nameplate RLA/FLA — over 10 % above FLA warrants investigation
• Check for supply voltage within ±10 % of nameplate
• Verify capacitor condition (single-phase motors)
• Check for mechanical binding — fan blades, bearings, compressor seizure
• Confirm airflow is adequate — an obstructed motor casing overheats

Voltage Imbalance (Three-Phase Systems)

Voltage imbalance in a three-phase supply causes unequal current in each winding, creating a hot winding and reducing motor efficiency and life. NEMA MG-1 limits imbalance to 1 %; a 3.5 % imbalance increases temperature rise by approximately 25 %.

Wiring Inspection

Loose or corroded wire connections are a leading cause of electrical efficiency loss and fire hazard. High-resistance connections under load cause voltage drop and localised heating. During any service visit, check:

• All terminal screws in the disconnect, contactor, control board, and motor terminal box are tight
• No signs of overheating (discoloration, melted insulation, carbon tracks)
• Wire gauge matches the circuit breaker rating and equipment nameplate
• Wiring is secured away from hot surfaces, moving parts, and sharp edges

6.3.4 — Supply Voltage Conditions

Both high and low supply voltage reduce system efficiency and equipment life. Most HVAC equipment is designed to operate within ±10 % of nameplate voltage.

6.3.5 — Additional Service Requirements

Beyond electrical and mechanical diagnostics, scheduled maintenance includes several service requirements that directly sustain system efficiency and indoor air quality.

Water & Chemical Treatment (Cooling Towers / Evaporative Condensers)

Open evaporative systems concentrate dissolved minerals as water evaporates. Without treatment, scale deposits insulate heat-transfer surfaces (even 1/32 in. of calcium scale reduces heat transfer efficiency by ~8 %), and biological growth (Legionella bacteria) presents a serious health risk.

• Measure conductivity (total dissolved solids) and compare to target cycles of concentration
• Bleed-down the sump to control mineral concentration when conductivity exceeds set point
• Add biocide on schedule; verify residual with test strips
• Inspect drift eliminators and distribution nozzles for blockage
• Record all water treatment activities in a log book

Coil Cleaning

Coil cleaning frequency depends on environment: commercial kitchen exhaust near condensers may require monthly cleaning; typical office buildings annually. Use the appropriate cleaner for the coil material:

Condensate Traps

Systems with negative-pressure drain pans (blow-through AHUs) require a condensate trap to prevent air from being drawn backward through the drain line, which would bypass the drain pan and spray condensate into the airstream. A properly sized trap must have a water seal height greater than the static pressure at the drain pan (in inches of water column).

Corrosion Testing

Coastal, industrial, and agricultural environments expose equipment to corrosive atmospheres (salt spray, ammonia, hydrogen sulphide). Testing methods:

• Visual inspection: Look for white or green oxidation on aluminium fins, green patina on copper, rust on steel cabinets
• Copper corrosion coupons: Standardised coupons placed in the airstream; measured mass loss quantifies corrosion rate
• Electronic corrosion monitor: Provides real-time data for critical applications
• Where corrosion is confirmed, consider coil coatings (phenolic, epoxy) and more frequent coil cleaning

Filter Changes

Air filters are the first line of defence for all downstream components. A loaded filter increases static pressure across the air handler, reduces airflow, and causes all the downstream problems discussed in Section 6.2.

Measure static pressure drop across the filter bank at every visit. Compare to the clean filter pressure drop from the manufacturer’s data. A filter loaded to 2× the clean drop is ready to change regardless of visual appearance — dark-coloured filters can be loaded with fine particles not visible to the eye.