Common Air Handler Issues Reference
Why this matters
Air handler is the indoor portion of split-system HVAC - blower, coil, filter, sometimes electric heat. Failures affect cooling + heating + airflow. The tech who diagnoses air handler issues fixes the indoor half of HVAC. This is the field card.
Air handler basics
Function:
- Move air through ductwork
- Cool air via evaporator coil (cooling)
- Heat air via electric strips OR heat pump (heating)
- Filter air
Components:
- Blower (motor + fan)
- Evaporator coil (cools/heats per refrigerant flow)
- Filter
- Drain pan
- Control board
- Sometimes electric heat strips
- Cabinet
Common air handler problems
Blower not running:
- Power issue
- Motor failure
- Control board issue
- Door safety switch
- Float switch (drain pan)
Blower runs but no cooling:
- Outdoor unit not running
- Refrigerant issue
- Coil iced
- Sometimes blower issue
Frozen evaporator coil:
- Low refrigerant
- Low airflow (filter, blower issue)
- Thermostat too low
- Customer's vents closed
Drain pan overflow:
- Clogged drain line
- Float switch fail
- Pan damaged
- Customer's water damage
Loud noise:
- Blower wheel imbalance
- Bearing failure
- Loose mounting
- Sometimes refrigerant noise
Customer's "weak airflow":
- Filter clogged
- Blower issue
- Duct restriction
- Wrong blower speed
Diagnostic sequence
For air handler issues:
- Power present? (Breaker, fuse, disconnect)
- Door safety switch engaged?
- Filter clean? (most common easy fix)
- Blower running? (or trying to?)
- Coil condition (iced? clean? mold?)
- Drain pan clear?
- Static pressure OK?
Blower types
PSC (Permanent Split Capacitor):
- Older standard
- Single-speed OR multi-tap
- Capacitor required
- Common diagnosis: capacitor failure
ECM (Electronically Commutated Motor):
- Variable speed
- Higher efficiency
- More complex
- Different diagnosis
Constant-speed:
- ECM motors that emulate single-speed
- Various designs
Blower motor diagnosis
For PSC motor:
- Capacitor test (microfarad)
- Continuity test (windings)
- Visual inspection
- Common: capacitor failure first
For ECM motor:
- Module communication
- Speed signal from control
- Sometimes "test mode"
- More complex
Common blower failures
PSC capacitor failed:
- Motor hums; won't start
- Replace capacitor (cheap)
Motor bearing failed:
- Loud noise
- Wobbly operation
- Replace motor
Motor coil shorted:
- Trips breaker
- Replace motor
ECM module failed:
- Won't respond to control signal
- Sometimes specific error code
- Replace ECM control OR motor
Filter issues
Clogged filter (most common):
- Reduced airflow
- High static pressure
- Reduced capacity
- Customer complaint
Wrong size filter:
- Air bypasses
- Dust to coil
- Reduced efficiency
Filter rack damaged:
- Filter doesn't seal
- Bypass air
- Sometimes after years
Filter recommendation
Frequency:
- 1-3 months typical
- More with pets, allergies
- Customer's habits + needs
MERV rating:
- MERV 8-11 standard
- MERV 13+ for allergies
- Higher MERV = higher static pressure (caution)
Evaporator coil issues
Coil iced over:
- Stop AC (let thaw)
- Diagnose cause:
- Low refrigerant (most common)
- Low airflow
- Thermostat set too low
- Address cause; don't just thaw
Coil dirty:
- Reduced heat transfer
- Higher pressures
- Cleaning required
- Sometimes deep service
Coil leaking refrigerant:
- Inside coil (formicary corrosion common)
- Hidden leak
- Sometimes whole coil replacement
- Customer's significant cost
Coil cleaning
Surface cleaning:
- Coil cleaner spray
- Self-rinsing OR rinse
- Customer's HVAC service includes
Deeper cleaning:
- Pull coil
- Specialty cleaner
- More expensive service
Drain pan issues
Clogged drain:
- Water in pan
- Float switch trips
- AC shuts off
- Common service call
Damaged pan:
- Rust through (older equipment)
- Crack
- Replace pan OR equipment
Float switch fail:
- Doesn't trip when water present
- Water overflow
- Replace switch
Electric heat strips
For heat pump systems:
- Auxiliary heat
- Backup for cold weather
- 5-20 kW typical
Common failures:
- Element burnout
- Sequencer failure
- Relay failure
- Limit switch trip
Diagnosis:
- Voltage at strips when calling
- Amperage draw
- Resistance test
Sequencer
Controls strip activation:
- Time-delayed engagement
- Multiple stages
- Prevents simultaneous high amp draw
Failure:
- Strips don't engage
- All engage at once
- Replace sequencer
Door safety switch
On many air handlers:
- Door must be closed for blower
- Safety device
- Common forgotten
Symptoms:
- Blower won't start
- Door slightly ajar
- Easy fix; just close
Float switch on drain pan
Detects water level:
- High water = trips switch
- Shuts off AC (sometimes whole system)
- Prevents flood
Diagnosis:
- Visible in drain pan
- Test by raising
- Verify shuts equipment
Control board on air handler
Sometimes:
- Coordinates blower + safety
- ECM motor interface
- Failure = various symptoms
Diagnosis:
- Like other control boards
- Power, signal, output verification
Common service requests
"My AC isn't cooling":
- Often indoor (air handler) issue
- Filter, blower, coil ice, etc.
- Diagnose systematically
"My AC blows warm air":
- Compressor not running
- Or air handler running without cooling
- Diagnose where
"My AC makes loud noise":
- Blower issue likely
- Bearing OR mounting
- Investigate
"Water on floor from AC":
- Drain pan overflow
- Clogged line
- Float switch failed
"AC keeps tripping breaker":
- Could be air handler (motor shorting)
- Could be outdoor unit
- Diagnose which
Refrigerant + air handler
Air handler holds:
- Evaporator coil (refrigerant)
- Connection to outdoor unit
- Service ports sometimes
Diagnosis:
- Refrigerant pressures (per related article)
- Superheat at coil
- Customer's whole system
Mold + bacteria
Common in air handlers:
- Cool damp coil
- Drain pan
- Customer's IAQ concern
Treatment:
- Cleaning
- UV light (preventive)
- Tablet treatments
Customer education
Maintenance:
- Change filter regularly
- Annual professional service
- Coil cleaning periodically
- Drain line treatment
Warning signs:
- Reduced airflow
- Water on floor
- Unusual noise
- Mold smell
Common installation issues
For new air handler:
- Proper mounting
- Adequate clearance
- Drain pan proper
- Combustion air (some)
- Electrical proper
- Refrigerant lines connected correctly
Variable-speed considerations
ECM blower benefits:
- Continuous low-speed
- Better dehumidification
- Energy efficient
- Comfort improvement
Customer's awareness:
- Higher cost
- Different operation
- Customer education
When to replace air handler
References
- ACCA Manual D (Duct Design)
- AHRI Equipment Standards
- Manufacturer service manuals
- Manuall internal: Common Cooling Diagnosis, HVAC Static Pressure Measurement