Common Evaporator Coil Issues Reference

Why this matters

Evaporator coil is the indoor coil where refrigerant evaporates + absorbs heat. Critical for cooling. Problems cause reduced cooling, mold, ice, leaks. The tech who can diagnose coil issues fixes core HVAC problems. This is the field card.

Evaporator coil basics

Function:

• Refrigerant enters as liquid
• Evaporates (absorbs heat from indoor air)
• Air across coil cools
• Refrigerant vapor returns to compressor

Location:

• Inside air handler OR furnace
• Above OR below depending design

Materials:

• Copper tubing
• Aluminum fins typically
• Some all-aluminum (newer)

Common evaporator problems

Dirty coil:

• Reduced airflow through
• Reduced heat transfer
• Higher pressures
• Reduced cooling

Iced coil:

• Low refrigerant
• Low airflow
• Thermostat too low
• Customer's complaint

Frozen + thawed cycle:

• Repeated ice + melt
• Water damage potential
• Customer's recurring issue

Mold growth:

• Cool damp surface
• Customer's IAQ issue
• "Dirty sock syndrome"

Coil leak:

• Formicary corrosion (common)
• Pinhole leaks
• Customer's significant repair

TXV / orifice issues:

• Refrigerant flow problems
• Affects coil function
• Different from coil itself

Diagnostic approach

For coil issues:

1. Visual inspection (filter off, coil visible)
2. Touch test (cool? frosted? iced?)
3. Pressure check (refrigerant)
4. Temperature check (supply vs return)
5. Airflow check (filter, static pressure)

Dirty coil

Common causes:

• No filter / wrong filter
• Filter loaded
• Long-term neglect
• Pet hair

Symptoms:

• Reduced cooling
• High pressures
• Sometimes ice formation
• Sometimes smell

Cleaning:

• Surface cleaning (mild)
• Pull-out + clean (heavy)
• Specialty chemicals

Coil cleaning options

Foam cleaner (no-rinse):

• Spray on
• Cleans gently
• Self-rinses with condensate

Acid cleaner:

• For heavy fouling
• Rinse required
• Sometimes harsh
• Drain protection needed

Power wash:

• For pulled coils
• More thorough
• More involved

Customer's regular maintenance:

• Annual cleaning
• Filter changes
• Prevents heavy fouling

Iced coil

Process when found:

1. Turn off AC (cooling)
2. Run fan only (helps thaw)
3. Wait (sometimes hours)
4. Investigate cause:
   • Low refrigerant?
   • Filter?
   • Blower issue?
   • Thermostat too low?

Don't restart cooling until investigated.

Causes of icing

Low refrigerant:

• Most common
• Leak somewhere
• Find + repair leak (don't just add)

Low airflow:

• Clogged filter (most common)
• Blower issue
• Duct restriction
• Closed registers

Thermostat set very low:

• Below 65°F sometimes
• System runs constantly
• Even working system ices

Customer's misuse:

• All registers closed
• Ducts blocked
• Stuff stacked against returns

When ice forms repeatedly

Address root cause:

• Find refrigerant leak
• Replace filter
• Fix blower
• Address airflow

Customer education:

• Don't keep restarting
• Address underlying

Mold on coil

Common modern issue:

• Cool damp surface
• Biological growth
• "Dirty sock syndrome"

Cleaning:

• Coil cleaner
• Sometimes deep clean
• Sometimes coil replacement

Prevention:

• UV light installation
• Coil treatments
• Regular cleaning

UV light

For mold prevention:

• Sterilizes coil
• Customer's IAQ benefit
• Bulb replacement annually
• See related article

Coil leak

Common cause:

• Formicary corrosion
• Microscopic ant-tunnel-like corrosion
• Sometimes formaldehyde-related
• Customer's home chemistry

Symptoms:

• Slow refrigerant loss
• Customer adds refrigerant repeatedly
• Eventually significant loss

Detection:

• Electronic leak detector
• UV dye
• Sometimes obvious
• Sometimes elusive

Coil leak repair

Options:

Repair specific location (sometimes):

• If leak found + accessible
• Sealant injection
• Limited success

Coil replacement:

• Major repair
• Sometimes whole air handler
• Customer's significant cost

Equipment replacement:

• If coil + age suggest
• Sometimes more cost-effective
• Customer's decision

Coil replacement cost

Significant:

• Coil itself
• Labor
• Refrigerant
• Sometimes air handler easier

Vs equipment replacement:

• Cost difference often
• Customer's age consideration
• Sometimes replace whole system

TXV (Thermostatic Expansion Valve) issues

Common on coil:

• Meters refrigerant
• Adjusts to load

Failure modes:

• Stuck open (overcharge to evaporator)
• Stuck closed (insufficient)
• Sensor issue

Diagnosis:

• Superheat measurement
• Sometimes replace
• TXV separate from coil

Capillary tube (older)

Some systems:

• Fixed flow restriction
• No active TXV
• Less common newer

Orifice (piston)

Many systems:

• Fixed restriction
• Simpler
• Per refrigerant + capacity
• Match to system

Pressure observations

Low refrigerant + coil cold:

• Low side too low
• Possible ice forming
• Investigate leak

Normal refrigerant + dirty coil:

• Both pressures elevated
• Higher head pressure
• Cleaning needed

TXV stuck open:

• Low side too high
• Liquid floods coil
• Possible compressor damage

TXV stuck closed:

• Low side too low
• Insufficient cooling
• Sometimes mistaken for low charge

Service prep

Before opening coil cabinet:

• Power off + LOTO
• Refrigerant pressures noted
• Customer's space protected
• Tools ready

Air handler vs furnace mount

Coil location varies:

• Above furnace (upflow)
• Below furnace (downflow)
• Horizontal (attic, etc.)
• Each has access considerations

Refrigerant lines to coil

Suction line:

• Larger
• Insulated
• Cold during cooling
• Brazed connection

Liquid line:

• Smaller
• Uninsulated typically
• Warm
• Brazed connection

Both to outdoor unit + coil.

Common coil installation

For replacement:

1. Recover refrigerant (per EPA)
2. Cut refrigerant lines
3. Remove old coil
4. Inspect drain pan + connections
5. Install new coil
6. Braze refrigerant connections (with nitrogen purge)
7. Pressure test (nitrogen)
8. Evacuate
9. Recharge
10. Test operation

Significant work; not DIY.

Customer's "AC not cooling"

Coil involvement possible:

• Ice
• Dirty
• Leak
• TXV issue

Diagnose properly:

• Don't assume coil
• Multiple causes

Customer's "musty smell from AC"

Mold on coil likely:

• Cleaning needed
• Sometimes UV light addition
• Customer's IAQ

Customer's "water on floor under AC"

Drain pan overflow:

• Clogged drain (see related)
• Sometimes pan damaged
• Sometimes float switch failed

Check pan condition:

• Rust
• Crack
• Replacement if needed

Customer's "refrigerant keeps leaking"

Coil leak suspected:

• Test specifically
• Locate
• Sometimes replace coil

When coil + age suggest replacement

Old equipment + coil leak:

• Customer's investment
• Repair vs replace
• Sometimes replace whole system
• Customer's decision

References

• AHRI equipment standards
• Manufacturer service manuals
• ASHRAE refrigeration guidance
• Manuall internal: Common Cooling Diagnosis, Common Air Handler Issues