COOLING COIL FREEZE-UP: The indoor coil develops either a coating of thick, white frost or thick ice.
- May only freeze a portion of the indoor coil or the entire coil.
- Ice may extend from the indoor coil all the way to the compressor in the outdoor unit.
- Generally causes a loss of airflow from the registers.
| Qty |
Possible Causes: |
| 2 |
Low Refrigerant Charge |
| 2 |
Low/No Airflow |
| 2 |
Bad Expansion Valve |
| 2 |
Stuck Compressor Contactor |
|
Cold Outdoor Air |
|
Cold Return Air |
CHECKOUT:
1. Check Contactor
- Turn the thermostat system switch to the "Off" position.
- Check the outdoor unit to make sure that it quits running.
- Units Still Running --> Check for 24V across the contactor coil
- Contactor Pulled-In + No Coil Voltage = Stuck Contactor
- Coil Voltage = Shorted Low Voltage Wire
2. Check Indoor Coil
- DO NOT attempt to check the refrigerant charge with ice on the coil.
- If possible, remove the coil access panel to check for ice.
- If ice is still present, set the thermostat fan switch "On" and wait for the coil to thaw.
3. Check the Blower
- Make sure that the fan motor is operating.
- Check the blower wheel for correct rotation.
- A blower wheel can be turning backwards and still blow air from the registers.
4. Check the Air Filter
- Make sure the air filter is clean.
- In some cases, it may not be possible to remove the filter until the coil thaws.
- Be cautious of "Ultra Allergen" air filters - highly restrictive.
5. Check Refrigerant Charge
| Normal Pressure Ranges |
| Refrigerant |
55-75 Deg O/D |
75-90 Deg O/D |
| R-22 |
- 55-70 Suction
- 150-225 Discharge
|
- 70-80 Suction
- 170-270 Discharge
|
| R410A |
- 100-125 Suction
- 250-320 Discharge
|
- 120-140 Suction
- 300-400 Discharge
|
Expansion Valve
Check Subcooling & Superheat
- If a Liquid-Line Sight Glass is installed, observe for bubbles in the sight glass (after the system has been operating for at least 5 min.)
- Low Subcooling = Bubbles in Sight Glass
- High Subcooling = Clear Sight Glass
| Suction |
Discharge |
Subcooling |
Superheat |
Cause |
| Low |
Low/Normal |
High |
Low |
Airflow/Heat Transfer |
| Low |
Low/Normal |
High |
High |
Bad Expansion Valve |
| Low |
Low |
Low |
High |
Low Refrigerant Charge |
Fixed Orifice
Check Superheat
| Suction |
Discharge |
Superheat |
Cause |
| Low |
Low |
High |
Low Refrigerant Charge |
| Low |
Low/Normal |
Low |
Airflow/Heat Transfer |
6. Check Supply/Return Air Temperature
- Check the Return Air Temperature at the furnace or air handler.
- Minimum acceptable return air temperature = 64 deg.
- Check the supply air temperature and calculate the coil temperature drop.
- Normal Temperature Drop = 15 - 18 deg.
- Low Temperature Drop = Low Refrigerant Charge or Bad Metering Device (Expansion Valve)
- High Temperature Drop = Low Airflow
- Dirty Air Filter
- Dirty Coil
- Dirty Blower Wheel
- Low Blower Speed
- Dirty Re-Coup Coil (90% furnaces)
- Mis-Matched Equipment