HVAC Diagnostic:
The Five Fundamentals
Suction Pressure / Low Side
- Suction pressure indicates the evaporating temperature of the refrigerant in the evaporator.
- Low pressure can lead to coil freezing.
- General rule: 35°F below the return air temperature (DTD) with 400 CFM/ton ±5°F.
- For 350 CFM/ton, DTD between 38°F and 40°F ±5°F.
- Oversized evaporators may slightly reduce DTD.
Discharge Pressure / High Side
- Helps determine compression ratio and condenser conditions.
- General rule: 15–20°F above outdoor ambient ±3°F.
- CTOA according to SEER:
- – 6–10 SEER (pre-1991): 30°F
- – 10–12 SEER (1992–2005): 25°F
- – 13–15 SEER (2006–present): 20°F
Superheat
- Indicates if liquid is entering the compressor and if the evaporator is properly fed.
- 0°F superheat = risk of slugging damage.
- With TXV: maintain between 5°F and 15°F (ideal 10°F ±5°F).
- With fixed orifice: follow manufacturer’s chart.
Subcooling
- Indicates if the liquid line contains only liquid refrigerant.
- 0°F = liquid and vapor mix.
- High subcooling = refrigerant flowing slowly in condenser.
- Typical values with TXV: 8–14°F (general rule: 10°F ±3°F).
- With fixed orifice can vary from 5–23°F.
Delta T / Temperature Differential
- Evaluates performance and airflow.
- Typical Delta T: 16–22°F.
- High Delta T = low airflow.
- Low Delta T = poor performance.
Diagnosis with the Five Fundamentals
- Correlate all parameters to identify probable cause.
- In TXV systems, superheat and suction pressure tend to be stable.
- Issues with TXV or 0°F subcooling may disrupt that balance.
Parameter Diagnosis Table
| Parameter | Possible Causes |
|---|---|
| Low Suction Pressure | Refrigerant shortage, low thermal load, evaporator or TXV blockage |
| High Suction Pressure | Refrigerant overcharge, hot air return, TXV stuck open |
| Low Discharge Pressure | Refrigerant shortage, cold ambient, over-open expansion valve |
| High Discharge Pressure | Overcharge, low condenser flow, mixed or non-condensable refrigerants |
| Low Superheat | Overcharge, TXV too open, restricted airflow |
| High Superheat | Refrigerant shortage, restricted TXV, very hot return |
| Low Subcooling | Refrigerant shortage, over-open TXV, poor compressor compression |
| High Subcooling | Overcharge, liquid line restriction, TXV closed or restricted |
| High Delta T | Insufficient airflow, low humidity |
| Low Delta T | Low load, excess flow, electric heat active, high humidity |
Graph: Relationship Between Delta T and Airflow
This graph shows how temperature differential (Delta T) varies based on airflow in CFM per ton. Lower airflow generates higher Delta T, which may indicate lower dehumidification capacity or potential evaporator underutilization.
