HVAC Diagnostics: The Five Fundamentals
Suction pressure / low side
- The suction pressure indicates the evaporating temperature of the refrigerant in the evaporator.
- Low pressure can cause freezing of the coil.
- Rule of thumb: 35°F below return (DTD) with 400 CFM/ton ±5°F.
- For 350 CFM/ton, DTD between 38°F and 40°F ±5°F.
- Oversized evaporators can slightly reduce the DTD.
Discharge pressure / High side
- Helps determine the compression ratio and conditions in the condenser.
- Rule of thumb: 15-20°F above outside 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
- 16+ SEER (2006-present): 15°F
Superheat
- Indicates if liquid is entering the compressor and if the evaporator is well fed.
- 0°F superheat = risk of slugging damage.
- With TXV: maintain between 5°F and 15°F (ideally 10°F ±5°F).
- With fixed hole: use manufacturer's table.
Subcooling (Subcooling)
- Indicates if the liquid line contains only liquid refrigerant.
- 0°F = mixture of liquid and vapor.
- High subcool = refrigerant flowing slowly in condenser.
- Typical values with TXV: 8-14°F (rule of thumb: 10°F ±3°F).
- With fixed orifice can range from 5-23°F.
Delta T / Temperature differential
- Evaluates performance and air flow.
- Typical Delta T: 16-22°F.
- - Delta T high = low air flow.
- Low Delta T = poor performance.
- In humid climates or low CFM, higher Delta T is expected.
Diagnosis with the Five Fundamentals
- Correlate all parameters to find the probable cause.
- In TXV systems, superheat and suction pressure tend to be stable.
- Problems with TXV or subcool = 0°F can break that equilibrium.
Diagnostic Table by Parameter
| Parameter | Possible causes |
|---|---|
| Low suction pressure | Lack of refrigerant, low heat load, evaporator clogging or TXV |
| High suction pressure | Refrigerant overcharge, hot air return, TXV failing open |
| Low discharge pressure | Lack of refrigerant, cold environment, excessively open expansion valve |
| High discharge pressure | Overcharge, low condenser flow, refrigerant mixture or non-condensables |
| Low reheat | Overload, TXV too open, restricted air flow |
| High reheat | Lack of coolant, TXV restricted, return too hot |
| Low subcooling | Lack of refrigerant, TXV open too much, compressor with poor compression |
| High subcooling | Overload, liquid line restriction, TXV closed or restricted |
| Delta T high | Insufficient air flow, low humidity |
| Delta T low | Low load, excess flow, activated electrical resistance, high humidity |
