Improper Metering Device

• Poor System Installation or Design Problems

Improper Metering Device

• A TXV (Thermal Expansion Valve) manufactured for one refrigerant cannot be used with another unless their boiling points are similar.
• R-12 and R-134A are examples of refrigerants with similar boiling points.
• R-22 and R-407C are examples of refrigerants with similar boiling points.
• R-410A and R-22 are examples of refrigerants that do not have similar boiling points.
• Using an R-410A TXV in an R-22 system results in low or no superheat, which can damage the compressor due to saturated refrigerant entering.
• Using an R-22 TXV in an R-410A system causes refrigerant pressures to hunt (fluctuate) and results in high superheat.
• Hunting action in an air conditioner is a fluctuating pressure reading on the low side, often accompanied by high-side fluctuations.
• Hunting occurs when a TXV struggles to maintain a superheat of 10-14° F due to low refrigerant charge, incorrect refrigerant or air in the system.
• Hunting happens when the TXV cannot establish a stable refrigerant flow because the external equalization pressure and bulb temperature are misaligned with its design.

Improperly Sized Metering Device or Evaporator Coil

• Technicians must calculate the heat load and loss of a building to properly size a system based on its BTU/HR capacity.
• The evaporator coil, metering device, and outdoor unit should be sized to work together and match in capacity.
• Components like the metering device or evaporator coil can have a range of capacities.
• Line set size depends on system BTU/HR capacity, refrigerant type, oil type, length, and rise, as specified in the manufacturer's installation literature.
• Mismatched component capacities lead to unequal heat rejection and absorption.
• Examples of mismatched equipment includes:
• Oversized Evaporator Coil, TXV sized according to Outdoor Unit size:
  • Using a 48,000 BTU/HR Evap Coil with an 18,000-36,000 BTU/HR TXV and a 2-ton condenser results in hunting action and high superheat due to excessive heat absorption.
  • Lowering the blower speed is a possible remedy.
• Oversized Evaporator Coil, Piston sized according to Outdoor Unit size:
  • Using a 42,000 BTU/HR Evap Coil with a 24,000 BTU/HR Piston and a 2-ton condenser results in high superheat due to excessive heat absorption.
  • Lowering the blower speed is a possible remedy.
• Undersized Evaporator Coil, TXV sized according to Outdoor Unit size:
  • Using a 30,000 BTU/HR Evap Coil with a 48,000 BTU/HR TXV and a 4-ton condenser results in low superheat and potential compressor damage.
• Undersized Evaporator Coil, Piston sized according to Outdoor Unit size:
  • Using a 30,000 BTU/HR Evap Coil with a 48,000 BTU/HR Piston and a 4-ton condenser results in low superheat and compressor damage.
• Undersized TXV:
  • Using an 18,000-36,000 BTU/HR TXV with a 48,000 BTU/HR Evap Coil and a 48,000 BTU/HR Condenser results in high superheat.
• Undersized Piston:
  • Using a 36,000 BTU/HR Piston with a 48,000 BTU/HR Evap Coil and a 48,000 BTU/HR Condenser results in high superheat.
• Oversized Piston:
  • Using a 48,000 BTU/HR Piston with a 30,000 BTU/HR Evap Coil and a 30,000 BTU/HR Condenser results in low superheat and compressor damage.
• Oversized TXV:
  • Using a 48,000 BTU/HR TXV with a 30,000 BTU/HR Evap Coil and a 30,000 BTU/HR Condenser results in low superheat, compressor damage, and insufficient refrigerant flow reduction by the TXV.

Finding Target Superheat in a Dry Climate

• Target superheat is determined by indoor wet bulb and outdoor dry bulb temperatures.
• Lower wet bulb or higher dry bulb temperatures result in lower target superheat.
• In hot, dry climates, the target superheat can be less than 5°F, potentially below the range of superheat charts, calculators, or digital manifold sets.
• Avoid setting superheat below 5°F, as the compressor requires superheated vapor and not saturated refrigerant.
• Setting superheat higher than optimal protects the compressor.
• Replacing a fixed orifice with a TXV is an effective solution in hot, dry climates as the TXV monitors and adjusts superheat, increasing efficiency and protecting the compressor.
• Installing an accumulator can further protect the compressor.
• TXVs are a cost-effective addition to split systems in dry climates, improving system performance.
• TXVs simplify refrigerant charge checks using the subcooling method.
• In situations where the chart indicates a temperature lower than what is shown, the target superheat must be set at a safe number that is certainly higher than what is efficient for the system to ensure that saturated refrigerant does not enter the compressor
• Replacing the fixed orifice with a TXV metering device handles systems in these climates efficiently.

Acidic Refrigerant

• Mixing water with refrigerant oil due to poor installation or service creates acid and alcohol, corroding internal system components.
• Acidic refrigerant oil degrades the resin on compressor electrical windings, creating a dangerous situation.
• Refrigerant and oil travel across these windings, with the refrigerant absorbing heat from the motor.
• If the resin breaks down, it can lead to a shorted out compressor.
• Resin is insulation around each electrical winding wire, separating each loop.
• Always add an oil treatment to neutralize or chemically bond to the acid in this state.
• Replace the filter drier when servicing the system and opening it to atmospheric pressure.
• Certain acid treatments require filter drier replacement afterward.
• Use an acid test kit to check for high acid levels.
• A bubbling of oil after detaching the refrigerant hose at the service port indicates an acid problem.
• Unchecked high acid levels will likely lead to a compressor burnout.