How to Diognose an Over-Feeding Metering Device

An evaporator coil that is receiving a higher than normal flow of refrigerant through the metering device will operate in a flooded condition.  The system may experience high humidity complaints, compressor starting problems, or compressor failure.

Causes

  • Piston too large in fixed metering device
  • Dirty piston seat not allowing metering device to seat properly
  • TXV sensing bulb not insulated or in good contact with the suction line

Symptoms

The metering device isolates the high side from the low side pressure.  When an overfeeding condition is present, the suction side of the system operates in a flooded state while the high side of the system operates in a starved state.  The suction pressure will be high and the superheat low.  The liquid pressure will be low and the subcooling will be low.

If the system is using a reciprocating type compressor, it may apear that the compressor valve plate is bad.

Procedure

  • Close the liquid service valve and pump the system down.  if the system does not hold a pump down and the liquid line service valve is properly closed.  Replace the compressor. (Recip)
  • If the system holds a pump down cycle, inspect the metering device to determine if it is seated properly or too large.
  • If the system has a TXV, make sure the sensing bulb is properly insulated and in good contact with the suction line.  If it is not, correct the problem.  If it is, replace the TXV.

How to Diognose Restricted Metering Device

Restrictions in the metering device will prevent the evaporator coil from receiving the proper amount of refrigerant.  The restriction will cause the suction pressure to be too low.  With low suction pressure present, the saturation temperature of the refrigerant in the evaporator is very low.  Because of low saturation temperature, freezing of the evaporator surface is likely.

Causes

  • Undersized fixed metering device
  • Failed expansion valve
  • Debris in metering device
  • Moisture freezing at metering device

Symptoms

Symptoms running with a metering restriction will have low suction pressure along with high superheat.  The liquid pressure will initially be low which will make the system appear undercharged.  When refrigerant is added to the system, the liquid pressure will rise but the suction pressure will remain low.  At this point you should be able to identify there is a problem in the liquid circuit.  If subcooling is measured, at this time, it will be high.  The high subcooling level is an indication that excess refrigerant is being stored in the condenser coil.

There will be no obvious sign of frosting between the consenser coil and the metering device.

Procedure

  • If the liquid and suction pressure are low, and no obvious sweating or frosting of the liquid line is present, add refrigerant.  If the system is undercharged, both pressures will begin to rise.  If the liquid pressure rises significantly, but the suction pressure remained low, the liquid circuit has a restriction.
  • Check the temperature of the liquid line at the outlet of the condenser coil and at the inlet to the metering device.  If you detect a large temperature drop, there is a pressure drop between the two test points.  Isolate the restriction and make the repair.
  • If there is no temperature drop between the outlet of the condenser and the inlet of the metering device, yet the liquid pressure is elevated, suction pressure is low, superheat and subcooling high, the problem is in the metering device.
  • Pump the system down or recover the refrigerant and inspect the metering device for proper size, operation or debris.  Replace if necessary

Diognosing Liquid Line Restrictions

Restrictions in the liquid line will prevent the evaporator coil from receiving the proper amount of refrigerant.  The restriction will cause a large pressure drop between the condenser and the metering device.  This large pressure drop can be detected by checking the temperature of the liquid line at the outlet of the condenser coil and at the inlet to the metering device.

Causes

  • Restricted liquid line dryer
  • Undersized liquid line
  • Excessive liquid line pipe fittings
  • Kinked liquid line.

Symptoms

Systems running with a liquid line restriction will have low suction pressure along with high superheat.  The liquid pressure will initially be low which will make the system appear undercharged.  When refrigerant is added to the system, the liquid pressure will rise but the suction pressure will remain low.  At this point, you should be able to identify there is a problem in the liquid circuit.  If subcooling is measures, at this time, it will be high.  The high subcooling level is an indication that excess refrigerant is being stored in the condenser coil.

Look for frosting or sweating of the liquid line or liquid line dryer.

Procedure

  • If the liquid and suction pressure are low, and no obvious sweating or frosting of the liquid line is present, begin to add refrigerant.
  • If the system is undercharged, both pressures will begin to rise.  If the liquid pressure rises significantly, but the suction pressure remains low, the liquid circuit has a restriction.
  • Check the temperature of the liquid line at the outlet of the condenser coil and at the inlet to the metering device.  No significant drop in temperature should be detected.  If you detect a large temperature drop, there is a pressure drop between the two test points.  Isolate the restriction to make the repair.
  • If there is no temperature drop between the outlet of the condenser and the inlet of the metering device, yet the liquid pressure is elevated, suction pressure is low, superheat and subcooling is high, perform diagnostics for a restricted metering device.

How to Diognose High Heat Load, Fixed Orface or TXV

High evaporator heat load occurs when there is excessive heat entering the refrigerant at the evaporator coil.  This condition can cause the compressor to shut off on it’s internal overload, or trip the electrical circuit breaker.

Causes

  • Infiltration of unconditioned air into the return duct
  • New systems started on a very hot day
  • Economizer/fresh air damper open on a hot day
  • Temporary high people loads
  • System undersized for heat gain

Symptoms

  • Systems running with high evaporator loading will run at pressures above factory required levels.  Abnormally high superheat and normal subcooling levels will be present when the evaporator coil is exposed to higher than allowed heat load.  The high heat load condition could cause the evaporator coil temperature to be very warm.  The compressor shell and hot gas temperature will be extremely high.

TXV  A system with a TXV may keep superheat level close to the required range of 10 to 15F.  In exteme cases of high load the superheat may go higher.

Procedures

  • Measure the temperature of the conditioned space and then check the temperature of the air entering the evaporator coil at the filter rack.  If the air entering the evaporator is warmer that the conditioned space, infiltration of unconditioned air is present.
  • If the system has an economizer or fresh air damper, make sure it is closed.
  • If the system is new and start up is on a hot day, allow time for the indoor temperature to fall within charging chart range.
  • If the system is still overloaded and the outdoor air temperature is within design range for your area, run a heat load to determine if the system is properly sized.

How to Identify Low Evaporator Heat Load, TXV

Low evaporator heat loading occurs when there is not enough heat entering the refrigerant at the evaporator coil.  This condition can cause the evaporator coil to form a coating of frost or ice on it’s surface.  In extreme cases, superheat could fall to zero degrees and liquid refrigerant can flood back to the compressor.

Causes:

  • Dirty return air filter
  • Dirty indoor fan blower
  • Indoor fan motor not on high speed
  • Undersized duct work
  • Dirty evaporator coil
  • Thermostat set-point too low

Symptoms

Systems running with low evaporator loading will run at pressures below factory required levels.  The TXV may hunt in an attempt to maintain proper superheat levels.

The low heat load condition could cause the evaporator coil temperature to be very cold.  Ice or frost may form on the evaporator surface and suction line surface.  The compressor shell may sweat and the hot gas temperature will be cool.

Repair Procedure

  • Thaw the evaporator coil if frost or ice has formed.
  • Inspect the return air filter and replace if dirty.
  • Inspect the evaporator coil and clean if dirty
  • Inspect the indoor blower and clean if dirty
  • Make sure the indoor blower motor is operating at high speed
  • Make sure the supply and return duct are sized properly
  • Measure indoor CFM for proper flow.  350 to 450 CFM per ton.

How To Diognose Low Evaporator Heat Load, Fixed Orifice

Low evaporator heat loading occurs when there is not enough heat entering the refrigerant at the evaporator coil.

This condition can cause the evaporator coil to form a coating of frost or ice on it’s surface.  In extreme cases, superheat could fall to 0 degrees and liquid can flood back to the compressor.

Symptoms

  • Fixed orifice.  Systems running with low evaporator loading will run at pressures below factory required levels.  Abnormally low superheat and subcooling levels will be present when the evaporator coil lacks heat load.  The low heat load condition could cause the evaporator coil temperature to be very cold.  Ice or frost may form on the evaporator surface and suction line surface.  The compressor shell may sweat and the hot gas temperature will be cool.

Procedure

  • Thaw out the evaporator coil if ice or frost are present.
  • Check airflow (350-450 CFM per ton)
  • Determine cause of low air flow, dirty filter, dirty evap coil, dirty blower, blower motor speed, restricted return duct.
  • Make sure thermostat setpoint is not too low.
  • Correct airflow problem.
  • Check for proper refrigerant charge.

How to Diognose Refrigerant Overcharge, TXV System

When a system with a TXV metering device is overcharged, the suction pressure and superheat may appear normal.  The normal conditionsare a result of the TXV regulating the refrigerant flow independent of the compressor.

The excess refrigerant is stored in the condenser coil and increases the head pressure and liquid subcooling.  If the charge is high enough the suction pressure may be elevated with a slightly low superheat.

Higher pressures increase power required for system operation.

Remove refrigerant in accordance with the manufacturers instructions to obtain the desired or target subcooling.

How to Diognose Refrigerant Overcharge, Fixed Orface

Overcharged systems will experience excessive energy consumption, possible compressor starting problems, internal overload tripping, indoor air humidity complaints and compressor failure due to liquid flood back.

Systems running with high charge will run at pressures above factory required levels.  Abnormally low superheat and high subcooling levels will be present. The compressor will experience liquid flood back problems.

Remove refrigerant to obtain manufacturers recomended level.

How to Diognose Low System Refrigerant Charge, TXV

When a system is undercharged and equipped with a TXV metering device, the TXV may open enough to maintain adequate suction pressure and suction vapor superheat.  The condensing pressure will be low along with low subcooling.  If the system charge is very low, the system will appear with the same symptoms as a fixed metering device.

  • Leak test the system
  • Charge to OEM requirements.
  • If OEM requirements are not available, a subcooling target of 10F can be used until specs are found.

When charging R-410A systems always charge in a liquid state only.

How-to Diognose Low System Refrigerant Charge, Fixed Expansion Device

Low system charge can be caused by a leak or improper charge when installed.
Low charge will result in low capacity, low EER, and a hot compressor.

Systems with a low charge will operate at pressures below the factory recommended pressures. High superheat and low subcooling will be evident. The compressor will run hot with insufficient colling for the motor windings. The low suction pressure may result in sub-freezing temperatures at the evaporator causing ice to form, further inhibiting air flow.

Check for leaks.
Charge the system in accordance with manufacturers instructions to the target superheat level. Make sure when charging R-410A to add refrigerant in the liquid state only.