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.

Measuring Airflow, Electric Heat Temperature Rise Method

Use this method with electric resistance heat.  You can also use a velometer or the gas heat method.  See Diognostics catagory.

  • With power to the air handler heater section on, call for heat.  Make sure all heating stages are energised.
  • Measure the supply voltage to the air handler and record the value.
  • Measure the total amperage being drawn by the heaters and record the value.
  • Multiply the measured voltage by the measured amperage.  multiply the result by 3.414.  This is your total BTUH output.  Record this value.
  • When the supply air temperature with all heat on has stabilized, measure the temperature of the supply air 36″ from the plenum.  Record this value.
  • Measure the temperature of the return air at the filter rack.  Record this value.
  • Subtract the return air temperature from the supply air temperature to obtain the difference (delta t).  Record this vallue.
  • Multiply the delta t by 1.08.  Record this value.
  • Divide your BTUH output by the value above .  The answer is your actual CFM.


  • Voltage = 235
  • Amps = 75
  • Supply air temp = 110F
  • Return air temp = 74F
  • 235V x 75a x 3.414 = 60171 BTUH
  • 110F -74F =36F
  • 60171 / 1.08 x 36 = 1548 CFM

How to Measure Airflow, Gas Heat Temperature Rise Method

The best method to measure airflow is with a vane anemometer.  Following is one of the alternate methods if you have gas heat.

  • Disconnect power to the furnace.
  • Set the indoor blower to run on high speed in heat mode.  Restore power.
  • Set to call for heat and confirm heat is running.  Make sure both stages of heat are on.
  • Determine the heating output capacity of the furnace in btuh.  record this value.
  • Run the heating system and allow time for the system to stabilize.  (blower on)
  • Measure the temperature of the return air at the filter.  Record this value.
  • Record the supply air temperature at least 36 inches from the plenum.  Record this value.
  • Disconnect the power and return the blower speed back to its original setting for heating mode.
  • Subtract your measured return air temperature from your supply air temperature.  This is the Delta-t.
  • Multiply your Delta-t by 1.08.  Record this measured BTUH value.
  • Divide your furnace label BTUH output by your measured BTUH value.  This is your CFM.


  • BTUH output label:  100,000
  • Supply air temp:  120F
  • Return air temp:  70F

100,000/1.08 x 50 = 1852 CFM

How to Measure SubCooling

Subcooling is measured to adjust the refrigerant charge on AC/Heat Pump Systems with a TXV expansion device.

  • Run the condensing unit until pressures and temperatures stabilize.
  • Read and record the liquid pressure at the liquid line pressure port fitting at the condensing unit.
  • Place a digital temperature probe against the liquid line near the liquid gauge pressure port.  Read and record the temperature.
  • Reference the face of your high pressure gauge or use a pressure / temperature chart and convert the measured liquid pressure to the coresponding condenser coil saturation temperature for the refrigerant being used.  Record this value.
  • Next, subtract the measured liquid line temperature from the saturation temperature.  The result is your actual liquid subcooling level.


  • Refrigerant: R-22
  • Liquid pressure:  200 PSIG
  • Corresponding condenser saturation temperature:  100F
  • Liquid line temperature:  90F
  • 100F – 90F =Liquid subcooling
  • Liquid subcooling = 10F

How to Measure Superheat

  • Run the condensing unit until suction pressure and suction line temperature stabilize.
  • Read and record the suction pressure.
  • Place a digital temperature probe against the suction line near the suction gauge pressure port.  Read and record the temperature.
  • Reference the face of your suction gauge or use the temperature/pressure chart and convert the measured suction pressure to the corresponding evaporator coil saturation temperature for the refrigerant being used.  Round this value.
  • Next, subtract the saturation temperature from the measured suction line temperature.  The result is your actual suction vapor superheat.


  • R22
  • Suction Pressure: 70 PSIG
  • Corresponding evaporator saturation temperature: 41F
  • Suction line temperature: 51F
  • 51F -41F = Superheat
  • Superheat = 10F

Refrigerant Charge for a Heat Pump or Air Conditioner

To purchase and use most refrigerants, you must hold an EPA Certification.  It is illegal and there are significant fines for handling refrigerants without the certification.

Heat pumps are air conditioners and need to be charged in the cooling season with an outside temperature above 70 degrees F.  Follow the manufacturer’s directions, usually on a label attached to the inside of the service panel.  The standard charge may be stamped on the model/serial label.  If the system is a split system, additional refrigerant for the line set piping is required.

Methods used to adjust the refrigerant charge:

  • All systems… Weigh-in the charge.  Remove all refrigerant and replace it with an accurate charge using scales or charging cylinder.
  • Systems with fixed orifice.  Measure superheat and add/remove refrigerant to obtain the target superheat.
  • Systems with TXV (thermal expansion valves).  Measure sub cooling and add/remove refrigerant to obtain the target sub cooling for the system.  (adjust the TXV for proper superheat)
  • Approach method (Lennox) See charging by the approach method on this site.
  • Beer Can Method.  Not recommended but very common.  Your energy consumption can double and system capacity can be reduced by 50% using this method.  You add/remove refrigerant until the suction line is as cold as a beer can.  We like 33F beer in Texas.

Superheat … degrees F added to the saturation temperature (temperature/pressure that the refrigerant changes from a liquid to a gas) of the refrigerant in the suction line as measured by the pressure.  A pressure gauge and digital thermometer are required to measure and calculate superheat.  Digital gauge sets are available that will do the calculations.  10F to 13F superheat is typical but follow manufacturer’s recommendations.

Sub cooling.. degrees F subtracted from the saturation temperature in the liquid line leaving the condenser and before the expansion device.  9F to 13F is typical but follow manufacturer’s recommendations.

What Size Air Conditioner Do I Need For a 3000 Square Foot House?

One of the questions most often ask is what size air conditioner do I need to heat cool a 3000 square foot house?  The problem is you are asking the wrong question.  The question should be how much heat gets into my 3000 square foot house that the AC System must remove. 

We could tell you that the average home needs one ton of air conditioning for each 500 square feet of conditioned space and we could tell you that the average man wears a size 7¼ hat.  If you buy yourself a 7¼ hat without measuring, it will probably be the wrong size.

Heat does enter the building through walls and ceilings, so there is a relationship between AC size and square feet.  The amount of heat entering through the ceiling and other building envelope components is highly variable depending on factors such as the amount of insulation in the attic. 

There are many sources of heat that must be measured or estimated to determine the AC System size.  They include: walls, ceilings, windows, floors, doors, air leaks as well as internal heat such as people and kitchen appliances. 

Other factors are also important to calculate heat gain such as the outdoor temperature on the hottest days and the temperature you want your AC System to maintain indoors on those days.

You can hire an engineer to measure and calculate heat gain for sizing an air conditioner or heating system and many HVAC Companies will provide this service for no charge.  There is no reason to guess.  See this article for additional information