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.

How Much Does Your Heat and Cool Unit, Heat and Cool?

If You Purchased a 3 Ton, 14 SEER Heat Cool Unit, What Size Unit Did You Really Get? 

Your system can be tested to find out.

Under extreme conditions your new 3 ton unit may only have a 1.5 ton and 7 SEER efficiency.  Under normal conditions your unit might only measure up to only 90% of its nameplate rating. If your unit is over 12 years old it may have lost significant capacity and efficiency.

About 70% of your heat and cool air conditioner units capacity is used to lower the temperature of the air.  The remaining 30% capacity is used to remove humidity or moisture from the air.  As an example a 3 ton unit should have a total capacity of 12000 btu’s per ton or 36000 btu’s.  Of the 36000 btu’s, 24000 btu’s would lower the temperature of the air and 12000 btu’s would remove moisture from the air.  In order to measure total capacity, we will need to measure both. 

A normal thermometer will measure the air temperature entering the return of your AC and the temperature of the colder air can be measured at the first supply vent.  If you multiply the difference in temperature by 1.08 btu for each cubic foot of air flow produced by the Heat Cool system, you have the capacity measure that lowers the temperature.  This measure is called sensible heat.

Another thermometer called a psychrometer or hygrometer is needed to measure the humidity that is removed by the air conditioner.  A psychrometer is a normal thermometer with a wet cotton sock over the bulb.  The temperature reading called wet bulb temperature can be converted to a value indicating the temperature and grains of moisture in the air.   This value is called latent heat and enthalpy.  You can purchase or make a wet bulb thermometer.  A very accurate digital hygrometer that measures % relative humidity can be purchased for around $100.  Using an enthalpy chart or enthalpy calculator available online, you can convert your measurements to btu’s.  It is very important to measure wet bulb temperature within one tenth of a degree F.

The more difficult measure to obtain is the amount of air flow in cubic feet per minute (CFM).  An instrument called a vane anemometer can be used as can an instrument to measure static pressure.  Instruction videos for measuring air flow in CFM can be found on

The Formula

Calculating total Btu’s involves measuring airflow and wet bulb air temperatures in and into a conditioned space. Total Btu’s are calculated using the formula:

Total Btu = 4.5 X CFM X Enthalpy Change

The 4.5 factor represents the weight of 1 cubic foot/minute (CFM) of standard air per hour.

CFM is the total supply airflow entering the building from the system.

Enthalpy change is measured using a hygrometer and converting the readings to Enthalpy.

Latent eat = Total Heat – Sensible Heat


  • Air and heat can leak into the system between your temperature readings and the indoor cooling coil.  If this happens, your calculation will indicate the cooling delivered to the space and not the cooling produced by the AC unit.
  • You can also determine equipment airflow by carefully measuring fan static pressure, then plotting the fan CFM based on the manufacturer’s fan data
  • An enthalpy change of about 6 is normal.
  • Airflow has a major effect on system performance. It must be measured accurately with top quality instruments and procedures.
  • A humidity level of 90% plus from the supply side of the system is common. As the air is cooled is can hold less moisture.
  • Normal sensible/latent ratio is 70/30.  A significant return air duct leak from a hot attic, produce a 55/45 sensible/latent BTU ratio.

Lennox Solar Assist Air Conditioner Heat Pump

According to Lennox literature, during  peak daylight hours, the SunSource Comfort System gets help from a solar panel to heat and cool your home.  It uses renewable solar energy to power the outdoor fan motor, reducing the utility-provided electricity needed for operation.  The solar assist increases the system efficiency, reducing the power consumed by as much as 8%.

The system supplies up to 170 watts solar PV assist to the condenser fan motor.