Refrigerant Charge, Adding Freon to Heat Cool Unit

Today’s high efficiency air conditioning units require the proper refrigerant charge to deliver low energy operating cost.  For this reason the consumer should use the most competent service and maintenance personnel available.  Overcharging and undercharging are common conditions and waste significant amounts of energy. 

A license issued by the EPA is required to purchase or handle most refrigerants.

However anyone can ballpark the refrigerant charge.  You will need a digital thermometer and some tubing insulation. 

  • Locate the un-insulated and smaller of the two copper tubes connecting the indoor and outdoor unit.  Select a location where the tube makes a bend, outdoor or indoor, so you can place the sensor of a digital thermometer on the tube.  Insulate the tubing / sensor and read the temperature.
  • Subtract the tubing temperature from the discharge air temperature of the outdoor condensing unit.  
  • As an example, under normal conditions, a condenser discharge temperature might be 110F, and a tubing temperature of 100F indicates 10F of subcooling.  10F to 12F subcooling is normal.  If it’s considerably less than 10F, the unit could be low on refrigerant or need a Tech to check it.  If the  subcooling is too far above 12-F, it’s overcharged.  If the subcooling is less or more than the 12F subcooling, it’s time to call a licensed service tech.

More Information

The magic of air conditioning is made possible by “phase change” of liquids to gases.  Water is a familiar phase change material.  As an example, a phase change occurs when water changes from a vapor or gas to a liquid or to a solid.  Each time the material changes phase significantly more heat or energy is transferred than when a simpe temperature change occurs.  It takes one BTU to increase or decrease one pound of water one degree.  During a phase change of water to ice, 144 BTU’s are required to change 32 degree water to 32 F. degree ice.  To change one pound of water to vapor at 212 F. requires 970 BTU. 

Refrigerants are selected based on their phase change qualities.  In your home, the refrigerant goes thru a phase change from a liquid to a gas in the evaporator coil, hence the name evaporate.  The heat in the air going over the coil is absorbed by the refrigerant and its increased BTU absorbing, phase change state. 

When a unit is undercharged or overcharged, energy waste occurs.  The refrigerant liquid is cooled beyond phase change or the gas is overheated beyond phase change.  The magic of the increased BTU capacity occurs with phase change, not changes in temperature.

One of the devices used to expand the liquid refrigerant into a gas is a fixed orifice expansion device; we use the superheat method to charge the unit equiped with a fixed orface.  Superheat is the amount of heat in excess of phase change.  We want our superheat change to be about 10F degrees above the phase change temperature.   If all the liquid is not evaporated in the evaporator coil, liquid refrigerant can reach the compressor and damage it.  Liquid in a gas compressor is bad news.

Superheat at the evaporator should be checked as close to the end of the coil as possible (preferably near the expansion valve thermal bulb).  Convert this to saturation temperature and compare it to the actual temperature obtained near the thermal bulb. Take the suction pressure at the service valve and convert it to saturation temperature. Compare this to the actual temperature obtained approximately six inches out on the suction line.

In AC units with a TXV or thermostatic expansion device, we want to use the subcooling method where the liquid refrigerant is 10F to 15F degrees below the phase change temperature.  This is the temperature after the compressor and condenser coil, where the gas has been changed back to a liquid.

Subcooling the refrigerant is desirable to insure we deliver the maximum amount of liquid to the evaporator.

Add refrigerant if the calculated subcooling temperature is lower than the recommended level. Remove and recover the refrigerant if the subcooling is higher than the recommended level.

Subcooling should be checked as close to the condenser as possible & then as close to the TXV as possible, noting the difference.

If you are not absolutely sure if the metering device is a TXV, fixed orifice device or cap tube, use the following method.  Hook up your manifold gauges, block off considerable condenser air intake for a short time.  If the suction pressure changes it’s a fixed piston or cap tube
If only the high-side goes up, you have a TXV.

One manufacturer uses the approach charging method when ambient temperatures are above 60F.  Measure the outdoor ambient temperature and liquid line temperature with the same thermometer.  Subtract the outdoor ambient temperature from the liquid line temperature to determine the approach temperature.  (Liquid line F – Outdoor Ambient F = Approach Temperature)  Compare to the charging table and add refrigerant to lower the approach temperature.

Liquid line temperature at the evaporator should be within 2 degrees of liquid line temperature at condensing unit. If not, there could be a restriction or the line set tubing is too long.
Service Technicians should charge the unit in the following sequence.  The system must be properly installed in order to be properly charged.

  1. Check the size of supply and return ductwork.  Make corrections.
  2. Seal any duct leaks.
  3. Clean the indoor blower to insure maximum air flow for heat exchange.
  4. Clean the evaporator coil for maximum heat exchange.
  5. Clean the condenser coil for maximum heat exchange.
  6. Install a new filter for maximum air flow.
  7. Measure and record duct internal static pressure. (wet coil)  Make any corrections needed.  Low amp draw for the blower motor would be an early indicator of a high static pressure problem and low CFM.  High amp draw would be an early indicator of a separated duct or high CFM problem.
  8. Measure the system CFM and set to 400CFM to 450CFM per ton. (wet coil).  A hot wire anemometer can be used to measure duct velocity.  See CFM calculations.  A direct reading CFM flow hood can also be used.  A magnehelic gauge can be used to read static pressures.  This information can be used with the fan curve or performance chart to calculate CFM.
  9. Record ambient temperature.

10.  Record temperature difference across the evaporator coil.  With a 75F room temperature and 50% RH the TD should be close to the following:

  1. 10 SEER  20F
  2. 12 SEER  18F
  3. 14 SEER  14F              

11.  Measure subcooling from the manifold gauge saturated temperature compared to measured temperature of the liquid line as close to condenser as possible.

12.  Measure superheat from the suction line temperature at the evaporator less the suction saturation temperature from the manifold gauge.  With an 85F ambient temperature the superheat should be in the 8F to 12F range.

13.  The TXV should be set for 8F superheat.