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

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.

Heat and Cool Service Call Must Measure Air Flow to Avoid Malpractice

Heat and Cool units get a little more complex every year.  With complexity comes more sensitivity to correct installation practices.  Today’s systems are much less forgiving of installation shortcuts than units of the past.  We can’t fix everything by adding refrigerant.

Measuring air flow has not been a skill possessed by the majority of service technicians.  However to properly charge today’s Heat Cool units, any air flow issues must be corrected first.  Possible issues include dirty filters, dirty evaporator coils, small return grills, too few return grills, closed dampers, and undersized ducts to name a few.

Tools are available that allow the technician to measure air flow in one or two minutes.  Using a rotating vane anemometer with a mean averaging function the technician can traverse the return air grills in less than a minute each.  If the air flow is significantly short of the 400 CFM target or higher than the 400 CFM target, additional diagnosis should be made.

To determine where the air flow problem is located, a digital monometer can be used to measure duct static pressure.  A probe can be placed in the return grill and in the supply grill to arrive at the Total External Static pressure. (TESP)  This test might take a couple of minutes. 

Most residential components, including the furnace or air handler, have blowers sized to produce .5 inches of static pressure.  Good practice is to design the air distribution system with no more than one third of the available static pressure in the return.  The problem is a normally sized common air filter might impose .2” of static pressure, exceeding our target.  Our target of .3” of supply duct static pressure for a furnace would include the evaporator coil.  Most coils when new would impose .2” to .25” of pressure drop leaving little for the supply duct system.  You should refer to the manufacturer’s data for TESP and where to measure it.

If the system TESP exceeds the manufacturer’s recommendations, normal superheat and subcooling may result in an improper charge and reduced efficiency.  The importance of examining the entire system before you diagnosis cannot be overemphasized.  Diagnosis without examining the entire system is malpractice.