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.