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4.6 CHECKING INDOOR AIR-FLOW
4.6.1 ESTIMATING CFM USING EXTERNAL STATIC PRESSURE
A common method of checking indoor is to measure the external static pressure that the
air-handler is working against and then referring to the air-flow data in Section 3.13.3.
Measuring external static pressure to a high degree of precision in the field is challeng-
ing, so keep in mind that the CFM determined by this method is an estimate, but is accu-
rate enough for all practical purposes.
To determine external static pressure, the static pressure should be measured in inches
of water column across the air-handler using an incline manometer, digital static pres-
sure meter, or a Magnahelic.
The static pressure inside the return plenum should be
measured as close to the air-handler as possible and must be measured between any
external filter rack and the unit so the pressure drop across the filter is accounted for.
The static pressure inside the supply plenum should be measured at a point about half-
way between the air-handler and the first elbow or the end of the plenum. Total exter-
nal static pressure is the sum of the return and supply plenum static pressures. Even
though the return plenum static pressure is a negative pressure, it must be added to the
supply plenum static pressure, ignoring the negative sign. The supply and return ple-
num static pressure tubing can also be connected to both pressure ports of the pressure
measuring device which will automatically add the two pressures together.
4.6.2 ESTIMATING CFM USING ELECTRIC HEAT TEMPERATURE RISE
If the air-handler is equipped with an electric heater, the CFM can be estimated using
the air temperature rise across the air-handler with the heater and blower both ener-
gized once the unit has run long enough for the temperatures to stabilize.
As with
determining CFM using external static pressure, the CFM determined by this method is
an estimate, but is accurate enough for all practical purposes.
Measure the return air
temperature as close to the unit as possible and the supply air temperature about half
way from the air-handler to the first elbow or end of the supply plenum. Use the follow-
ing formula to calculate CFM once the temperature rise is determined.
CFM = Heating BTUH / (Elevation Factor × Temp Rise °F)
Note: Refer to Section 4.5 to determine Heating BTUH and the following chart for
Elevation Factor.
4.7 CHECKING REFRIGERANT CHARGE
System refrigerant charging should only be performed after the indoor air-flow is con-
firmed to be correct for the application. Once the air-flow is confirmed, refer to the man-
ufacturer's outdoor unit charging chart and installation manual for the proper charging
procedure for the system.
Note: The charging method for systems with a fixed orifice (piston) will likely be differ-
ent than systems with TXV's.
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