Friedrich VERT-I-PAK VHA-18K75RTP, Service & Parts Manual

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36 PB
External Static Pressure can best be described as the pressure difference (drop) between the Positive Pressure (discharge)
and the Negative Pressure (intake) sides of the blower. External Static Pressure is developed by the blower as a result of
resistance to airflow (Friction) in the air distribution system EXTERNAL to the VERT-I-PAK cabinet.
Resistance applied externally to the VERT-I-PAK (i.e. duct work, filters, etc.) on either the supply or return side of the system
causes an INCREASE in External Static Pressure accompanied by a REDUCTION in airflow.
External Static Pressure is affected by two factors.
1.Resistance
2.Blower Speed (Changing to a higher or lower blower speed will raise or lower the External Static Pressure accord
ingly).
These affects must be understood and taken into consideration when checking External Static Pressure/Airfl ow to insure that
the system is operating within design conditions.
Operating a system with insufficient or excessive airflow can cause a variety of different operating problems. Among these are
problems such as, reduced capacity, freezing evaporator coils, premature compressor’ heating component failures, and/ or
other air local distribution issues..
System airflow should always be verified upon completion of a new installation, or before a change-out, compressor
replacement, or in the case of heat strip failure to insure that the failure was not caused by improper airflow.
Checking External Static Pressure
The airflow through the unit can be determined by measuring the external static pressure of the system, and consulting the
blower performance data for the specific VERT-I-PAK.
1. Set up to measure external static pressure at the supply and return air.
2. Ensure the coil and filter are clean, and that all the registers are open.
3. Determine the external static pressure with the blower operating.
Use an incline or dual port manometer to measure. Measurement should be taken roughly 3-6” from the Vert-I-Pak
collar and the center of the indoor coil with the filter installed.
4. Refer to the Air Flow Data for your VERT-I-PAK system to find the actual airflow for factory-selected fan speeds.
5. If the actual airfl ow is either too high or too low, the blower speed will need to be changed to appropriate setting
or the ductwork will need to be reassessed and corrections made as required.
6. Select a speed, which most closely provides the required airflow for the system.
7. Recheck the external static pressure with the new speed. External static pressure (and actual airflow) will have
changed to a higher or lower value depending upon speed selected. Recheck the actual airflow (at this “new” static
pressure) to confirm speed selection.
8. Repeat steps 8 and 9 (if necessary) until proper airfl ow has been obtained.
EXAMPLE:
Airflow requirements are calculated as follows: (Having a wet coil creates additional resistance to airflow. This
additional resistance must be taken into consideration to obtain accurate airflow information.
Figure 604
EXTERNAL STATIC PRESSURE