External Static Pressure; Checking External Static Pressure - Friedrich VERT-I-PAK VHA09K25RTQ Service Manual

Single package vertical heat pump system r-410a refrigerant

Table of Contents

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 accordingly).

These affects must be understood and taken into consideration when checking External Static Pressure/Airflow 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 a digital 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 airflow 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 7 and 8 (if necessary) until proper airflow 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.

Determining the Indoor CFM

MODEL

VHA 09/12

FAN SPEED

LOW

ESP (")

CFM

0.0"

470

0.05"

460

*0.10"

430

0.15"

410

0.20"

360

0.25"

310

0.30"

260

0.35"

0.40"

* values indicate rated performance point