Checking External Static Pressure - Friedrich H)A09K25L Service Manual

External Static Pressure
External Static Pressure can best be defi ned 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 airfl ow (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, fi lters, etc.) on either the supply or return side of the
system causes an INCREASE in External Static Pres-
sure accompanied by a REDUCTION in airfl ow.
External Static Pressure is affected by two (2) factors.
1. Resistance to Airfl ow as already explained.
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/Airfl ow to insure that
the system is operating within design conditions.
Operating a system with insuffi cient or excessive airfl ow
can cause a variety of different operating problems.
Among these are reduced capacity, freezing evaporator
coils, premature compressor and/or heating component
failures. etc.
System airfl ow should always be verifi ed 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 airfl ow.

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 specifi c
VERT-I-PAK.
1. Set up to measure external static pressure at the
supply and return air.
2. Ensure the coil and fi lter are clean, and that all the
registers are open.
3. Determine the external static pressure with the
blower operating.
4. Refer to the Air Flow Data for your VERT-I-PAK
system to fi nd the actual airfl ow 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 airfl ow for the system.
7. Recheck the external static pressure with the
new speed. External static pressure (and actual
airfl ow) will have changed to a higher or lower value
depending upon speed selected. Recheck the actual
airfl ow (at this "new" static pressure) to confi rm
speed selection.
8. Repeat steps 8 and 9 (if necessary) until proper
airfl ow has been obtained.
EXAMPLE: Airfl ow requirements are calculated as follows:
(Having a wet coil creates additional resistance to airfl ow.
This addit ional resistance must be taken into consideration
to obtain accurate airfl ow information.
Determining the Indoor CFM: Chart A – CFM
VEA09/VHA09
ESP (") Low High
.00" 340 385
.10" 300 340
.20" 230 280
.30" 140 190
Highlighted values indicate rated performance point.
Rated performance for
* VEA12
Rated Performance for
** VHA12
VEA24/VHA24
ESP (") Low
.00" 690
.10" 610
.20" 560
.30" 510
.40" 450
Highlighted values indicate rated performance point.
Model
VEA12/VHA12 VEA18/VHA18
Low High Low
420 470 430
350 * 420 ** 400
290 350 340
250 300 290
Model
High
740
700
640
580
520
High
480
450
400
330
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