Friedrich H)A09K25 User Manual

15

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.  Drill holes in the supply duct for pressure taps, pilot 

tubes or other accurate pressure sensing devices.

  3.  Connect these taps to a level inclined manometer 

or Magnehelic gauges.

  4.  Ensure the coil and fi lter are clean, and that all the 

registers are open.

  5.  Determine the external static pressure with the 

blower operating.

  6.  Refer to the Air Flow Data for your VERT-I-PAK 

system to fi nd the actual airfl ow for factory-selected 
fan speeds.

  7.  If the actual airfl ow is either too high or too low, the 

blower speed will need to be changed.

  8.  Select a speed, which most closely provides the 

required airfl ow for the system.

  9.  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.

10.  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.

1 ½ TON SYSTEM ( 18,000 Btu) 

Operating on high speed @ 230 volts with dry coil

measured external static pressure .20

Air Flow = 500 CFM

In the same SYSTEM used in the previous example but 
having a WET coil you must use a correction factor of 
.94 (i.e. 500 x .94=470 CFM) to allow for the resistance 
(internal) of the condensate on the coil.

It is important to use the proper procedure to check external 
Static Pressure and determine actual airfl ow. Since in 

the case of the VERT-I-PAK, the condensate will cause 
a reduction in measured External Static Pressure for the 
given airfl ow.

It  is  also  important  to  remember  that  when  dealing  with 
VERT-l-PAK units that the measured External Static 
Pressure increases as the resistance is added externally 
to the cabinet.  Example:  duct work, fi lters, grilles.

If  an  inclined  manometer  or  Magnehelic  gauge  is  not 
available to check the External Static Pressure, or the 
blower performance data is unavailable for your unit, 
approximate air fl ow call be calculated by measuring the 
temperature rise, then using tile following criteria.

KILOWATTS x 3413

=  CFM

Temp Rise x 1.08

The approximate CFM actually being delivered can be 
calculated by using the following formula:

DO NOT simply use the Kilowatt Rating of the heater (i.e. 
2.5, 3.4, 5.0) as this will result in a less-than-correct airfl ow 
calculation.  Kilowatts may be calculated by multiplying 
the measured voltage to the unit (heater) times the 
measured
current draw of all heaters (ONLY) in operation to obtain 
watts. Kilowatts are than obtained by dividing by 1000.

EXAMPLE: Measured voltage to unit (heaters) is 230 volts.  
Measured Current Draw of strip heaters is 11.0 amps.
 
230 x 11.0 = 2530
2530/1000 = 2.53 Kilowatts
2.53  x  3413 = 8635

Supply Air 

 

95

°

F

Return Air 

  

75

°

F

Temperature Rise 

20

°

20 x 1.08 = 21.6

8635

= 400 CFM

21.6