268894-UIM-A-0407
26
Unitary Products Group
APPLYING FILTER PRESSURE DROP TO
DETERMINE SYSTEM AIRFLOW
To determine the approximate airflow of the unit with a filter in place, fol-
low the steps below:
1.
Select the filter type.
2.
Select the number of return air openings or calculate the return
opening size in square inches to determine the proper filter pres-
sure drop.
3.
Determine the External System Static Pressure (ESP) without the
filter.
4.
Select a filter pressure drop from the table based upon the number
of return air openings or return air opening size and add to the
ESP from Step 3 to determine the total system static.
5.
If total system static matches an ESP value in the airflow table (i.e.
0.20 w.c. (50 Pa), 0.60 w.c. (150 Pa), etc.,) the system airflow cor-
responds to the intersection of the ESP column and Model/Blower
Speed row.
6.
If the total system static falls between ESP values in the table (i.e.
0.58 w.c. (144 Pa), 0.75 w.c. (187 Pa), etc.), the static pressure
may be rounded to the nearest value in the table determining the
airflow using Step 5 or calculate the airflow by using the following
example.
Example: For a 60,000 BTUH (38.06 kW) furnace operating on high-
speed blower, it is found that total system static is 0.58” w.c. To deter-
mine the system airflow, complete the following steps:
Obtain the airflow values at 0.50 w.c. (125 Pa) & 0.60 w.c. (150 Pa)
ESP.
Airflow @ 0.50”: 1240 CFM (35.1 m
3
/min)
Airflow @ 0.60”: 1170 CFM (23.1 m
3
/min)
Subtract the airflow @ 0.50 w.c. (125 Pa) from the airflow @ 0.60 w.c.
(150 Pa) to obtain airflow difference.
1170 - 1240 = -70 CFM (2.0 m
3
/min)
Subtract the total system static from 0.50 w.c. (125 Pa) and divide this
difference by the difference in ESP values in the table
0.60 w.c. (150 Pa) - 0.50 w.c. (125 Pa), to obtain a percentage.
(0.58 - 0.50) / (0.60 - 0.50) = 0.8
Multiply percentage by airflow difference to obtain airflow reduction.
(0.8) X (-70) = -56 CFM (0.8) x (2.0) = 1.6 m
3
/min
Subtract airflow reduction value to airflow @ 0.50 w.c. (125 Pa) to
obtain actual airflow @ 0.58 in. w.c. (144 Pa) ESP.
1240 (35.1) - 56 (1.6) = 1184 (33.5)
TABLE 16:
Field Installed Accessories - Non Electrical
MODEL NO.
DESCRIPTION
USED WITH
1NP0347
PROPANE (LP) CONVERSION KIT
ALL MODELS
1PS0301
HIGH ALTITUDE PRESSURE SWITCH KIT*
(Does Not Include Orifices)
40, 60, 80 MBH
1PS0311
100, 115, 130 MBH
1CB0314
COMBUSTIBLE FLOOR BASE FOR DOWNFLOW UNITS
14-1/2” (”A”) CABINETS
1CB0317
17-1/2” (“B”) CABINETS
1CB0321
21” (“C”) CABINETS
1CB0324
24-1/2” (“D”) CABINETS
