035-17477-001 Rev. A (801)
Unitary Products Group 33
APPLYING FILTER PRESSURE DROP TO DETERMINE
SYSTEM AIRFLOW
To determine the approximate airflow of the unit with a filter in
place, follow 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 pressure 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 a ESP value in the airflow
table (i.e. 0.20, 0.60, etc,) the system airflow corre-
sponds 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, 0.75, 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 130,000 Btuh furnace with 2 return openings
and operating on high speed blower, it is found that total sys-
tem static is 0.58" w.c. To determine the system airflow, com-
plete the following steps:
1. Obtain the airflow values at 0.50" & 0.60" ESP.
Airflow @ 0.50": 2125 CFM
Airflow @ 0.60": 2035 CFM
2. Subtract the airflow @ 0.50" from the airflow @ 0.60" to
obtain airflow difference.
2035 - 2125 = -90 CFM
3. Subtract the total system static from 0.50" and divide this
difference by the difference in ESP values in the table,
0.60" - 0.50", to obtain a percentage.
(0.58 - 0.50) / (0.60 - 0.50) = 0.8
4. Multiply percentage by airflow difference to obtain airflow
reduction.
(0.8) x (-90) = -72
5. Subract airflow reduction value to airflow @ 0.50" to
obtain actual airflow @ 0.58" ESP.
2125 - 72 = 2053
OPERATION AND MAINTENANCE
SEQUENCE OF OPERATION
The following describes the sequence of operation of the fur-
nace. Refer to the schematic wiring diagrams in the back of
this manual for component location.
CONTINUOUS BLOWER
On cooling/heating thermostats with fan switch, when the fan
switch is set in the ON position, a circuit is completed
between terminals R and G of the thermostat. The blower
motor is energized through the cool terminal and runs on the
selected speed. This allows constant air circulation at lower
flow rate.
INTERMITTENT BLOWER - COOLING
On cooling/heating thermostats with fan switch, when the fan
switch is set in the auto position and the thermostat calls for
cooling, a circuit is completed between the R, Y and G termi-
nals.
The motor is energized through the cool fan terminal and
runs on the selected speed. The fan off setting is fixed at 60
seconds for SEER enhancement.
HEATING CYCLE
When the system switch is set on HEAT and the fan is set on
AUTO, and the room thermostat calls for heat, a circuit is
completed between terminals R and W of the thermostat.
When the proper amount of combustion air is being provided,
a pressure switch activates the ignition control.
The ignition control provides a 17-second warm-up period.
The gas valve then opens for 10 seconds.
As the gas starts to flow and ignition occurs, the flame sensor
begins its sensing function. If a flame is detected during the
10 second flame stabilization period the circulating blower will
energize 30 seconds after the gas valve opens (20 seconds
after the flame stabilization period ends). Normal furnace
operation will continue until the thermostat circuit between R
and W is opened. When the thermostat circuit opens, the
ignition control is de-energized. When the ignition control is
de-energized, the gas flow stops, and the burner flames are
extinguished. The ventor continues to operate for 15 seconds
after the gas flow stops.
Label all wires prior to disconnecting when servic-
ing controls. Wiring errors can cause improper and
dangerous operation. Verify proper operation after
servicing.
To Next Page
Loading...
Need help?
General
