Unitary products group G8C Installation Manual Download

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66832-UIM-D-1205

Unitary Products Group

The temperature rise, or temperature difference between the return air
and the heated supply air from the furnace, must be within the range
shown on the furnace rating plate and within the application limitations
as shown in Table 8.
After about 15 minutes of operation, determine the furnace temperature
rise. Take readings of both the return air and the heated air in the ducts,
about six feet (1.83 m) from the furnace where they will not be affected
by radiant heat. Increase the blower speed to decrease the temperature
rise; decrease the blower speed to increase the rise.
All direct-drive blowers have multi-speed motors. The blower motor
speed taps are located in the control box in the blower compartment.
Refer to Figure 39, and the unit wiring label to change the blower
speed. To use the same speed tap for heating and cooling, the heat ter-
minal and cool terminal must be connected using a jumper wire and
connected to the desired motor lead. Place all unused motor leads on
Park terminals. Two are provided.

ADJUSTMENT OF FAN CONTROL SETTINGS

This furnace is equipped with a time-on/time-off heating fan control. The
fan on delay is fixed at 30 seconds. The fan off delay has 4 settings (60,
90, 120 and 180 seconds). The fan off delay is factory set to 120 sec-
onds. The fan-off setting must be long enough to adequately cool the
furnace, but not so long that cold air is blown into the heated space. The
fan-off timing may be adjusted by positioning the jumper on two of the
four pins as shown in Figure 39.

FILTER PERFORMANCE

The airflow capacity data published in Table 17 represents blower per-
formance WITHOUT filters. To determine the approximate blower per-
formance of the system, apply the filter drop value for the filter being
used or select an appropriate value from the Table 16.
The filter pressure drop values in Table 16 are typical values for the
type of filter listed and should only be used as a guideline. Actual pres-
sure drop ratings for each filter type vary between filter manufacturers.

* Hogs Hair Filters are the type supplied with furnace (if supplied).

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:

Select the filter type.

Select the number of return air openings or calculate the return
opening size in square inches to determine the proper filter pres-
sure drop.

Determine the External System Static Pressure (ESP) without the
filter.

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.

If total system static matches a 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.

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 75,000 BTUH (21.98 kW) furnace with 2 return open-
ings and operating on high-speed blower, it is found that total system
static is 0.38” w.c. To determine the system airflow, complete the follow-
ing steps:
Obtain the airflow values at 0.30 w.c. (75 Pa) & 0.40 w.c. (99.6 Pa) ESP.

Airflow @ 0.30”: 1408 CFM (39.8 m

/min)

Airflow @ 0.40”: 1343 CFM (38.0 m

/min)

Subtract the airflow @ 0.30 w.c. (75 Pa) from the airflow @ 0.40 w.c.
(199.6 Pa) to obtain airflow difference.

1343 - 1408 = -65 CFM (1.89 m

/min)

Subtract the total system static from 0.30 w.c. (75 Pa) and divide this
difference by the difference in ESP values in the table, 0.40 w.c.
(99.6 Pa) - 0.30 w.c. (75 Pa), to obtain a percentage.
(0.38 - 0.30) / (0.40 - 0.30) = 0.8
Multiply percentage by airflow difference to obtain airflow reduction.
(0.8) X (-65) = -52
Subtract airflow reduction value to airflow @ 0.30 w.c. (75 Pa) to obtain
actual airflow @ 0.38 inwc (94.6 Pa) ESP.
1408 - 52 = 1356

Do not energize more than one motor speed at a time or damage to
the motor will result.

FIGURE 39: Typical Heat/Cool Speed Tap Connections

120

180

PARK

LINE

COOL

HEAT

FAN OFF
ADJUSTMENT
SWITCHES

TABLE 17: Filter Performance - Pressure Drop Inches W.C. and (kPa)

Airflow Range

Minimum Opening Size