Bryant 548D User Manual

Page 19

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Mechanical Cooling Lockout

Determine the outdoor-air temperature at which you want
the mechanical cooling (compressors) to be disabled. Locate
the mechanical cooling lockout (MECH CLG LOCKOUT)
potentiometer. To disable this feature, turn the potentiome-
ter counterclockwise (CCW) to the OFF position. Otherwise,
set the value between 10 and 60 F. Mechanical cooling will
not operate when the outdoor-air temperature is below this
value. See Fig. 34.

Dry Bulb Changeover Set Up

Determine the dry bulb changeover set point from Table 4.
The settings are A, B, C and D. Locate the ECON SP potenti-
ometer and set the dry bulb changeover set point. See
Fig. 34. When the OAT is above this set point, the damper is
limited to minimum position setting.

Table 4 — Changeover Set Points

*Field-installed accessory.

If a potentiometer fails, its setting will default to the values
in Table 5.

Table 5 — Default Potentiometer Settings

Ventilation Air (Minimum Position Set Up)

If ventilation air is not required, proceed to Step 5. If ventila-
tion air is required, perform the following:

1. The indoor fan must be on to set the ventilation air.

Either put the thermostat in the continuous fan mode
or jumper the R and G terminals at the rooftop unit
connection board.

2. Locate the minimum position (MIN POS) potentiome-

ter. Turn the potentiometer full CCW to fully close
the outdoor air dampers. Turn the potentiometer
gradually clockwise (CW) to the desired position. See
Fig. 34.

3. Replace the filter access panel. See Fig. 11. Ensure

the filter access panel slides along the tracks and is
securely engaged.

4. Calculate the minimum airflow across the

EconoMi$er.

a. Calculate % of outside air using the following

formula.

% Outdoor air through EconoMi$er

b. Divide total CFM by percentage outdoor air, this

gives outdoor air volume in CFM.

5. Turn on base unit power.

NOTE: The EconoMi$er begins operation three minutes after
power up.

See Fig. 35 for barometric relief damper characteristics.

VII. STEP 7 — ADJUST INDOOR-FAN SPEED

Adjust indoor-fan speed to meet jobsite requirements.

For units with electric heat, required minimum cfm is 2250
for 548D090, 2550 for 548D102 and 3000 for 548D120 with
the following exceptions:

Table 6 shows indoor-fan motor data. Table 7 shows fan rpm
at motor pulley settings for standard and alternate motors.
Refer to Tables 8-13 to determine fan rpm settings. Fan
motor pulleys are factory set for speed shown in Table 1.

To change fan speeds:

1. Shut off unit power supply and tag disconnect.

2. Loosen belt by loosening fan motor mounting nuts

See Fig. 36 and 37.

3. Loosen movable pulley flange setscrew (see Fig. 38).

4. Screw movable flange toward fixed flange to increase

fan rpm or away from fixed flange to decrease speed.
Increasing fan rpm increases load on motor. Do not
exceed maximum speed specified in Table 1.

SETTINGS

Dry Bulb (°F)

Single Enthalpy* (Btu/lb)

Differential Temperature*
(°F, Not Adjustable)

Differential Enthalpy*
(Btu/lb, Not Adjustable)

POTENTIOMETER

DEFAULT SETTING

SP (PPM)

1,000

MECH CLG LOCKOUT

47°

ECON SP

MIN POS (%)

% Outdoor air =

Mixture Temp – Return Air Temp
Outdoor Temp – Return Air Temp

WARNING: PERSONAL INJURY HAZARD. Avoid

possible injury by keeping fingers away from damper
blades.

UNIT

VOLTAGE

HEATER

UNIT

CONFIG-

URATION

REQUIRED

MINIMUM

CFM

548D120

208/230

42.4

Horizontal

3200

208/230

50.0

Horizontal

3200

460

50.0

Horizontal

Vertical

3200

575

17.0

Horizontal

Vertical

2800

51.0

2350

Fig. 34 — EconoMi$er Control Adjustment

Potentiometers (Factory Settings)

200

400

600

800

1000 1200 1400 1600

FLOW (CUBIC FEET/MINUTE)

0.5

0.4

0.3

0.2

0.1

TIC PRESSURE (IN. WG)

Fig. 35 — Barometric Relief Capacity