Bryant 548D Changeover Set Points, Default Potentiometer Settings, EconoMi$er Control Adjustment

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

SETTINGS	A	B	C	D
Dry Bulb (°F)	73	69	66	63
Single Enthalpy* (Btu/lb)	27	25	24	22
Differential Temperature*	2	2	2	2
(°F, Not Adjustable)
Differential Enthalpy*	1	1	1	1
(Btu/lb, Not Adjustable)
*Field-installed accessory.

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

Table 5 — Default Potentiometer Settings

POTENTIOMETER	DEFAULT SETTING
CO2 SP (PPM)	1,000
MECH CLG LOCKOUT	47°
ECON SP	D
MIN POS (%)	20

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

%Outdoor air = Mixture Temp – Return Air Temp Outdoor Temp – Return Air Temp

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.

WARNING: PERSONAL INJURY HAZARD. Avoid possible injury by keeping fingers away from damper blades.

See Fig. 35 for barometric relief damper characteristics.

Fig. 34 — EconoMi$er Control Adjustment

Potentiometers (Factory Settings)

WG)	0.5
	0.4
(IN.
PRESSURE	0.3
	0.2
STATIC	0.1
	0
		200	400	600	800	1000	1200	1400	1600
	0
			FLOW (CUBIC FEET/MINUTE)
	Fig. 35 — Barometric Relief Capacity

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:

	UNIT	HEATER	UNIT	REQUIRED
UNIT			CONFIG-	MINIMUM
	VOLTAGE	kW
			URATION	CFM
	208/230	42.4	Horizontal	3200
	208/230	50.0	Horizontal	3200
			Horizontal
548D120	460	50.0	or	3200
			Vertical
		17.0	Horizontal	2800
	575	51.0	or	2350
			Vertical

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.

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