for your safety for your safety important caution warning

start- uP Procedure control operating sequence

IMPORTANT

To prevent premature heat exchanger failure, with all control systems, a blower starting mechanism must be provided so that the blower is running or energized within 45 seconds of the gas control operation.

Indoor duct furnaces are supplied with intermittent pilot systems with continuous retry, which both the main burner and pilot burner are turned off 100% when the thermostat is satisfied.

On a call for heat, the system will attempt to light the pilot for 70 seconds. If the pilot is not sensed for any reason, the ignition control will wait for approximately six minutes with the combination gas control closed and no spark. After six minutes, the cycle will begin again. After three cycles, some ignition controllers lockout for approximately one hour before the cycle begins again. This will continue indefinitely until the pilot flame is sensed or power is interrupted to the system.

note: Gas Control Options (see page 19) could change the listed sequence of operation based on their function.

The descriptions given are for the basic duct furnace.

single furnace controls staged control (digit 12=1 or 2):

These units utilize a single- or two-stage combination gas valve, an ignition control, and a low voltage thermostat.

electronic modulating control (digit 12=4, 7, or 8):

These units utilize a single-stage combination gas valve, an electronic modulating gas valve, a modulating amplifier, an ignition control, and one of the following:

• Modulating room thermostat

• Modulating duct thermostat with remote temperature set point adjuster

• Building Management system (BMs) signal by others (an inverted signal where 0 VDC or 4 mA is high fire and 10 VDC or 20 mA is low fire).

The control operating sequence for all units is as follows:

1. The thermostat calls for heat. For BMS controlled units, the

BMS closes a heat enable contact at the unit.

2. model HBP/HcP/HdP/HPP only - The power exhauster relay is energized starting the power exhauster motor. Once the motor has reached full speed, the differential pressure switch closes. The power exhauster pre-purge time delay relay then closes after 20 to 40 seconds and energizes the gas control circuit.

3. The pilot valve opens and the spark igniter sparks in an attempt to light the pilot. (If the unit was not provided with a time delay relay, the blower starts).

4. Once the pilot is lit, the flame sensor proves the pilot and stops the spark igniter from sparking.

5. The main gas valve is opened and the main burner is controlled as follows:

a. single-stage units: The main burner is lit to 100% full fire.

b. two-stage units: The main burner is lit to 50% fire. If the temperature at the thermostat continues to fall, the thermostat will call for high stage heat and the main burner is lit to 100% full fire.

c. modulating thermostat (room or duct): The main gas valve is opened 100% and the burner firing rate is modulated between 40% and 100% full fire. A resistance signal (8000 to 12000 ohms) in the thermostat is converted

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by the modulating amplifier to an inverted DC voltage

(0VDC for high fire to 12 VDC for low fire). The output voltage is applied to the modulating gas valve to control the gas flow to the main burner. The modulating valve is modulated open or closed based on the voltage from the amplifier (less gas flow required = higher voltage, more gas flow required = lower voltage).

note: When modulating duct sensing is utilized, a room override thermostat can be added. When the room override calls for heat, the burner modulates to full fire operation until the room override is satisfied. The unit then reverts back to duct sensing control. When equipped with both, either the duct sensor or the room override thermostat can call for heat.

d. Bms signal: The main gas valve is opened 100% and the burner firing rate is modulated between 40% and

100% full fire. A BMS 0-10VDC or 4-20mA signal (inverted, such that 0 VDC or 4 mA is high fire and 10 VDC or 20 mA is low fire) is converted by the signal conditioner/ modulating amplifier into an inverted DC voltage (0VDC for high fire to 12 VDC for low fire). The output voltage is applied to the modulating gas valve to control the gas flow to the main burner. The signal conditioner can accept a 0-10 VDC signal when all the dip switches are in the “OFF” position and 4-20 mA signal when all the dip switches are in the “ON” position. The modulating valve is modulated open or closed based on the voltage from the amplifier (less gas flow required = higher voltage, more gas flow required = lower voltage), which correlates to the control signal from the BMS.

note: For further information regarding the operation of any of the electronic modulating system options above, consult the literature provided with the unit.

6. If the unit was provided with a time delay relay, the blower starts after 30 to 45 seconds.

7. The unit continues to operate until the thermostat is satisfied,

Once satisfied:

a. single-stage units: Both the main and pilot valves close

100%.

b. two-stage units: Once the high stage of the thermostat is satisfied, the main valve closes to 50% fire. The unit continues to operate until the low stage thermostat is satisfied, at which time both the main and pilot valves close 100%.

c. electronic modulation units: The unit continues to operate in this manner until the thermostat is satisfied or the BMS heat enable contact opens. Power is then cut to both the main and pilot valves, closing them 100% and stopping gas flow to the main and pilot burners.

8. If the unit was not provided with a time delay relay, the blower stops immediately. If the unit was provided with a time delay relay, the blower stops after 30 to 45 seconds.

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start-uP Procedure multiple furnace controls staged control (digit 12=1):

For control of multiple staged units, each furnace would be individually controlled. Refer to the section for Single Furnace

Controls, Staged Control (Digit 12=1 or 2).

electronic modulating control (digit 12=4):

These units are the same as Electronic Modulating Gas

Controls – Single Furnace (Digit 12=4) except the Master unit features a modulating amplifier capable of driving multiple modulating gas valves for systems with a Master and up to two

Slave units. Slave units do not have a modulating amplifier.

The units would be controlled by one of the following:

• Modulating room thermostat

• Modulating duct thermostat with remote temperature set point adjuster

The sequence of operation for Electronic Modulating Gas

Controls - Master/Slave is the same as Electronic Modulating

Gas Controls - Single Furnace. The modulating amplifier sends an equal voltage signal to all of the modulating gas valves so that they modulate at the same percentage, between 40% and

100% full fire.

electronic modulating control (digit 12=7, or 8):

For control of multiple electronic modulation units for BMS control, each furnace would be individually controlled. Refer to the section for Single Furnace Controls, Electronic Modulation

Control (Digit 12=7 or 8).

variable air movement applications

Units may be supplied with variable frequency drives for applications where variable air volume is required. The minimum air flow may be varied between 30 and 100% of the full speed air flow depending on the controls selection of the unit. Due to the extra restrictions of the controller all selections must be performed with the AccuSpec configuration software.

Within AccuSpec, three variable frequency drive speed control changeover options are available:

1. Two speed which may be controlled by a manual high/low switch which may be factory mounted on the control panel or shipped loose for field installation or by exhaust fan interlocks.

2. Floating building pressure sensing which utilizes a photohelic pressure controller to adjust the building pressure by varying the amount of makeup air supplied to the the space.

3. Building management control which allows for an external signal of 0-10VDC of 4-20mA to adjust the unit airflow.

The allowable minimum CFM of the system can be 66% of the minimum listed CFM in Table 27.1 if the unit is applied as follows:

1. The unit is provided with 2-stage or electronic modulating gas controls.

2. The unit is provided with a discharge air thermostat.

3. The system does not include a room thermostat.

The discharge air thermostat will prevent the unit from firing above the allowable 100°F rise when the unit is at or above the minimum CFM by monitoring the discharge air and going to low fire. A room thermostat, because it is located remote from the unit, could cause the unit to over-fire.

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