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Multiple Furnace Controls
Multiple furnace units are available as two furnace units (Model
Digits 4-6=500 through 800) and three furnace units (Model
Digits 4-6=840 or 960) and with different types of gas control:
•
Staged Control (Model Digit 12=1): For control of multiple
staged furnaces, each furnace would be individually
controlled. Refer to the section for Single Furnace
Controls, Staged Control (Model Digit 12=1).
»
Two furnace units would be controlled as 2-stage
(50%/50%).
»
Three furnace units can be controlled as 3-stage
(33%/33%/33%) or 2-stage (33%/67%).
•
Electronic Modulating Control (Model Digit 12=4): For
control of multiple electronic modulating furnaces, one
furnace is the Primary furnace featuring a modulating
amplifier capable of driving modulating gas valves on
Secondary furnaces. Because all valves are driven by one
amplifier, all valves modulate at the same rate, from 40%
to 100% full fire. Refer to the section for Single Furnace
Controls, Electronic Modulating Gas Controls (Digit 12=4).
•
Electronic Modulating Control (Model Digit 12=7 or 8):
For control of multiple electronic modulation furnaces for
BMS control (0-10VDC or 4-20mA control signal), each
furnace would be individually controlled. Refer to the
section for Single Furnace Controls, Electronic Modulation
Control (Model Digit 12=7 or 8).
•
Modulating with Modine Control System (Model Digit
12=9): For control of multiple furnaces, one furnace
features a modulating gas valve while additional furnaces
are two stage furnaces. The Modine Control system
seamlessly stages furnaces and modulates the primary
furnace based on demand.
Variable Air Volume Applications
Units may be supplied with variable frequency drives for
applications where variable air volume is required. The minimum
air flow may be reduced to as low as 30% of the full speed
air flow, but not lower than the minimum airflow listed in Table
25.1 and 36.1, unless certain conditions are followed. Refer
to the following section titled "Extended VAV Airflow Limits" for
additional information.
To determine the actual minimum allowable airflow, all unit
selections must be performed with the AccuSpec configuration
software. Within AccuSpec, multiple variable frequency drive
speed control changeover options are available, which include
these common selections:
•
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.
•
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 space.
•
Building management control which allows for an
external signal of 0-10VDC of 4-20mA to adjust the unit
airflow.
Additional options are available when the unit is configured with
the Modine Control System option (Model Digit 12=9).
Extended VAV Airflow Limits
If certain conditions are followed, the allowable minimum CFM
of the system can be extended to:
•
75% of the minimum listed CFM in Table 36.1 for high
air temperature rise units (Model Digit 10=H)
•
66% of the minimum listed CFM in Table 36.1 for low
air temperature rise units (Model Digit 10=L).
Refer to Table 25.1 for a summary of the reduced minimum
airflows, indicated in the column "Extended Range". To allow
the reduced airflows, the unit must be applied as follows:
1.
The unit has 2-stage or modulating gas controls.
2.
The unit is provided with a discharge air thermostat.
3.
The system does not include a room thermostat.
Exception: See note below for units equipped with the
Modine Control System option (Model Digit 12=9).
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.
Note for units equipped with the Modine Control System option
(Model Digit 12=9): A space temperature sensor (Carel pAD) is
permitted, as a call for heating from the space results in a reset
of the supply air temperature setpoint that will still maintain a
supply temperature that is at or below the maximum allowable
discharge air temperature.
Table 25.1 - Extended Range VAV Minimum Airflow
Input Rating
(Digit 4-6)
Standard
75
563
100
750
125
938
150
1,125
175
1,313
200
1,500
225
1,688
250
1,875
300
2,250
350
2,625
400
3,000
500
-
600
-
700
-
800
-
840
-
960
-
j Extended range minimum can be 75% of standard minimum.
k Extended range minimum can be 66% of standard minimum.
Cooling Coil Operation
1. Proper air distribution is vital to coil performance. Air flow
anywhere on the coil face should not vary by more than 20%.
2. Air velocities should be maintained between 200 and 550 feet
per minute.
3. For chilled fluid coils, fluid velocities should be maintained
within the recommended values of 1 to 8 feet per second (fps)
for Water and 1 to 6 fps for Glycol solutions.
5-594.10
Minimum Airflow (CFM)
High ATR
Low ATR
(Digit 10=H)
(Digit 10=H)
Extended
Range j
Standard
422
938
563
1,250
703
1,563
844
1,875
984
2,188
1,125
2,500
1,266
2,813
1,406
3,125
1,688
3,750
1,969
4,375
2,250
5,000
-
3,125
3,750
-
4,375
-
5,000
-
6,562
-
7,500
-
Extended
Range k
619
825
1,031
1,238
1,444
1,650
1,856
2,063
2,475
2,888
3,300
2,063
2,475
2,888
3,300
4,331
4,950
25
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