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39MUA-PRC001-EN
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Mixing Box —
Building Pressure Control
Purpose
To provide a way for recirculating return
air and letting building pressure control
the amount of air which is recirculated.
Operation
If building pressure is less than desired,
the RA damper modulates to a more
closed position and the OA damper
modulates to a more open position.
More OA is brought into building.
If building pressure is greater than
desired, the RA damper modulates to a
more open position and the OA damper
modulates to a more closed position.
Less OA is brought into building.
The minimum OA damper position is
factory set with a potentiometer. The
minimum OA damper position is set at 4
cfm per 1 MBh of burner rated input, or
25% supply air, whichever is larger.
The desired pressure differential is set at
the photohelic.
Motorized Inlet Damper
Purpose
To provide a shut-off damper.
Operation:
When the fan switch is on, the inlet
damper begins to open. As the inlet
damper approaches the full open
position, a built-in end switch located
inside the damper motor closes. The
closed end switch starts the fan motor.
When the fan switch is turned to the
“off” position, or if there is a power
failure, the inlet damper closes.
Option Includes:
• Damper
• Two position damper motors with built-
in end switch and spring return
• Linkage
Option Includes:
• Mixing box with V-bank filter and
permanent filters
• Dampers
• Modulating damper motor(s)
• Photohelic differential pressure gauge