Manual

modulating valve to maintain desired discharge temperature.

Additional Sequence of Operation – All VFD Motor Control

1. During lower speeds, the low-speed potentiometer increases the resistance going to the modulating

valve to limit temperature rise.

2. VFD contacts are included in the burner control circuit to disable the burner should a fault condition

occur in the VFD.

VFD by Others and VAV Choke Off Profile Bypass Damper

As the VFD increases and decreases speed the profile air flow switch modulates the VAV bypass

damper motor to maintain proper pressure drop across the burner profile.

Factory VFD Profile Bypass Damper

The VFD produces a 0-10vdc or a 4-20ma signal to the profile damper motor which modulates

the profile damper’s opening to maintain the burner profile pressure drop. The range of this

modulation must coincide with the VFD operating range.

VFD Preset Speeds

1. Inputs are made to the VFD to engage programmable pre-set speeds. A factory-supplied VFD wiring

and programming instruction sheet comes with every unit. Consult the as-built schematics and the

VFD manual for additional information.

Manual Potentiometer Adjustment – VFD and Choke-Off Dampers

1. A 10k-15k ohm potentiometer may be field supplied and installed if remote manual control is required

for the VFD. See VFD manual for proper installation.

2. A 135 ohm potentiometer is used for remote manual control of choke-off vav dampers.

Static Pressure Control (Photohelic) – VFD and Choke-Off Dampers

The VFD or Choke-off dampers can be controlled by a building static pressure control. This controller will

sense the difference between pressure inside the building, and pressure outside the building and will

control the operating speed of the VFD or position of choke-off dampers to maintain the pressure setting

on the controller. The controller has two set points and an indicator. The two set points are a minimum

desired static pressure point, and a maximum static pressure point.

When building static pressure is below the minimum setting, the VFD will increase the blower RPM or

choke-off damper will open to increase airflow. If the building static is above maximum setting, the VFD

will decrease the blower RPM or close choke-off dampers. When building static pressure is between the

minimum and maximum settings, no signal is given to change VFD speed or choke-off damper position.

See additional wiring and installation information on the static pressure controller and outdoor sensor.

The static pressure controller can be ordered to be shipped loose or mounted inside the unit to reduce

field wiring and assembly.