General Information
20 OAU-SVX007A-EN
is energized, the NO contact is closed and the NC contact is
opened, thus opening the outdoor air damper, and closing the
return air damper.
Mixed Air Setup on 2-Position Dampers
2-Position outdoor and return air dampers can be set up for a
mixed airflow using mechanical stops at any position between
0 and 90deg. The mechanical stops are factory provided,
installed on the actuators. With the damper closed, loosen the
set screw on the stop and set it in position. Depending on static
pressures, setting the position on the return air damper may
not be necessary to achieve proper mixed airflows. Depending
on access, the clamp may need to be loosened and re-
positioned to set the mechanical stop in position.
2-Position Return Air Damper
2-position return air damper without an outdoor air damper is
an open/close damper that is energized with 24Vac when the
unit begins its startup sequence and energies the coil on the
RADR relay. There is an end switch wired back to the
controller, factory set to close at 30-50% damper position,
using a rotary dial on the actuator, to begin the supply fan
sequence. The damper should remain closed while the unit is
off, with or without power.
No Damper (100% Return Air)
No return or outdoor air damper installed and supply fan
startup sequence begins immediately.
Exhaust Dampers
Barometric Relief Exhaust Dampers
Barometric relief dampers are weighted backdraft dampers
with a counterweight that requires a small amount of back
pressure before the dampers are opened. The counterweights
are adjustable such that the pressure required to open them
can be set for site requirements. When the exhaust fan is
operating (if installed), the backdraft dampers open
automatically as the fan speed increase.
Primary Heaters
Indirect-Fired Gas Heat
Indirect Gas Heater Sequencing
Primary gas heat typically has a minimum turndown of 5°F to
15°F, depending on heater size and airflow. A resetting
deadband is utilized to prevent cycling due to minimum
turndown of the modulating heater, but still allows precise
control when the heat demand is not between stages and
minimum turndown. The deadband ranges from 0.5°F to 2°F
for the discharge air setpoint.
Air stratification can occur whenever there is a duct tee
immediately after the unit or when the discharge air sensor is
installed too closely to the outlet. When there's more than a
single stage of heat, the primary heater is split manifold and
there are instances where only half the gas heater is ignited,
and only half of the air stream is conditioned immediately
downstream of the unit.
Modulating 1-Stage Gas Heat
Heat 1 is enabled whenever the Heating Capacity Primary
Status is above 0%, and the modulating heat output is directly
proportional to the heating capacity. Stage 1 heat command is
turned off once the heat capacity returns to 0% for 3 continuous
minutes and has a minimum on time of 5 minutes.
Modulating 2-Stage Gas Heat
With modulating 2-stage gas heat, there is a single heater with
a split manifold, with each stage having 50% of the total
capacity. The first stage is on one side, and the second stage
is on the other side.
Heat 1 is enabled whenever the Heating Capacity Primary
Status is above 0%, and heat 2 is enabled whenever the
heating capacity reaches 50%. The modulating heat output is
scaled from 0 to 100% with the heating capacity between 0-
50% and again at 50-100% see, Figure 1.
Heat 1 heat command is turned off once the heat capacity
returns to 0% for 3 continuous minutes and has a minimum on
time of 5 minutes. When starting heat 2, the heat capacity is
initially locked at 50% to prevent overshooting setpoint.
Figure 1. Mechanical stop on a 2-position actuator
Figure 2. Barometric relief damper with counterweight
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