5168276-XIM-B-0216
22
Johnson Controls Unitary Products
Vent And Combustion Air
Two vent hoods and a combustion air hood (with screens) are
shipped attached to the blower housing in the blower
compartment. For units with factory installed VFD option, the
hoods and accompanying hardware are shipped inside the
gas heat section. These hoods must be installed to assure
proper unit function. All hoods must be fastened to the
outside of the gas heat access panel with the screws
provided in the bag also attached to the blower housing.
The screen for the combustion air intake hood is secured to
the inside of the access panel opening with four fasteners and
the screws used for mounting the hood to the panel. The top
flange of this hood slips in under the top of the access panel
opening when installing. Refer to Vent and Combustion Air
Hood Figure 18.
Each vent hood is installed by inserting the top flange of the
hood into the slotted opening in the access panel and securing
in place.
The products of combustion are discharged horizontally
through these two screened, hooded vent openings on the
upper gas heat access panel.
Figure 18: Vent and Combustion Air Hood
Options/Accessories
Economizer/Motorized Outdoor Damper Rain Hood
The instruction for the optional economizer/motorized damper
rain hood can be found in the rain hood kit. Use these
instructions when field assembling an economizer rain hood
onto a unit. The outdoor and return air dampers, the damper
actuator, the damper linkage, the outdoor and return air divider
baffles, and all the control sensors are factory mounted as part
of the “Factory installed” economizer option.
Power Exhaust/Barometric Relief Damper and Rain Hood
The instructions for the power exhaust/barometric relief damper
and rain hood can be found in the rain hood kit. The exhaust
fan, all supporting brackets, angles, and the wiring are factory
installed as part of the power exhaust option.
Economizer Sequences
Several functions can drive the economizer, including: minimum
position, free cooling, economizer loading, and minimum
outdoor air supply.
Economizer Minimum Position
The economizer minimum position is set during occupied mode
when outside air is not suitable for free cooling. The position of
the damper is set proportionally between the "Economizer
Minimum Position and the Economizer Minimum Position Low
Speed Fan" set points, in relationship to the VFD output
percentage. On a constant volume single speed supply fan
system both set-points should be set to the same value.
Free Cooling
Four types of free cooling options are available: dry bulb
changeover, single enthalpy, dual enthalpy changeover, and
Auto.
Dry Bulb Changeover
For dry bulb economizer operation, the outside air is suitable for
free cooling if the outside air temperature is 1°F below the
Economizer OAT Enable Setpoint
and
1°F below the Return Air
Temperature.
Free cooling is no longer available if the outside air temperature
rises above
either
the Economizer OAT Enable setpoint
or
the
return air temperature.
Single Enthalpy Changeover
For single enthalpy economizer operation, the outside air is
suitable for free cooling if the outside air enthalpy is at least 1
BTU/lb below the Economizer Outside Air Enthalpy Setpoint
and
the outside air temperature is no greater than the RAT plus
9°F.
If the outside air temperature rises above the RAT plus 10°F,
free cooling is no longer available. The outside air temperature
must drop to no greater than RAT plus 9°F to enter free cooling
again.
Free cooling is no longer available if the outside air enthalpy
rises above the Economizer Outside Air Enthalpy Setpoint.
Dual Enthalpy Changeover
For dual enthalpy economizer operation, the outside air
enthalpy must be lower than the return air enthalpy by 1 btu/lb
AND the outside air temperature is no greater than the RAT
plus 9°F.
Auto
The control determines the type of free cooling changeover
based on which sensors are present and reliable. Conditions
include:
• Return and outside air dry bulb = dry bulb changeover
Vent Air
Outlet Hoods
Slotted Openings in
Access Panel
Combustion Air
Intake Hood
Gas Heat
Access Panels
