Figure 63: High gas heat flue exhaust hood
installation
Item Description
A Flue exhaust
B Four mounting screws
C Combustion air intake louvers
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 setpoints, in relationship to the VFD
output percentage. On a constant volume single
speed supply fan system, set both setpoints to the
same value.
Free cooling operation
When the control determines that the outside air
is suitable, the first stage of cooling is always free
cooling.
Thermostat
In free cooling, with a thermostat input to Y1,
the dampers modulate to control the supply air
temperature to the economizer setpoint +/- 1°F
(default 55°F).
If the thermostat provides an input to Y2 and the
parameter COMPRESSORS OFF IN FREE COOLING
is turned OFF, a compressor output energizes.
The economizer dampers continue to modulate
to control the supply air temperature to the
economizer setpoint.
If the supply air temperature cannot be maintained
within 5°F of the economizer setpoint, the first
stage compressor (C1) is turned on. The second
stage compressor (C2) is added as needed to keep
the supply air temperature within the 5°F of the
economizer setpoint.
Sensor
In free cooling, with a demand from the zone/return
sensor for the first stage of cooling, the dampers
modulate to control the supply air temperature to
the economizer setpoint +/- 1°F.
If the economizer output is at 100% and the SAT
is greater than the economizer setpoint + 1°F,
the control starts a 12-minute timer to energize a
compressor output.
If at any time the economizer output drops
below 100%, the timer stops and resets when the
economizer output returns to 100%.
When a compressor output is turned ON, the
economizer dampers continue to modulate
to control the supply air temperature to the
economizer setpoint.
At no time is a compressor output turned ON if the
economizer output is less than 100%, even if the
differential between zone (or return) temperature
and the current cooling setpoint is great enough to
demand more than one stage of cooling.
If the economizer output goes to minimum position
and the SAT is less than economizer setpoint -1°F,
the control starts a 12-minute timer to de-energize a
compressor output.
If at any time the economizer output goes above
the minimum position, the timer stops and resets
when the economizer output returns to minimum
position.
If the demand for cooling from the space/return
is satisfied, the economizer output modulates to
minimum position and the compressor outputs are
de-energized as long as their minimum run timers
have expired.
Installation Manual: YORK
®
Sun
™
Select YV28 to YV50
71
Johnson Controls Ducted Systems
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