York ECO R-22 User Manual page 55

ELECTRIC HEATING OPERATION
For units equipped with electric heaters, the unit can
control up to six stages of electric heat which are staged
on based on heating demand calculates by the
OptiLogic controller.
ECONOMIZER OPERATION
The unit can be equipped with one of three types of
optional economizers, dry bulb, single enthalpy, or com-
parative enthalpy. When the unit controller determines
that Outside Air is suitable for economizing, the unit
controller will control the outside air damper(s) open to
provide economizer cooling. If economizer cooling alone
is insufficient for the cooling load, the unit controller
shall stage up compressors, one at a time, to meet
demand.
The control logic for the three types of economizers is
as follows:
Dry Bulb Economizer
The dry bulb economizer is the default economizer
control scheme. With the dry bulb economizer, the
unit controller monitors the Outside Air temperature
only and compares it to a reference temperature set-
ting. Outside Air is deemed suitable for economizing
when the Outside Air temperature is determined to
be less than the reference temperature setting. This
method of economizing is effective, but is prone to
some change-over inefficiencies due to the fact that
this method is based on sensible temperatures only
and does not take Outside Air moisture content into
consideration.
Single Enthalpy Economizer
With the optional single enthalpy economizer, the unit
controller monitors the Outside Air enthalpy in addition
to the Outside Air temperature and compares it to a
reference enthalpy setting and a reference tempera-
ture setting. Outside Air is deemed suitable for econo-
mizing when the Outside Air enthalpy is determined to
be less than the reference enthalpy setting and the
Outside Air temperature is less than the reference tem-
perature setting. This method of economizing allows
the reference temperature setting to be set higher than
the DB Economizer and is consequently a more effi-
cient packaged rooftop economizer.
YORK INTERNATIONAL
Comparative (Dual) Enthalpy Economizer
With the optional comparative enthalpy economizer, the
unit controller monitors and compares the Outside Air
and Return Air enthalpies in addition to comparing the
Outside Air temperature to the reference temperature
setting. Outside Air is deemed suitable for economiz-
ing when the Outside Air enthalpy is determined to be
less than the Return Air enthalpy and the Outside Air
temperature is less than the reference temperature
setting. This method of economizing is the most accu-
rate and provides the highest degree of energy effi-
ciency for a packaged rooftop economizer.
VENTILATION CONTROL SEQUENCES
Minimum OA Damper Position (CV Units)
When the unit goes into the Occupied mode of opera-
tion, the unit controller shall open the Outside Air
Damper to a fixed minimum position. The damper shall
remain at this position as long as the unit is in the
occupied mode, and the economizer is not suitable
for cooling.
Minimum OA Damper Position (VAV Units)
With Variable Air Volume units, there are two Minimum
OA Damper Positions, one when the unit is at full speed
and the second when the unit is at approximately half
speed. These two points allow the control to linearly
reset the position of the OA damper in response to fan
speed.
When the unit goes into the Occupied mode of opera-
tion, the unit controller shall monitor the speed of the
supply fan and open the Outside Air damper to a calcu-
lated minimum position based on the fan speed. This
minimum position shall vary as the speed of the fan
changes. The damper shall remain at this calculated
position as long as the unit is in the occupied mode,
and the economizer is not suitable for cooling.
Air Measurement Stations
When the unit is equipped with an air measurement
station, the unit controller shall control the Outside Air
damper to a measured flow rate through the Air Mea-
surement Station.
When the unit goes into the Occupied mode of opera-
tion, the unit controller shall control the Outside Air
damper to maintain the Minimum AirFlow Setpoint
FORM 100.50-EG3 (802)
55