York MILLENNIUM Y22 Installation Manual Download Page 88

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102777-YIM-E-0206

Unitary Products Group

The choice of a method with highest preference is automatic
(“self-configuration”) based on availability of appropriate sen-
sors. If a sensor fails/ goes unreliable, a fault is indicated and,
again, the next highest preference method will be automati-
cally selected (“fault tolerance”).

There are two ON/OFF programmable parameters related to
the choice of an economizer method:

•

“OAH Sensor Enable”

•

“RAH Sensor Enable”

These parameters are set to reflect the installed sensors that
can be used by the “self-configuration” feature and control
sensor failure alarms.

The OAH sensor, if available, allows use of Outside Enthalpy
method for deciding on free cooling availability.

The RAH sensor, if available in addition to the OAH sensor,
allows use of Differential Enthalpy method for deciding on
free cooling availability.

See paragraphs OAH Sensor Enable and RAH Sensor
Enable in Settable Parameters, Table 52 in this document.

If the selected method is using an enthalpy, the enthalpy is
calculated in the controller from sensed temperature and
humidity of the respective air stream.

Differential Enthalpy Method

: is set by parameter and used

only when sensors for Outdoor Air temperature, Outdoor Air
humidity, Return Air temperature and Return Air humidity are
all installed and reliable.

Outside Enthalpy Method

: will be configured by setting ON

the parameter for the Outdoor Humidity Sensor [OAH], and
will be the default if the unit defined as Differential Enthalpy
cannot read the Return Air Humidity sensor.

Outside Temperature Method

: will be self-configured and

used only when differential enthalpy or outside enthalpy
methods are not available, and sensor for Outside Air tem-
perature is installed and reliable.

Economizer is suitable when OAT is less than SAT se
10

F. Use a 2

F differential on both sides of this limit. As the

SAT setpoint value, use only one of the programmed 1

stage economizer setpoints (depending on what cooling

stage is called), not any “transient” setpoints that may be
temporarily used during process of staging. Note that this rule
does not reflect any consideration of geographical location
and weather conditions, but rather reflects the average
expected SAT temperature drop obtained from DX cooling
stages, i.e. the highest outdoor air temperature that the DX
cooling can still reliably reduce to the SAT setpoint

Outside Enthalpy Method:

Economizer is suitable when OA

Enthalpy is less than Outside Enthalpy number AND OAT is
less than SAT se 10

F: Use a 2

F and 1 BTU/LB dif-

ferentials respectively on both sides of these limits. The
Enthalpy number is a programmed parameter (range 22-40
BTU/LB, default 30 BTU/LB. The Enthalpy Number can be
viewed as the maximum outdoor air enthalpy with which the

outside air can still be considered suitable for DX cooling, or,
in comparison to the Differential Enthalpy Method described
below, as a “best guess” on actual return air enthalpy (which
in this method is not being sensed). The temperature limit
reflects the average expected SAT temperature drop
obtained from DX cooling stages.

Differential Enthalpy Method:

Economizer is suitable when

OA Enthalpy is less than the RA Enthalpy AND OAT is less
than SAT setpoint plus 10

F (+/- 2

and 1 BTU/LB): Use a 2

and 1 BTU/LB differentials respectively on both sides of these
limits. This is similar to the Outside Enthalpy method, except
instead of a programmed Enthalpy Number, an actually
sensed return air enthalpy is used.

SAT Control with Economizer

If the economizer is “suitable” (free cooling is available) and
cooling is required (the unit is not in satisfied state), the algo-
rithm will be active and modulate economizer position in
order to control SAT to the active SAT setpoint. If the econo-
mizer is not suitable, the algorithm is deactivated and the
economizer is placed at its programmed minimum position.
The economizer control algorithm will typically be cycled
ON/OFF several times an hour (will be activated and deacti-
vated) under control of a zone thermostat, or a zone sensor.
A zone control algorithm will activate the economizer algo-
rithm when cooling is required, and will switch between Econ-
omizer 1

and 2

stage SAT setpoints, and will deactivate

the economizer algorithm when the zone is satisfied.

The PI algorithm is direct acting. This economizer control
algorithm is always active during economizer operation (as
long as economizer is “suitable”) and will control SAT to an
active (1

or 2

stage) Economizer setpoint. This means

that this control loop not only modulates the Outside Air
damper open to add free cooling and decrease mixed air
temperature to maintain SAT at setpoint, but, when DX cool-
ing is running, also may modulate the Outside Air damper
closed to increase mixed air temperature (use more return
air) and thus add load on the DX coil to maintain SAT at set-
point (“economizer loading”). This represents a trade-off
between energy and compressor cycling.

Control of compressors with Economizer

Compressors are turned ON / OFF individually, regardless of
grouping them into “stages”. The stage groups are used only
for control of compressors with external signals, such as from
a thermostat.

Turning ON of compressor #1:

Never operate compression if the Economizer can maintain
the SAT setpoint with free cooling. If no compressors are on,
and the economizer controller is saturated High (i.e. the
economizer is 100% open and can no longer maintain the
SAT setpoint by just free cooling),

•

Temporarily override SAT setpoint to increase it by 5

•

When the SAT is within 0.5

F of this temporary setpoint,

turn on compressor #1

•

Resume SAT control at active setpoint