Specifications

52
Outdoor Dewpoint Limit Check
— If an outdoor relative
humidity sensor is installed, the control is able to calculate the
outdoor air dewpoint temperature and will compare this
temperature against the outside air dewpoint temperature
limit configuration (Configuration
→ ECON → O.DEW). If the
outdoor air dewpoint temperature is greater than O.DEW, free
cooling will not be allowed. Fig. 9 shows a horizontal limit line
in the custom curve of the psychrometric chart. This is the out-
door air dewpoint limit boundary.
Custom Psychrometric Curves
— Refer to the psychrometric
chart and the standard Honeywell A-D curves in Fig. 8. The
curves start from the bottom and rise vertically, angle to the left
and then fold over. This corresponds to the limits imposed by
dry bulb changeover, outdoor enthalpy changeover and
outdoor dewpoint limiting respectively. Therefore, it is now
possible to create any curve desired with the addition of one
outdoor relative humidity sensor and the options for
changeover now available. See Fig. 9 for an example of a
custom curve constructed on a psychrometric chart.
UNOCCUPIED ECONOMIZER FREE COOLING — This
Free Cooling function is used to start the supply fan and use
the economizer to bring in outside air when the outside
temperature is cool enough to pre-cool the space. This is done
to delay the need for mechanical cooling when the system
enters the occupied period. This function requires the use of a
space temperature sensor.
When configured, the economizer will modulate during an
unoccupied period and attempt to maintain space temperature
to the occupied cooling set point. Once the need for cooling has
been satisfied during this cycle, the fan will be stopped.
Configuring the economizer for Unoccupied Economizer
Free Cooling is done in the UEFC group. There are three
configuration options, FC.CF, FC.TM and FC.LO.
Unoccupied Economizer Free Cooling Configuration
(FC.CF) — This option is used to configure the type of unoc-
cupied economizer free cooling control that is desired.
0 = disable unoccupied economizer free cooling
1 = perform unocc. economizer free cooling as available dur-
ing the entire unoccupied period.
2 = perform unocc. economizer free cooling as available,
FC.TM minutes before the next occupied period.
Unoccupied Economizer Free Cooling Time Configuration
(FC.TM) — This option is a configurable time period, prior to
the next occupied period, that the control will allow unoccu-
pied economizer free cooling to operate. This option is only
applicable when FC.CF =2.
Unoccupied Economizer Free Cooling Outside Lockout
Temperature (FC.LO) — This configuration option allows
the user to select an outside-air temperature below which unoc-
cupied free cooling is not allowed. This is further explained in
the logic section.
Unoccupied Economizer Free Cooling Logic
— The follow-
ing qualifications that must be true for unoccupied free cooling
to operate:
• Unit configured for an economizer
• Space temperature sensor enabled and sensor reading
within limits
• Unit in the unoccupied mode
• FC.CF set to 1 or FC.CF set to 2 and control is within
FC.TM minutes of the next occupied period
• Not in the Temperature Compensated Start Mode
• Not in a cooling mode
• Not in a heating mode
• Not in a tempering mode
• Outside-air temperature sensor reading within limits
• Economizer would be allowed to cool if the fan were
requested and in a cool mode
•OAT>FC.LO (1.0° F hysteresis applied)
• Unit not in a fire smoke mode
• No fan failure when configured to for unit to shut down
on a fan failure
If all of the above conditions are satisfied:
Unoccupied Economizer Free Cooling will start when both of
the following conditions are true:
{SPT > (OCSP + 2)} AND {SPT > (OAT + 8)}
The Unoccupied Economizer Free Cooling Mode will stop
when either of the following conditions are true:
{SPT < OCSP} OR {SPT < (OAT + 3)} where SPT = Space
Temperature and OCSP = Occupied Cooling Set Point.
When the Unoccupied Economizer Free Cooling mode is
active, the supply fan is turned on and the economizer damper
modulated to control to the supply air set point (Setpoints
→
SASP) plus any supply air reset that may be applied (Inputs →
RSET → SA.S.R).
ECONOMIZER OPERATION CONFIGURATION — The
configuration items in the E.CFG menu group affect how
the economizer modulates when attempting to follow an
economizer cooling set point. Typically, they will not need
adjustment. In fact, it is strongly advised not to adjust these
configuration items from their default settings without first
consulting a service engineering representative.
In addition, the economizer cooling algorithm is designed to
automatically slow down the economizer actuator’s rate of
travel as outside air temperature decreases.
Fig. 8 — Psychrometric Chart for
Enthalpy Control
CONTROL CURVE
CONTROL POINT
(approx Deg) AT 50% RH
A 73
B 68
C 63
D 58