Honeywell PC8900A Manual page 37

current setpoint and system settings. They can monitor the
outdoor temperature, humidity, current setpoint, system mode,
alarm out on High-Low temperature, filter status and
equipment failure. See separate instructions on CM8900A and
W88735B for more detailed information.
Minimum-Off Timing
A minimum-off timer in the Comfort Center™ control systeml
assures that the cooling compressor or heat pump
compressor does not come on again for at least five minutes
after it turns off. The minimum-off timer is triggered when the
compressor goes off and when the system switch is changed.
If the compressor turns off when the setpoint is changed, then
the minimum-off timer is triggered. The minimum-off timer
operates during the first stage of cooling on the W8900A
and C. The minimum-off timer operates during the first stage
of both heating and cooling on the W8900B. The minimum-off
timer can be disabled in the installer setup mode and can be
overridden in the checkout mode.
Fan Control
The fan comes on when any of these conditions are present (it
can take up to 20 seconds for the fan to turn on).
NOTES:
FAN ON:This setting means the fan runs 100 percent of
the time during this period.
FAN AUTO:The AUTO setting means the fan runs only
with the heating or cooling system.
FAN CIRC:When the fan is set to CIRC, the fan runs at
least 30 percent of the time, including the time the
fan is on while the heating or cooling system is
running. When the heating or cooling system is
not running at all, the fan runs for ten minutes on,
and 20 minutes off. Protection against fan short
cycling is built into the device.
The installer setup has the ability to extend fan operation
following a cooling cycle. When selected, the fan run time at
the end of a cooling cycle is extended 80 seconds.
Ventilation Control
When the Comfort Center™ program is configured for
ventilation, the ventilator contacts are closed, activating high
speed ventilation equipment in response to any of the
following conditions:
• Ventilation request from the user.
• High CO condition (measured by CO
2
• High humidity condition while in the heating mode.
• Programming.
The ventilation control is set for a 20-second cycle so the user
can wait up to 20 seconds for the ventilation to start after the
request is made.
When the Comfort Center™ program is configured for no
ventilation, the system fan is activated in response to a call for
ventilation. When the Comfort Center™ system is configured
for a VNT damper, calls for ventilation close the VNT damper
contacts and activate the system fan.
NOTE: Each period can have a ventilation program.
Humidity Control (W8900A and B)
IMPORTANT
Heat Mode
Heat humidification is activated when the sensed RH is below
the setpoint RH and the fan is on. When a ventilator is
installed and configured and the RH is above the setpoint, the
Comfort Center™ program calls for ventilation to lower the
humidity.
Cool Mode
Cool dehumidification is activated when the sensed RH is
above the setpoint. This can cause overcooling; see W8900C
Humidity Control section.
Dehumidification equipment can be controlled in many ways.
See Installation section for hookup drawings.
Humidity Control with Fan
Select models of the W8900A-C can be configured to control
humidity levels while the fan is energized. This type of
humidity control does not require the heat output to be
energized. In order for the humidity terminals ("HUM") to
energize, the following conditions must be met:
Frost Control
The frost Control needs an outdoor sensor to be activated.
The Comfort Center™ control system calculates the
possibility of condensation or frost forming by using the
outside temperature and inside temperature to determine the
dewpoint. When the dewpoint would cause frost or
condensation, the humidification request will not be honored.
The frost setting has a value range of zero to ten with zero
representing glass with poor insulation and ten representing
glass with perfect insulation.
NOTE: Set the frost setting to 10 to severely limit the frost
monitor).
2
W8900C Humidity Control
The W8900C can be configured to control humidity levels in
both the heat and cool modes. A humidifier is connected to
the HUM terminals. The HUM contacts close and activate a
humidifier when the sensor RH is below the setpoint RH.
Dehumidification equipment is controlled by the Y1,Y2, and G
terminals. These contacts close to activate the air conditioning
equipment when the sensed RH is above the setpoint RH.
The reheat feature is designed to keep the occupants
comfortable. If the cooling setpoint is reached, but the
dehumidification setpoint is not satisfied, the Comfort
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The humidity configuration can only be set for heat or
cool by the installer. If configured for both, the
Comfort Center™ control system ignores both
settings and no humidity control is activated.
1. System switch must be in the heat mode or in Auto
mode with heat being the last call.
2. There must be a call for humidity (sensed relative
humidity [RH] is below RH setpoint).
3. Fan terminal ("G") must be energized through
programming or by pressing the fan button on the
PC8900A.
control.
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