11
CAPACITY CONTROL AND STAGING
The HVAC system is sized to handle the load of the entire
home or building. Because of this, when all the zones are not
calling, the load to the HVAC system can diminish below its
designed capacity. Left unchecked, the HVAC unit could
freeze up or overheat. To compensate for this, the Digitract 4
is furnished with a built in Capacity Controller. The basic
function of the Capacity Controller is to monitor the leaving
Y1 Cool Operation
- Upon a cool call, the controller will ener-
gize Y1 and G. If the leaving air temperature drops below the
low limit setpoint, Y1 will be de-energized, the PWR LED will
flash indicating a stage 1 capacity cutout. After 4 minutes the
leaving air temperature will be checked again; if the leaving
air temperature has risen above the cutout setpoint, then Y1
will re-energize and PWR LED will stop flashing. If controller is
for Heat Pump, and the reversing valve is in the O position,
Upon a heat call, the controller will energize W1 LED if the
leaving air temperature is less than High Limit setpoint. This
setpoint is determined by adjusting the potentionmeter
located in the upper right corner of the controller, HI. W1 well
de-energize if LAT exceeds the High setpoint, W1 LED will
de-energize and the PWR LED will flash slowly indicating a
stage 1 capacity cutout. If the LAT is below the High limit, W1
will energize, W1 LED will illuminate.
Y1 Heat Operation
- Upon a call for heat, the controller will
energize Y1 LED, if the LAT rises above the High Limit (poten-
tiometer located on controller top right corner, marked HI),
then Y1 LED will de-energize and PWR LED will flash indicat
-
ing a stage 1 capacity cutout. After a 4 minute time delay the
LAT will be checked; if LAT is less than HI cutout then Y1 LED
will re-energize and PWR LED will stop flashing. If the O/B
jumper is in the B position, the O/B LED will illuminate with
Y1.
Y2 Heat Operation
- After 4 minutes of continuous Y1 opera-
tion, the leaving air temperature will be checked. If the LAT is
less than high limit (HI) minus 25° (HI of 120 - 25 = 95°), then
Y2 will energize, indicated by the Y2 LED. Y2 will cycle off
when LAT is HI minus 5° (120 - 5 = 115) or when all heat calls
are satisfied.
Fossil Fuel Operation
- For Fossil Fuel Operation, FF jumper
must be placed in the FF position.
For greater efficiency and comfort, an Outdoor Air Sensor,
Part # FOAS, should be used in the Fossil Fuel application.
The FOAS is used to lock out compressor operation when
outside air temperature has dropped below Balance Point
setpoint, the temperature at which that heat pump is no
longer efficient. When the outside temperature is less than
Balance Point setpoint and there is a call for heat, Y1 and Y2
air temperature and cycle the unit off when the air is out of
operating range and, after a minimum 4 minute time delay,
turn the unit back on when the air temperature has returned
within operating range. Additionally, for heat pump the
Capacity Controller will turn on the heat pump auxiliary heat
if the coil leaving air temperature is not hot enough in heat
mode.
the O/B output will be energized simultaneously with the Y1
indicated by the O/B LED. After 8 minutes of continuous Y1
run time, LAT (leaving air temperature) will be checked. if LAT
is greater than low limit (LO) plus 12° (LO of 45° + 12° = 57°),
then Y2 will energize and Y2 LED will illuminate. Y2 will cycle
off when LAT drops below 5° plus low limit (45° + 5° = 50° ).
Y2 and G will de-energize when all calls are satisfied, and the
controller will go into a 4 minute purge cycle
.
W2 Heat Operations
- After W1 has operated continuously
for 4 minutes and LAT is less than High Limit minus 25° (if HI
of 145° - 25° = 120°), then W2 will energize by the W2 LED. If
LAT rises to High Limit minus 5°, then W2 will de-energize
indicted by W2 LED off and PWR LED flashing quickly. If all
heat calls are satisfied both W1 and W2 will de-energize and
controller will go into a 4 minute purge cycle.
will be locked out; and W2 will energize indicated by W2 LED
illuminating. The HI limit will be reset automatically to 135° to
provide HI limit protection for furnace. Calls for heat are
recognized from all thermostats; dampers for zones calling
for heat will open, indicated by DPR LEDs off, while zones not
calling will close, indicated by illuminated DPR LEDs. The
Balance Point setpoint is adjustable from 25° to 41° by
moving Balance Point potentiometer to desired setpoint.
Refer to page 9 for potentiometer location (J) on DT4U.
Applications without FOAS will stage on time and tempera-
ture. With the Fossil fuel jumper in the FF position, after 8
minutes of continuous heat operation, if leaving air is less
than 90°, then W2 will energize. Y1 and Y2 will de-energize
indicated by LEDs off; W2 LED will illuminate and furnace will
start. The HI limit will reset automatically to 135° to protect
furnace heat exchanger. DPR LED for calling zones will be off,
while DPR LEDs for non-calling zones will illuminate. All
thermostats are able to make calls for heat.
Auxiliary Heat (Electric Strip Heat)
- After 8 minutes of contin-
uous operation the leaving air temperature is checked. If LAT
is below 90° W2 will energize and W2 LED will illuminate. W2
will cycle off above 100°, or when all calls are satisfied.
COOLING OPERATION
HEAT OPERATION - GAS/ELECTRIC
HEAT OPERATION - HEAT PUMP
