Product data

54
connected to a coil on the discharge side of the unit and
supplied by a boiler or a single-stage ducted electric heater
in order to maintain the desired heating set point. Should
the compressor capacity be insufficient or a compressor
failure occurs, the auxiliary heat will be used. Unless the
compressor fails, the auxiliary heat will only operate to
supplement the heat provided by the compressor if the
space temperature falls more than one degree below the
desired heating set point (the amount is configurable). The
heat will be controlled so the SAT will not exceed the max-
imum heating SAT limit.
Auxiliary modulating hot water/steam heating reheat
—
The control can modulate a hot water or steam valve con-
nected to a coil on the discharge side of the unit and sup-
plied by a boiler in order to maintain the desired heating
set point should the compressor capacity be insufficient or
a compressor failure occurs. Unless a compressor fault
condition exists, the valve will only operate to supplement
the heat provided by the compressor if the space tempera-
ture falls more than one degree below the desired heating
set point. The valve will be controlled so the SAT will not
exceed the maximum heating SAT limit.
Two-position hot water/steam heating reheat
— The con-
trol can operate a two-position, NO or NC, hot water or
steam valve connected to a coil on the discharge side of the
unit and supplied by a boiler in order to maintain the de-
sired heating set point should the compressor capacity be
insufficient or a compressor failure occurs. Unless a com-
pressor fault condition exists, the valve will only open to
supplement the heat provided by the compressor if the
space temperature falls more than one degree below the
desired heating set point. The valve will be controlled so
the SAT will not exceed the maximum heating SAT limit.
The heat stage will also be subject to a 2-minute minimum
OFF time to prevent excessive valve cycling.
Single stage electric auxiliary heat
— The control can oper-
ate a field-installed single stage of electric heat installed on
the discharge side of the unit in order to maintain the de-
sired heating set point should the compressor capacity be
insufficient or a compressor failure occurs. Unless a com-
pressor fault condition exists, the heat stage will only oper-
ate to supplement the heat provided by the compressor if
the space temperature falls more than one degree below
the desired heating set point. The heat stage will be con-
trolled so the SAT will not exceed the maximum heating
SAT limit. The heat stage will also be subject to a 2-minute
minimum OFF time to prevent excessive cycling.
Indoor air quality (IAQ) and demand controlled ven-
tilation (DCV) — If the optional indoor air quality sensor
is installed, the WSHP Open controller can maintain in-
door air quality via a modulating OA damper providing de-
mand controlled ventilation. The control operates the mod-
ulating OA damper during occupied periods. The control
monitors the CO
2
level and compares it to the configured
set points, adjusting the ventilation rate as required. The
control provides proportional ventilation to meet the re-
quirements of ASHRAE specifications by providing a base
ventilation rate and then increasing the rate as the CO
2
lev-
el increases. The control will begin to proportionally in-
crease ventilation when the CO
2
level rises above the start
ventilation set point and will reach the full ventilation rate
when the CO
2
level is at or above the maximum set point.
A user-configurable minimum damper position ensures
that proper base ventilation is delivered when occupants
are not present. The IAQ configurations can be accessed
through the configuration screen. The following conditions
must be true in order for this algorithm to run:
• Damper control is configured for DCV.
• The unit is in an occupied mode.
• The IAQ sensor reading is greater than the DCV start
control set point.
The control has four user adjustable set points: DCV
start control set point, DCV maximum control set point,
minimum damper position, and DCV maximum damper
position.
Two-position OA damper
— The control can be config-
ured to operate a ventilation damper in a two-position ven-
tilation mode to provide the minimum ventilation require-
ments during occupied periods.
Waterside economizer — The WSHP Open controller
has the capability of providing modulating or two-position
water economizer operation (for a field-installed economiz-
er coil mounted to the entering air side of the unit and con-
nected to the condenser water loop) in order to provide
free cooling (or preheating) when water conditions are op-
timal. Water economizer settings can be accessed through
the equipment status screen. The following conditions
must be true for economizer operation:
• SAT reading is available.
• EWT reading is available.
• If occupied, the SPT is greater than the occupied cool-
ing set point or less than the occupied heating set point
and the condenser water is suitable.
• Space temperature reading is valid.
• If unoccupied, the SPT is greater than the unoccupied
cooling set point or less than the unoccupied heating set
point and the condenser water is suitable.
Modulating water economizer control
— The control has
the capability to modulate a water valve to control con-
denser water flowing through a coil on the entering air side
of the unit.
Cooling — The purpose is to provide an economizer cool-
ing function by using the water loop when the entering wa-
ter loop temperature is suitable (at least 5° F below space
temperature). If the water loop conditions are suitable,
then the valve will modulate open as required to maintain a
supply air temperature that meets the load conditions.
Should the economizer coil capacity alone be insufficient
for a period greater than 5 minutes, or should a high hu-
midity condition occur, then the compressor will also be
started to satisfy the load. Should the SAT approach the
minimum cooling SAT limit, the economizer valve will
modulate closed during compressor operation.
Heating — Additionally, the control will modulate the wa-
ter valve should the entering water loop temperature be
suitable for heating (at least 5° F above space temperature)
and heat is required. The valve will be controlled in a simi-
lar manner except to satisfy the heating requirement.
Should the economizer coil capacity alone be insufficient
Controls (cont)