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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 supplied
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 temperature falls more than one degree
below the desired heating set point. The valve will be con-
trolled 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 desired heating set
point should the compressor capacity be insufficient or a com-
pressor failure occurs. Unless a compressor fault condition ex-
ists, 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 op-
erate a field-installed single stage of electric heat installed on
the discharge side of the unit in order to maintain the desired
heating set point should the compressor capacity be insufficient
or a compressor failure occurs. Unless a compressor fault con-
dition exists, the heat stage 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
heat stage will be controlled so the SAT will not exceed the
maximum heating SAT limit. The heat stage will also be sub-
ject to a 2-minute minimum OFF time to prevent excessive
cycling.
INDOOR AIR QUALITY (IAQ) AND DEMAND CON-
TROLLED VENTILATION (DCV) — If the optional in-
door air quality sensor is installed, the WSHP Open controller
can maintain indoor air quality via a modulating OA damper
providing demand controlled ventilation. The control operates
the modulating OA damper during occupied periods. The con-
trol 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 requirements of
ASHRAE (American Society of Heating, Refrigerating and
Air Conditioning Engineers) specifications by providing a base
ventilation rate and then increasing the rate as the CO
2
level in-
creases. The control will begin to proportionally increase venti-
lation 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 min-
imum damper position ensures that proper base ventilation is
delivered when occupants are not present. The IAQ configura-
tions 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 configured
to operate a ventilation damper in a two-position ventilation
mode to provide the minimum ventilation requirements during
occupied periods.
DEHUMIDIFCATION — The WSHP Open controller will
provide occupied and unoccupied dehumidification only on
units that are equipped with the modulating hot water reheat
(HWR) option. This function requires an accessory space rela-
tive humidity sensor. When using a relative humidity sensor to
control dehumidification during occupied or unoccupied times,
the dehumidification set points are used accordingly. When the
indoor relative humidity becomes greater than the dehumidifi-
cation set point, a dehumidification demand will be acknowl-
edged. Once acknowledged, the dehumidification output will
be energized, bringing on the supply fan (medium speed), me-
chanical cooling, and the integral hot water reheat coil. The
controls will engage Cooling mode and waste heat from the
compressor cooling cycle will be returned to the reheat coil si-
multaneously, meaning that the reversing valve is causing the
compressor to operate in the Cooling mode. During Cooling
mode, the unit cools, dehumidifies, and disables the HWR coil;
however, once the call for cooling has been satisfied and there
is still a call for dehumidification, the unit will continue to op-
erate using the reheat mode and HWR coil.
WATERSIDE ECONOMIZER — The WSHP Open control-
ler has the capability of providing modulating or two-position
water economizer operation (for a field-installed economizer
coil mounted to the entering air side of the unit and connected
to the condenser water loop) in order to provide free cooling
(or preheating) when water conditions are optimal. Water econ-
omizer settings can be accessed through the equipment status
screen. The following conditions must be true for economizer
operation:
• SAT reading is available.
• LWT reading is available.
• If occupied, the SPT is greater than the occupied cooling
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 condenser
water flowing through a coil on the entering air side of the unit.
Cooling — The purpose is to provide an economizer cooling
function by using the water loop when the entering water 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 ca-
pacity alone be insufficient for a period greater than 5 minutes,
or should a high humidity condition occur, then the compressor
will also be started to satisfy the load. Should the SAT ap-
proach the minimum cooling SAT limit, the economizer valve
will modulate closed during compressor operation.
Heating — Additionally, the control will modulate the water
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 similar manner ex-
cept to satisfy the heating requirement. Should the economizer
coil capacity alone be insufficient to satisfy the space load con-
ditions for more than 5 minutes, then the compressor will be
started to satisfy the load. Should the SAT approach the maxi-
mum heating SAT limit, the economizer valve will modulate
closed during compressor operation.
Two-Position Water Economizer Control
— The control has
the capability to control a NO or NC, two-position water valve
to control condenser water flow through a coil on the entering
air side of the unit.
Cooling — The purpose is to provide a cooling economizer
function directly from the condenser water loop when the en-
tering water loop temperature is suitable (at least 5 F below
space temperature). If the optional coil is provided and the wa-
ter loop conditions are suitable, then the valve will open to pro-