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 subject 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 CO2 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 CO2 level in- creases. The control will begin to proportionally increase venti- lation when the CO2 level rises above the start ventilation set point and will reach the full ventilation rate when the CO2 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.
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- vide cooling to the space when required. Should the capacity be insufficient for a period greater than 5 minutes, or should a high humidity condition occur, then the compressor will be started to satisfy the load. Should the SAT reach the minimum cooling SAT limit, the economizer valve will close during compressor operation.
Heating — Additionally, the economizer control will open the water valve should the entering water loop temperature be suit- able for heating (at least 5° F above space temperature) and heat is required. The valve will be controlled in a similar man- ner except to satisfy the heating requirement. Should the coil capacity be insufficient to satisfy the space load for more than 5 minutes, then the compressor will be started to satisfy the load. Should the SAT reach the maximum heating SAT limit, the economizer valve will close during compressor operation.
DEMAND LIMIT — The WSHP Open controller has the ability to accept three levels of demand limit from the network. In response to a demand limit, the unit will decrease its heating set point and increase its cooling set point to widen the range in order to immediately lower the electrical demand. The amount of temperature adjustment in response is user adjustable for both heating and cooling and for each demand level. The re- sponse to a particular demand level may also be set to zero.
CONDENSER WATER LINKAGE — The control pro- vides optimized water loop operation using an universal con- troller (UC) open loop controller. Loop pump operation is auto- matically controlled by WSHP equipment occupancy sched- ules, unoccupied demand and tenant override conditions. Positive pump status feedback prevents nuisance fault trips. The condenser water linkage operates when a request for con- denser water pump operation is sent from each WSHP to the loop controller. This request is generated whenever any WSHP is scheduled to be occupied, is starting during optimal start (for warm-up or pull down prior to occupancy), there is an unoccu- pied heating or cooling demand, or a tenant pushbutton over- ride. At each WSHP, the water loop temperature and the loop pump status is given. The WSHP will NOT start a compressor until the loop pumps are running or will shutdown the com- pressors should the pumps stop. This prevents the WSHP from operating without water flow and thus tripping out on refriger- ant pressure, causing a lockout condition. The WSHP Open controller control will prevent this from occurring. Also, the loop controller can be configured to start the pumps only after a configurable number of WSHPs are requesting operation (from 1-"N"). This can be used to prevent starting the entire loop op- eration for only one WSHP. Meanwhile, the WSHPs will not
