Guide specification - Heat Pump Control Logic

General

1. The heat pump control shall consist of pre-programmed microprocessor-based control with the capability to co-ordinate, manage, and operate a heat pump system, and ancillary equipment including circulation pumps, valves, and other heat equipment.
2. The heat pump control shall provide loop control functions with stand-alone capability, it can be operating independently with normally installed thermostats.
3. The heat pump control shall be a direct digital control (DDC) type of operation with factory-mounted controls.
4. The heat pump control display shall be factory mounted on the front side of the enclosure, the display shall provide access to all operating and configuration parameters, in the loop controller panel not limited to but including loop water temperatures, control valve position, number of stages active for heat pump outlet water temperature, water pumps active, indication of water flow, and heat pump protection parameters.
5. The heat pump control panel shall include a set of staged output transducers, relays, and sensors for stages of ancillary equipment operation in either start/stop or lead/lag operation.
6. The heat pump control panel shall coordinate the start and stop of the stage compressors, ancillary equipment, and control the outlet water temperature of the heat pump regardless of the inlet water temperature delivered to the heat pump.
7. The heat pump control panel shall coordinate the shut down of the system in case of loss of water flow, or excessively high or low water temperatures. The system ancillary equipment will be controlled as needed to maintain the specified loop water temperature.
8. The heat pump control panel shall control water pumps to maintain water flow to the heat pumps and maintain the loop temperatures within the required temperatures for the heat pumps to operate whenany schedule is occupied.
9. Alarms and diagnostics shall include reporting of alarm/alerts and return to normal conditions on DDC controller.

Sequence of operation

1. When the heat pumps are operating, there are built-in water temperature sensors which connected to the DDC and control shall compare the system water temperature to determine the required stages in operation to meet the demand load.
2. The DDC controls the outlet water temperature of the heat pumps at set-point level, it regulates the number of compressors in running and valves motion to maintain the outlet water temperature at set-point. The heat pump oultet water temperature is independent of the inlet water temperature, no matter the inlet water temperature varies, the heat pump controls the outlet water temperature at constant level.
3. Each stage within heat pump is an independent refrigerant circuit which consists of compressor, condenser, evaporator, expansion valve, protection sensors, vale and accessories. Whenever there is failure in a circuit, the other circuit shall not be affected and can be operate normally. The DDC controller shall remove any failure circuit from the running list and prevent the compressor of failure circuit from start-up again until manual reset by authorized person.
4. When the water flow varies to maintenance the constant outlet water temperature of heat pump unit, the DDC output control type is "variable" and the outputs will be calculated using a PID calculation based on the low inlet water temperature and outlet water set point. Both outlet and inlet shall be variable.
5. Whenever the runtime of the lead compressor exceeds the runtime of the lag compressor by 50 hours or more, then the compressor with the lesser runtime will become the lead compressor.
6. At each subsequent star-up, the heating and cooling water valves shall be opened first, the flow switch sense water flow inside the heat pump and then command the running of compressors. Whenever a running compressor needs to be turned off, the shut off of cooling side water valve shall be delayed to prevent freezing temperature inside the heat exchanger. The control shall provide signal to water pump to start again and turn off the water pump if the controller detects this state of conditions.
7. In the event that the heat pumps have been commanded to provide water flow but either the flow switch does not indicate flow or the differential pressure sensor does not indicate pressure is pressure is present, then the DDC controller shall assume the pumps have failed and command the heat pumps to turn off.
8. When the heat pump encounter abnormal situation, the DDC controller shall shut down the relevant circuit. The protection device in the circuit shall include:

• High pressure protection
• Low pressure protection
• Compressor overheat protection
• Running current overload protection
• Compressor discharge gas overshoot temperature protection
• Phase and voltage protection
• No water flow protection
• Water freezing temperature protection
• Time delay to avoid frequency start-stop of compressors.