When units are in cooling/heating mode, the control uses the cooling/heating capacity value from proportional integral (PI) logic to turn compressor(s) on and off to meet setpoint. The PI calculates the cooling/heating capacity value based on space temperature measurement compared to the active space temperature setpoint +/- the offset. The active space temperature setpoints are different for cooling and heating modes. The setpoints and offsets applied are different for occupied and standby modes and are user adjustable. The PI capacity value will increase if cooling/heating load in the space increases beyond setpoint +/- the offset.
A non-adjustable PWM algorithm calculates the on off time based on PI capacity value and the preconfigured cycle period (1200 second as default). On time will be longer if PI capacity is larger, and vice versa.
A one stage unit will turn off, if it is on before, when on time is longer than desired on time; it will be turned on, if it is off before, when off time is longer than desired off time (Figure 1). Following exceptions will be observed:
- Compressor obeys 3 minutes minimum on time and 3 minutes minimum off time to avoid short cycling.
- Compressor will stay on, if it is on before, when PI capacity is larger than 90%.
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Compressor will stay off, if it is off before, when PI capacity is smaller than 10%.
On off time calculation for one stage units:
Desired on time =PI capacity *Cycle Period/100
Desired off time =Cycle Period – Desired on time
Figure 1Below is from the WSHP IOM
Compressor Cooling (DX)
The Symbio 400-B/500 & UC400 Controller supports two (2) stages of DX cooling.
The control is proportional and based on an error rate of 3°F degrees for single compressor operation and 5°F degrees for 2- compressor units. Zone temperature is compared against active setpoints for compressor operation: OFF, pulse width modulation (PWM), or ON.
Note, when the control is in the dehumidification mode, only OFF and ON are valid compressor states.
At startup or during mode transition, if both compressors are requested to run, Compressor 1 is energized first and Compressor 2 waits until the next control cycle (10 seconds) to energize.
WARNING
Information in this article is intended for use by individuals possessing adequate backgrounds of electrical and mechanical experience and who comply with all federal, state, and local laws, rules, orders, or regulations related to the installation, service, or repair of a heating or central air conditioning product. Any attempt to install, service, or repair a heating or central air conditioning product may result in personal injury and/or property damage. The manufacturer or seller cannot be responsible for the interpretation of the information contained herein, nor can it assume any liability in connection with its use.