Precedent with Symbio 700 Controls Dehumidification: Thermostat with Relative Humidity Sensor

Thermostat with Relative Humidity Sensor

Sequence of Operation

Initiation

When configured for Relative Humidity control, dehumidification initiates when the Space Humidity exceeds the Space Dehumidification Setpoint. A Y1 call is not required to initiate dehumidification.

If there is no thermostat call for cooling, heating, or supply fan, the unit will be off. If Space Relative Humidity exceeds the Space Dehumidification Setpoint, the Symbio 700 controller will start the unit and provide dehumidification.

When dehumidification activates, compressors stage to full capacity, economizer cooling is disabled. The reheat valve will open targeting the max allowed position, then modulate to an adjustable Discharge Air Reheat Setpoint (70°F default). This setpoint is adjustable in the Symbio Service & Installation Mobile App in SETTINGS>REFRIGERATION>DISCHARGE AIR REHEAT SETPOINT BAS and has an adjustable range between 65°F & 80°F. 

Termination

Any condition listed below will terminate dehumidification:

• Space Relative Humidity drops below the Space Dehumidification Setpoint BAS - Space Dehumidification Setpoint Offset BAS 
• Thermostat Y1 + Y2 call (dehumidification is allowed with a Y1 OR Y2 only call)
• Thermostat Heat Call
• Occupancy Input Closes and Unoccupied Dehumidification is disabled
• Entering Evaporator Temperature below 30°F or failed Entering Evaporator Temperature Sensor
• Outdoor Air Temperature below 40°F or above 100°F
• Stepper Motor Module (SMM) communication failure
• Any emergency stop condition that stops the compressors or supply fan

Wiring 

Thermostat wiring connects to the Symbio 700 on J21:

Relative Humidity Sensor Wiring Connects to the Symbio 700 on J23:

For more details on wiring a Relative Humidity Sensor to Symbio 700 see: How do I wire a BAYSENS036/037 Humidity / Temp Sensor for Symbio 700 and Reliatel Controls?

Troubleshooting

Unit will not enter dehumidification

• Is the OAT between 40°F and 100°F ?
  • Once the OAT exceeds these values, there is a 5°F dead band to allow dehumidification to work again, OAT would have to be >45°F or <95°F to re-enable dehumidification
• Does the unit have Y1 + Y2 calls ?
  • If the Y2 call drops, the dehumidification will begin per the normal sequence of operations
• Is the thermostat calling for Heat ?
  • A call for heat will disable dehumidification instantly
• Does the Relative Humidity Sensor have a valid reading and is it above the current dehumidification setpoint

Unit overheating the space

• Typically a thermostat will stop this from happening when the Y2 calls is added, but the Discharge Air Reheat Setpoint BAS (default 70°F) may be set too high for the space and is add too much heat during dehumidification. 
• If you have a single stage cooling unit (Standard Efficiency 3-5T) and only have a Y1 call wired to the unit, the same scenario above could be in play and there is not a Y2 call to terminate dehumidification. Adding a Y2 call to a single stage unit is a good idea for space temperature protection during dehumidification, but also the Y2 call is needed during economizer operation to allow the compressor to operate to supplement the outdoor air damper. 

Unit overcooling the space

• If the Discharge Air Reheat Setpoint BAS is set to the minimum of 65°F, it may be providing enough cooling to allow the space temperature to drop. 

Symbio 700 Dehumidification Points

Values in ORANGE are adjustable 

*Other dehumidification/reheat points are available in the mobile app or through BAS, these points either do not affect thermostat with relative humidity sensor, are feedback/active values or are not recommend to be changed without Tech Support direction.