Bryant F, 580J*08--14D appendix Cooling, Economizer, Power Exhaust

580J

Cooling

The compressor outputs are controlled by the Cooling Control PID Loop and Cooling Stages Capacity algorithm. They will be used to calculate the desired number of stages needed to satisfy the space by comparing the Space Temperature (SPT) to the Occupied Cool Setpoint plus the T56 slider offset when occupied and the Unoccupied Cool Setpoint (UCSP) plus the T56 slider offset, if unoccupied. The economizer, if available, will be used for cooling in addition to the compressors. The following conditions must be true in order for this algorithm to run:

S Indoor Fan has been ON for at least 30 seconds.

S Heat mode is not active and the time guard between modes equals zero.

S If occupied and the SPT >(occupied cool setpoint plus the T56 slider offset).

S Space Temperature reading is available.

S If it is unoccupied and the SPT > (unoccupied cool setpoint plus the T56 slider offset). The indoor fan will be turned on by the staging algorithm.

S If economizer is available and active and economizer open > 85% and SAT > (SAT low limit + 5_F) and SPT > effective setpoint + 0.5_F.

OR

Economizer is available, but not active

OR Economizer is not available

S OAT > DX Lockout temperature.

If all of the above conditions are met, the compressors will be energized as required, otherwise they will be de-energized.

There is a fixed 3-minute minimum on time and a 5-minute off time for each compressor output and a 3-minute minimum time delay between staging up or down.

Any time the compressors are running the RTU-MP will stage down the compressors if the SAT becomes less than the cooling low supply air setpoint.

After a compressor is staged off, it may be started again after a normal time-guard period and the supply air temperature has increased above the low supply air setpoint.

Economizer

The Economizer dampers are used to provide free cooling and Indoor Air Quality, if optional CO2 sensor is installed, when the outside conditions are suitable.

The following conditions must be true for economizer operation:

S Indoor Fan has been on for at least 30 seconds. S Enthalpy is Low if the Enthalpy input is enabled. S SAT reading is available.

S OAT reading is available. S SPT reading is available.

S OAT <= High OAT economizer lockout configuration (default = 75).

S OAT <= SPT

If any of the mentioned conditions are not true, the economizer will be set to its configured minimum position. The minimum damper position can be overridden by the IAQ routine described later in this section.

If the above conditions are true, the Economizer Control Master Loop will calculate a damper position value based on the following calculation:

Damper Position = minimum position + PID (SPT - econ setpoint). Econ setpoint is half way between the effective cool and heat setpoints. If the SAT drops below the cooling low supply air setpoint (+ 5_F), the economizer will ramp down to minimum position.

Power Exhaust

If RTU-MP is also controlling an exhaust fan, it can be enabled based on damper position or by occupancy. If configured for continuous occupied operation, it will be energized whenever the controller is in the occupied mode and disabled when in the unoccupied mode. If configured for damper position control, it will be energized whenever the economizer exceeds the power exhaust setpoint and disabled when the economizer drops below the setpoint by a fixed hysteresis of 10%.

Heating

The heating outputs are controlled by the Heating Control PID Loop and Heating Stages Capacity algorithm. They will be used to calculate the desired number of stages needed to satisfy the space by comparing the SPT to the Occupied Heat Setpoint plus the T56 slider offset when occupied and the Unoccupied Heat Setpoint plus the T56 slider offset if unoccupied. The following conditions must be true in order for this algorithm to run:

S Indoor Fan has been ON for at least 30 seconds.

S Cool mode is not active and the time guard between modes equals zero.

S If occupied and SPT <(occupied heat setpoint plus T56 slider offset)

S SPT reading is available

S If it is unoccupied and the SPT < (unoccupied heat setpoint plus T56 slider offset). The indoor fan will be turned on by the staging algorithm.

S OAT < High OAT lockout temperature.

If all of the above conditions are met, the heating outputs will be energized as required, otherwise they will be de-energized. If the SAT begins to exceed the high supply air setpoint, a ramping function will cause the Heat Stages Capacity algorithm to decrease the number of stages until the SAT has dropped below the setpoint.

There is a fixed one minute minimum on time and a one minute off time for each heat output. Heat staging has a 3 minute stage up and 30 second stage down delay.

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