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Heating
The specific heating sequence is determined by the controller's application and configuration. The RTU Open
controls up to two stages of gas or electric heating with an additional output for a Reversing Valve (Heat Pump
applications).
The following conditions must be true for the heating algorithm to operate:
•
The Outdoor Air Temperature is less than the Heating Lockout Temperature setpoint
•
The indoor fan has been ON for at least 30 seconds
•
The unit has a valid Supply Air Temperature input
•
The unit has a valid Space Temperature input
•
Neither Cool mode nor economizer are active and the time guard between modes has expired
The heating relays are controlled by the Heating Control PID Loop and Heating Stages Capacity algorithm, which
calculate the desired number of stages to satisfy the space by comparing the Space Temperature to the:
•
Effective Occupied Heating Setpoint when occupied
•
Effective Unoccupied Heating Setpoint when unoccupied
When the heating algorithm preconditions have been met, the heating is energized in stages. Anti-recycle timers
are employed to protect the equipment from short-cycling. There are fixed one minute minimum on and off times
for each heating output.
During heating operation, the RTU Open may reduce the number of active stages if the rooftop Supply Air
Temperature exceeds the Maximum Heating SAT setpoint. A heat stage turned off in this fashion may be started
again after the normal time-guard period has expired, if the Supply Air Temperature has decreased below the
Maximum Heating SAT setpoint.
Heat Pump operation
The RTU Open can control heat pumps HP O/B and Y1/W1.
HP O/B provides a separate output (BO-7) to control a reversing valve. The reversing valve control may be
configured to be energized with a call for heating (B), or energized with a call for cooling (O).
The sequence of operations are as previously described for heating and cooling except that the Y1 and Y2 outputs
are compressor outputs, energizing mechanical heating or cooling, depending on the state of the reversing valve.
W1 and W2 are used for auxiliary heat. Up to two stages are available.
Selection Y1/W1 is for heat pumps that do not require a O terminal to energize the reversing valve. The
sequences of operations are as described for Heating (page 51) and Cooling (page 45). The reversing valve output
is not used in this application. W1 and W2 are used for auxiliary heat. Up to two stages are available.
RTU Open v3
Installation and Start-up Guide
51
Sequence of Operation
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