Trane Tracer AH540 Setup And Operation page 44

Chapter 4 Sequence of operation
34
Auto mode
A communicated request of Auto or the controller default operation (Auto)
can place the unit into heating or cooling mode. When the controller auto-
matically determines the heating or cooling mode while in Auto mode, the
unit switches to the desired mode based on the control algorithm.
If the Tracer AH540/541 controller is operating space temperature con-
trol, it uses the space temperature and space temperature setpoint to
automatically determine heat or cool mode of operation. When the con-
troller first powers up or after a reset, it makes an initial determination if
the heat/cool mode should be heat or cool. If the controller is configured as
heating and cooling, the controller determines the appropriate mode.
For example, if the initial space temperature is less than the occupied
space heat setpoint then the initial heat/cool mode is heating. The heat/
cool mode for a cool-only unit is always cool. The heat/cool mode for a
heat-only unit is always heat.
When the controller is allowed to automatically determine its space heat-
ing and cooling mode, the unit changes from cool to heat or from heat to
cool, when the integrated error between the active space setpoint and the
active space temperature is 900°F
error is calculated once every ten seconds.
See Figure 10 for an example of the controller changing from space cool-
ing (unit mode = cool) to space heating (unit mode = heat). In this exam-
ple, the initial unit mode is cool because the space temperature is above
the cool setpoint, and the cooling capacity is greater than 0%.
Following the curve from left to right, the space temperature falls below
the cool setpoint, and the controller reacts by lessening its cooling capac-
ity. When the space temperature reaches 1, the cooling capacity is 0%.
The rate at which the controller reaches 0% capacity depends on the
space temperature rate of change.
Figure 10. Automatic heat/cool changeover logic example
Point 1 on the curve in Figure 10 indicates the point at which the cooling
capacity equals 0%, space temperature is less than 0.5°F below the cool-
ing setpoint, and the error integrator starts to add up. Error integration
does not begin until the capacity is 0%. See the "Heat/cool changeover:
error integration example" on page 35.
seconds or greater. The integrated
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