Issue:
How do setpoints work on Space Comfort Controllers (SCC)? My communicated setpoint does not match the active setpoint.
Resolution:
The following discussion pertains to ZN510/511, VV550/551, AH540/541, MP501, and constant volume LCI-I for IntelliPak, CSC (Commercial Self-Contained), and FAU (Fresh Air Unit) controllers (not the constant volume LCI-R....it behaves slightly different).
Even though it appears that the active setpoint while the unit is in occupied cooling should be equal to the occupied cooling setpoint as seen through Rover, this is not always the case. When there is a BCU on the site, a communicated value from the BCU will always be the active setpoint subject to an offset equal to one half of the difference between the occupied cooling and the occupied heating setpoints. The bottom line for this particular site was that he needed to set the Space Temperature Setpoint field in the SCC setup tab in Summit to Not Used in order for the SCC to use it's own setpoints.
The following is the Summit help screen for Space Temperature Setpoint from the Setup tab from the UCM editor for an SCC.
Enter a value to be communicated to the SCC as the active setpoint or use a referencer. If this field is set to Not Used, the local wired setpoint is the active setpoint, or the default setpoint in the SCC is the active setpoint. The heating and cooling setpoints are derived from the single space temperature setpoint as follows:
When the space temperature setpoint is used to determine the setpoints, all unit setpoints are calculated based on the space temperature setpoint value, the configured setpoints, and the active mode of the controller.
Example
Assume the controller is configured with the following default setpoints:
- Unoccupied cooling setpoint: 85 F
- Occupied standby cooling setpoint: 76 F
- Occupied cooling setpoint: 74 F
- Occupied heating setpoint: 70 F
- Occupied standby heating setpoint: 66 F
- Unoccupied heating setpoint: 60 F
Absolute setpoint offset = Default occupied cooling setpoint - Mean setpoint
From the default setpoints in this example, the mean of the occupied cooling and heating setpoints is 72 F ((74+70)/2); this is the mean setpoint. The absolute setpoint offset is the difference between the default occupied cooling setpoint and the mean setpoint. For this example, the absolute occupied setpoint offset is two degrees (74 - 72 = 2)
In Cooling mode, the controller adds the absolute setpoint offset to the communicated or hardwired cooling setpoints (listed in order of precedence) to derive the active cooling setpoint. In Heating mode, the controller subtracts the absolute setpoint offset from the communicated or hardwired heating setpoints (listed in order of precedence) to derive the active heating setpoint. The standby setpoints will retain the original standby offsets.
The following example, with a communicated or hardwired setpoint of 72 illustrates how this is done:
- Unoccupied cooling setpoint: 85 F (same as default)
- Occupied standby cooling setpoint: 78 F (OCS+2=78)
- Occupied cooling setpoint (OCS): 74 F (72+2 = 74)
- Occupied heating setpoint (OHS): 70 F (72-2=70)
- Occupied standby heating setpoint: 66 F (OHS-4=66)
- Unoccupied heating setpoint: 60 F (same as default)
The following example, with a communicated or hardwired setpoint of 73 illustrates how this is done:
- Unoccupied cooling setpoint: 85 F (same as default)
- Occupied standby cooling setpoint: 77 F (OCS+2=78)
- Occupied cooling setpoint (OCS): 75 F (73+2 = 75)
- Occupied heating setpoint (OHS): 71 F (73-2=71)
- Occupied standby heating setpoint: 67 F (OHS-4=67)
- Unoccupied heating setpoint: 60 F (same as default)
The SCC controller determines the active setpoint based on the following:
- Local hardwired setpoint
- Space temperature setpoint
- Heating and cooling shifts
- Default setpoints
- Occupancy mode
- Heating or cooling mode
- Setpoint high and low limits
When Tracer Summit communicates a setpoint to the controller, the controller ignores the hardwired setpoint input and uses the communicated value in its determination of the active setpoint, which includes the absolute setpoint offset . In unoccupied mode, the exception, the controller always uses the stored default unoccupied setpoints.
After the controller completes all setpoint calculations, based on the requested setpoint, the occupancy mode, the heating and cooling mode, and other factors, the calculated setpoint is validated against the following setpoint limits:
- Heating setpoint high limit
- Heating setpoint low limit
- Cooling setpoint high limit
- Cooling setpoint low limit
These setpoint limits apply only to the occupied and occupied standby heating and cooling setpoints. These setpoint limits do not apply to the unoccupied heating and cooling setpoints stored in the controller configuration.
Unit configuration also exists to enable or disable the local (hardwired) setpoint. This parameter provides additional flexibility to allow you to apply communicated, hardwired, or default setpoints without making physical changes to the unit.
The difference between the default occupied cooling setpoint and the default occupied heating setpoint is always the difference between the active cooling setpoint and the active heating setpoint. The communicated setpoint and the thumbwheel will always be in the middle of the difference.
For example, if the default occupied cooling setpoint = 74 deg F and the default occupied heating setpoint = 70 deg F, the difference is 4 deg F. If the communicated setpoint or the thumbwheel is 75 deg F, the active cooling setpoint is 2 deg above at 77 deg F. The active heating setpoint is 2 deg F below at 73 deg F.
From the Tracer Summit online help
An SCC typically uses an asymmetrical method to determine the active setpoint relative to the communicated Space Temperature Setpoint. This method is defined as follows:
- Absolute Setpoint Offset = Space Temperature Setpoint 0.5*(Default Occupied Cooling Setpoint + Default Occupied Heating Setpoint)
- Effective Occupied Cooling Setpoint = Default Occupied Cooling Setpoint + Absolute Setpoint Offset
- Effective Occupied Heating Setpoint = Default Occupied Heating Setpoint + Absolute Setpoint Offset
- Effective Standby Cooling Setpoint = Default Occupied Standby Cooling Setpoint + Absolute Setpoint Offset
- Effective Standby Heating Setpoint = Default Occupied Standby Heating Setpoint + Absolute Setpoint Offset
If you have checked everything and space setpoint arbitration is not working correctly, then check nciSetpoints.
nciSetpoints is a structure of 6 setpoints in the following order,
- occupied cool setpoint (OCSP)
- occupied standby cool setpoint (SCSP)
- unoccupied cool setpoint (UCSP)
- occupied heat setpoint (OHSP)
- occupied standby heat setpoint (SHSP)
- unoccupied heat setpoint (UHSP)
The values of the 6 setpoints have to be in the following order.
UCSP >= SCSP >= OCSP >= OHSP >= SHSP >= UHSP
The unit controller does not check this.
The unit controller trusts Summit and Rover to write the setpoints correctly.
When Summit has the thumbwheel enabled but is sending a value for the setpoint the controller will use the communicated value rather than the local thumbwheel. If the controller loses comm with the BCU, it will revert to the thumbwheel value rather than the defaults. If the thumbwheel is disabled or not functioning during the comm loss, the controller will revert to the default setpoints set up in configuration.
This is as designed.
This is correct operation for the LCI-R SCC and the LCI-RX.
The LCI-R treats the communicated setpoint like all other Trane LonTalk devices.
The local thumbwheel is treated differently.
The local thumbwheel is the cool setpoint.
And the heat setpoint is the whole differential (default occupied cool setpoint - default occupied heat setpoint) below the cool setpoint.
ReliaTel with the LCI-R is the only Trane control to do Space Temp Setpoint Arbitration like this.
All other Trane devices treat the local setpoint just like the communicated setpoint.
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This is NOT a bug 3/27/15. I tested with LCI-RX version 6.01.021 and RTRM version 16.1. Tested with an active setpoint of 72.2 from a thumbwheel. With the default Occ Cool = 74 and default Occ Heat = 71 the active setpoint = 72.2 when in the cool mode. Changed the default occ cool to 82 and default occ heat to 62. The active setpoint did not change. This is correct operation for the hardwired thumbwheel setpoint. When I send a value of 70 from BAS the correct 10 degree offset is applied for an active setpoint of 80.
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Notes:
Another "easier" way of explaining this is that the active setpoint is equal to the thumbwheel (or communicated) setpoint +/- the deadband/2. The deadband is the difference between the default occupied cooling and heating setpoints.
If the unit is in a occupied standby mode this active setpoint will be offset by the difference associated with the default setpoints.
Example: Default Setpoints
Unoccupied cooling setpoint 85F
Occupied standby cooling setpoint 76F
Occupied cooling setpoint 74F
Occupied heating setpoint 70F
Occupied standby heating setpoint 66F
Unoccupied heating setpoint 60F
DB= 4 degrees
Active Occupied Setpoint = TSP +/- 2 degrees
Standby cooling add 2 degrees
Standby heating subtract 4 degrees