Unable to change setpoint on stat connected to Xenta 102-AX

[Product_Support]

Issue

Cannot adjust the setpoint up or down on STR-250, STR-202 or M/STAT that is connected to Xenta 102-AX. Also, effective setpoint may not be changing when device goes from unoccupied to occupied modes.

Product Line

TAC Vista

Environment

• Xenta 102-AX Plug-in
• STR-250
• M/STAT
• STR-202

Cause

This problem is most often caused by a scenario similar to the following example:

1. A 102-AX is in cooling mode with an effective cooling setpoint of 70°F. An STR-250 stat is connected to the 102-AX and has a stat adjustment range (nviSetpoint.SCPTmaxRnge) of 6°F. This allows the occupant to adjust the setpoint ±3°F from the stat.
2. The occupant is hot and adjusts the stat down to its minimum of 67°F. (3°F less than the cooling setpoint)
3. The building operator decides that ±3°F is too much adjustment range and changes nviSetpoint.SCPTmaxRnge to 4°F (±2 degrees F) from the front end.
4. The effective setpoint is still 3°F less than the cooling setpoint but now the stat cannot adjust the setpoint up or down since it steps in .1°F increments and both 66.9°F and 67.1°F are outside of the adjustment range.
5. Since the stat adjusted setpoint offset is not a visible point and cannot be adjusted from the front end, the setpoint offset will be stuck until the stat adjustment range is expanded to include the effective setpoint and the setpoint offset is changed from the stat.

Resolution

1. To correct this problem the nviSetpoint.SCPTmaxRnge should be set high enough to include the effective setpoint. This can be done from Vista and can be found in the VAV_Controller object of the 102-AX or it can be done from the 102-AX Plug-in where it is called Stat Adjustment Range and is found on the Operating Parameters page.
2. The setpoint should be changed at the stat to match the setpoint of the mode in which the 102-AX controller is currently operating. (eg occupied cooling setpoint)
3. The stat adjustment range can then be returned back to its previous setting and the occupant can adjust the setpoint up or down and it will stay within the limit.