Mounting and Wiring VMA1400 Series Controllers Technical Bulletin
35
Troubleshooting
Troubleshooting VMA Room Temperature Drift
Several job sites have reported that VMA zone temperature readings
randomly drifted to extremely high values. Such readings cause the
VAV box to drive to its full cooling level, resulting in occupant
discomfort and unnecessary energy expenditure. The primary cause of
this condition is unreliable field wiring connections either at the VMA
or at the sensor.
For any site experiencing temperature drift, do not replace the VMA.
It is most likely functioning correctly in response to false readings
caused by faulty field connections. Correcting the faulty connections
eliminates erratic temperature readings.
The following describes the connection problems found and the
suggested solutions.
Problem 1
VMAs are wired using 0.40 mm (26 AWG) solid wire. The problem
occurs when 0.51 mm or 0.40 mm diameter (24 or 26 AWG) solid
wire is crimped into a 0.82 mm² or 0.32 mm² total cross-sectional area
(18 to 22 AWG) stranded wire spade terminal.
Note: Spade terminals are designed to accept only stranded wire of
the correct size.
Solution 1
Terminate any small-gauge, solid or stranded (preferred) wire with a
plug-on screw terminal at the VMA. Table 14 shows available
terminal blocks.
Table 14: Screw Terminals
Product Code Number Description
AP-TBK1002-0
Removable 2-position screw terminal kit (100 pieces)
AP-TBK1003-0
Removable 3-position screw terminal kit (100 pieces)
M9000-106
Removable 4-position screw terminal kit (1 piece each)
Problem 2
When pulling phone cable through a conduit or wire tray, the phone
plug is cut off to allow for easier installation. One or more of the
replacement phone plug connections is improperly crimped to the end
of the cable.
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