Trane CVHE Troubleshooting Manual page 115

The chilled water reset function can provide energy savings and/or better critical zone control
and/or better indoor air quality when applied to the right system type.
The chilled water setpoint is reset based on a temperature (return chilled water, zone, outside
air), or other input such as humidity or a customer supplied analog signal.
The most common use of chilled water reset is to provide constant return chilled water
temperature. UCP2 makes this particular form of chilled water reset very easy to set up. One way
to do this is to make it the default setting.
Constant Return Reset is a good starting point in a comfort cooling application. If you think about
it, at full load you both cool (sensible) and dehumidify (latent) the air at the coil. The coil is
designed to do so much sensible and latent cooling at a given entering water temperature. If you
can get sufficient sensible and latent cooling at full load, it follows then that by maintaining a
constant leaving water temperature off the coil (entering to the chiller) at part load you have a
good chance of satisfying both your sensible and latent loads. Of course, if at part load the
customer finds the air is either too wet or too dry, the Chilled Water Reset Ratio can be adjusted
up or down to compensate.
Due to extremely low usage, CWR based on Zone Temperature has been eliminated from
the basic UCP2.
UCP2 resets the chilled water temperature setpoint based on either return water
temperature, or outdoor air temperature. Return Reset is standard, Outdoor Reset is
optional.
The following are selectable:
1 One of three RESET TYPEs: no CWR, RETURN WATER TEMPERATURE RESET,
OUTDOOR AIR TEMPERATURE RESET, or CONSTANT RETURN WATER
TEMPERATURE RESET. The UCP2 will not permit more than one type of reset to be
selected.
2 RESET RATIO Setpoints.
For outdoor air temp. reset there shall be both positive and negative reset ratios.
3 START RESET Setpoints.
4 MAXIMUM RESET Setpoints.
The maximum reset's are with respect to the chilled water setpoint.
The equations for each type of reset are:
Return
CWS' = CWS + RATIO (START RESET - (TWE - TWL))
and CWS' > or = CWS
and CWS' - CWS < or = Maximum Reset
CVHE-SB-32A
Chilled Water Reset
110