BCXC-SVX01A-EN
37
Electric Heat Operation
Tracer ZN controllers support 1-stage
electric heat. Also, Tracer ZN520
supports 2-stage electric heat. Tracer
ZN520 cycles the electric heat to control
the discharge air temperature. The rate
of cycling is dependent upon the load in
the space and the temperature of the
incoming fresh air from the economizer (if
any). Two-pipe changeover units with
electric heat use the electric heat only
when hot water is not available.
Economizer Damper (Tracer
ZN520 Only)
With a valid outdoor air temperature
(either hardwired or communicated),
Tracer ZN520 uses the modulating
economizer damper as the highest
priority cooling source. Economizer
operation is only possible using a
modulating damper during the occupied,
occupied standby, unoccupied, and
occupied bypass modes.
The controller initiates the economizer
function if the fresh air temperature is
cold enough for use as free cooling
capacity. If the fresh air temperature is
less than the economizer enable setpoint
(absolute dry bulb), the controller
modulates the fresh air damper (be-
tween the active minimum damper
position and 100%) to control the amount
of fresh air cooling capacity. When the
fresh air temperature rises 5°F above the
economizer enable point, the controller
disables economizing and moves the
fresh air damper back to its predeter-
mined minimum position based on the
current occupancy mode or communi-
cated minimum damper position.
Operation
Sequence of
Operation
Table O-SO-8. Unit Mode as Related to Water Temperature
Unit Type
EWT Sensor Required? Coil Water Temperature
2-pipe changeover
Yes
• Can cool if: space temp - EWT
≥
5 deg F
• Can heat if: EWT - space temp
≥
5 deg F
4-pipe changeover
Yes
• Can cool if: space temp - EWT
≥
5 deg F
• Can heat if: EWT - space temp
≥
5 deg F
2-pipe heating only
No
Hot water assumed
2-pipe cooling only
No
Cold water assumed
4-pipe heat/cool
No
• Cold water assumed in main coil
• Hot water assumed in aux. coil
Two and Four-Pipe
Changeover Operation
Tracer ZN controllers offer accurate and
reliable unit changeover using 2-way
valves and the controller’s entering water
temperature sampling function. Only
units using the main hydronic coil for both
heating and cooling (2-pipe and 4-pipe
changeover units) use the entering water
temperature sampling function.
Two-pipe and 4-pipe changeover
applications require an entering water
temperature sensor to allow the main
coil to be used for heating and cooling.
This sensor is factory-provided and
should be field-installed on the entering
water pipe.
The entering water temperature sam-
pling function periodically opens the two-
way valve to allow temporary water
flow, producing reliable entering water
temperature measurement. To ensure
accurate unit changeover without
sacrificing the benefits of 2-way, 2-
position valves, Tracer ZN controllers
periodically test the entering water
temperature on all hydronic main coil
changeover units. Hydronic heating/
cooling changeover operation requires
central plant operation, and the unit
controller must use an entering water
temperature sensor to verify delivery of
the correct water temperature from the
central plant.
Entering Water Temperature Sampling
Function
The entering water temperature (EWT)
must be five degrees above the space
temperature for hydronic heating and
five degrees below the space
temperature for hydronic cooling. When
water flows normally and frequently
through the coil, the controller does not
invoke the sampling function because the
EWT is satisfactory.
However, when the controller detects an
incorrect water temperature based on
heating or cooling mode, it invokes the
entering water temperature sampling
function. For example, when the mea-
sured EWT is too cool to heat or too
warm to cool. For cooling the EWT needs
to be five degrees below the measured
space temperature. For heating, the EWT
should be five degrees above the
measured space temperature.
After the controller invokes the function,
the unit opens the main hydronic valve
for no more than three minutes before
considering the measured EWT. The
controller allows an initial stabilization
period, equal to 30 seconds plus ½ the
valve stroke time, to flush the coil. Once
the temperature stabilization period
expires, the controller compares the
EWT against the effective space tem-
perature (either hardwired or communi-
cated) to determine whether the EWT is
correct for the desired heating or cooling
mode. If the EWT is not usable for the
desired mode, the controller continues to
compare the EWT against the effective
space temperature for a maximum of
three minutes.
The controller automatically disables the
entering water temperature sampling
and closes the main hydronic valve
when the measured EWT exceeds the
high EWT limit (110°F). When the EWT is
warmer than 110°F, the controller
assumes the EWT is hot because it is
unlikely the coil would drift to a high
temperature unless the actual loop
temperature was very high.
If the EWT is unusable — too cool to heat
or too warm to cool — the controller
closes the hydronic valve and waits 60
minutes before initializing another
sampling. If the controller determines the
EWT is valid for heating or cooling, it
resumes normal heating/cooling control
and effectively disables entering water
temperature sampling until it is required.
