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Tracer
®
ZN.520
70 UNT-IOM-6
warm enough to heat when it is five degrees above the measured
space temperature.
When the controller invokes the entering water temperature sampling
function, the unit opens the main hydronic valve for no more than
three minutes before considering the measured entering water
temperature. An initial stabilization period is allowed to flush the coil.
This period is equal to 30 seconds plus ½ the valve stroke time. Once
this temperature stabilization period has expired, the controller
compares the entering water temperature against the effective space
temperature (either hardwired or communicated) to determine whether
the entering water can be used for the desired heating or cooling. If
the water temperature is not usable for the desired mode, the control-
ler continues to compare the entering water temperature 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
entering water exceeds the high entering water temperature limit (110
F). When the entering water temperature is warmer than 110 F, the
controller assumes the entering water temperature is hot because it
is unlikely the coil would drift to a high temperature unless the actual
loop temperature was very high.
If the entering water temperature 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 entering water temperature is valid for heating or
cooling, it resumes normal heating/cooling control and effectively
disables entering water temperature sampling until it is required.
The Tracer
®
ZN.520 controller supports 1- or 2-stage electric heat
operation for heating. To control the space temperature, electric heat
is cycled 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.
Manual
Units with the manual fresh air damper option ship with the damper in
the closed position, which is adjustable from zero to 100 percent in
25 percent increments. To adjust the position, first remove the air filter
Electric Heat Operation
Fresh Air Damper
Options