Sequence of operation
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water temperature to ensure reliable heating/cooling control decisions. The
entering water temperature may be communicated or may be hardwired to
analog input 1 of the controller (AI 1). If the unit is not applied as a coil
changeover unit, the controller uses the entering water temperature (when
present) for information only.
If the entering water temperature is expected and the valid value does not exist,
the controller allows only hydronic heating operation (hydronic cooling
capacity always equals 0%, DX and economizing are allowed). In the heating
mode, the controller is only permitted to heat (when required) and cannot cool.
Entering water temperature sampling function
The Tracer ZN.520 zone controller can sample the entering water temperature
for all hydronic main coil changeover units. The entering water temperature
value is important for reliable heating and cooling control. The entering water
temperature must be five degrees above the space temperature for hydronic
heating and five degrees below the space temperature for hydronic cooling.
The controller invokes entering water
temperature sampling only when the
measured entering water temperature is
too cool to heat or too warm to cool.
Entering water is cold enough to cool
when it is five degrees below the
measured space temperature. Entering
water is warm enough to heat when it is
five degrees above the measured space
temperature.
When water flows normally and frequently through the coil, the controller does
not invoke the sampling function because the water temperature is satisfactory
for the desired heating or cooling. During changeover, the zone controller
determines its ability to deliver heating or cooling. The controller invokes
entering water temperature sampling if the entering water temperature is not
adequate for delivering the desired heating or cooling.
Only units using the main hydronic coil for both heating and cooling (2-pipe
changeover and 4-pipe changeover units) use the entering water temperature
sampling function. For all applications other than 2-pipe changeover and 4-pipe
changeover, the entering water temperature does not affect unit operation, but
can be used for information.
Using three-way control valves, the central water loop continuously flows,
either through the coil or bypassed around the coil. Because the water is
continuously flowing, the entering water temperature sensor can be installed
on the pipe where to flow is constant. For both three-way valves and bleed
lines, continuous waterflow, combined with proper sensor location, gives a
reliable measurement of the entering water temperature.
