Two-pipe changeover
CNT-SVX05B-EN 53
Two-pipe changeover
This sequence applies only space temperature control.
The controller provides a two-pipe changeover function when an air-han-
dling unit has one hydronic coil for heating and cooling operation. Two-
pipe changeover allows the controller to provide heating or cooling to the
space depending on the entering water temperature.
When a two-pipe changeover unit has secondary electric heat the control-
ler uses the electric heat if the entering water temperature is not appro-
priate for heating. If entering water temperature is appropriate for
heating, the controller uses hydronic heating and disables electric heat
operation.
Two-pipe changeover units without an auxiliary source of heat, like elec-
tric heat, determine their mode based on the following sequence:
1. If the controlled space requires heating or cooling, the controller
changes from heating to cooling and cooling to heating based on the
space temperature, active setpoint, and integrated error between the
two. Once the controller changes modes, it verifies the entering water
temperature.
2. If the controller does not have a valid entering water temperature
(hard-wired or communicated), the controller assumes hot water is
present.
3. When the entering water temperature is not appropriate for the
desired capacity (either heating or cooling), the controller remains at
0% hydronic capacity. Economizer cooling would remain available if
needed regardless of the entering water conditions.
4. If the entering water temperature is appropriate for heating or cool-
ing, the controller energizes the appropriate output to control the
space temperature at the heating setpoint in heating mode or cooling
setpoint in cooling mode.
Entering water temperature sampling
This sequence applies to controllers configured for space temperature
control.
If configured for space temperature control, the controller can sample the
entering water condition for air-handling units with a single hydronic
coil. The entering water temperature is important for reliable heating
and cooling control. The entering water temperature must be at least 5ºF
above the space temperature for hydronic heating and 5ºF below the
space temperature for hydronic cooling for satisfactory capacity control.
Three-way valve applications
When using three-way control valves, the central water supply flows con-
tinuously to the unit valve and is either directed through the coil or
bypassed around the coil. Because the water flow is continuous, the enter-
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