Chapter 4 Sequence of operation
54 CNT-SVX05B-EN
ing water temperature sensor can be installed on the pipe where the flow
rate is constant. For both three-way valves and bleed lines, continuous
water flow when combined with proper sensor location gives a continuous
and reliable measurement of the entering water temperature.
Two-way valve applications
The AH540/541 controller offers a control solution for two-way valve
applications. The entering water temperature sampling function (purge)
periodically opens the two-way valve to allow temporary water flow, pro-
ducing a reliable entering water temperature measurement.
When water flows normally and frequently through the coil, the control-
ler does not initiate the entering water temperature sampling function
because the water temperature measurement is valid for determining the
entering water condition. During unit startup or changeover, the control-
ler determines its ability to deliver heating or cooling. The controller ini-
tiates the entering water temperature function to determine if the
entering water temperature is adequate for delivering the desired heat-
ing or cooling. The measurement must indicate that the water is warm
enough to heat the space or cool enough to cool the space.
When the controller initiates the entering water temperature sampling
function, the controller turns Off the supply fan and opens the hydronic
valve for no more than the maximum sampling time while measuring
entering water temperature. An initial stabilization period is allowed to
open the valve to a configured position (50% default) and to flush the pip-
ing. When this temperature stabilization period has expired, the control-
ler compares the entering water temperature to the effective space
temperature (either hard-wired or communicated) to determine if it can
be used for the desired mode. The controller continues to compare the
entering water temperature to the effective space temperature for the
maximum sampling time (see Table 52 on page 80).
Whenever the entering water temperature is warmer than 110°F, the con-
troller assumes the entering water temperature is hot because it is
unlikely the coil would drift to a high temperature unless the actual tem-
perature was very high. See Figure 15 on page 105 for a detailed descrip-
tion of the entering water sampling process.
If the entering water temperature is not usable for the required space
demand, the controller closes the water valve and starts the supply fan
until the next sampling period (configurable). If the controller determines
the entering water temperature is adequate for heat or cooling, it
resumes normal heating/cooling control and effectively disables entering
water sampling until it is required.
Note:
Entering water sampling (also referred to as purge) is not
required for three-way valve applications.
To Next Page
Loading...
Need help?
General
