Honeywell RP908A Series Service Data page 5

Two separate parameters must be known in order to
properly calibrate a reset system, Reset Schedule and
Throttling Range. Proportional Band and Authority
may be calculated from these parameters
RESET SCHEDULE
The reset schedule is plotted by determining the out-
door air temperature at which designed maximum heat-
ing water temperature is needed and the outdoor air
temperature at which no heating is required.
THROTTLING RANGE (TR)
Throttling Range indicates the change in water tem-
perature at the sensor to cause the valve to move from
fully open to fully closed (full heat to no heat). It is desir-
able to have a throttling range as narrow as possible for
efficient control and yet wide enough to produce stable
operation. A throttling range of 10F (5C) is a recom-
mended trial setting. If the system will not stabilize after
being in operation for a short period of time, the throt-
tling range should be increased (this requires recalcula-
tion of the proportional band, authority, and
recalibration). An unstable system can be recognized by
frequent, repetitive changes in branchline pressure,
known as hunting or cycling.
When the throttling range has been selected, it must be
converted to a proportional band value that can be set on
the Controller.
PROPORTIONAL BAND (PB)
Proportional Band is similar to throttling range except
it is expressed in terms of percent of temperature change
rather than degrees. Proportional band can be deter-
mined by using the following formula:
TR
x 100 = PB%
Sensor Span
EXAMPLE:
When a sensor having a 200-degree span is used and a
IO-degree TR has been selected:
10
AUTHORITY
Authority indicates the amount of effect that the change
in outdoor air temperature will have on the control point.
The formula must take into account the spans of Sensor 1
and Sensor 2. The complete formula is:
Authority =
Span SI
X
Where:
TR = Throttling Range
EXAMPLE:
Determine settings for a system where hot water should
be controlled at 180F (82C) when outdoor air
temperature is -lOF (-23C), and reduced to 9OF (32C)
when outdoor air is 65F (18C). Assume a TR of IOF
(5C). Hot Water Sensor 40/24OF (Span = 200F); 0-A.
Sensor -40/160F (Span = 2OOF).
Reset Schedule
TR = 10F
P B = & x100=5%
200
Authority = 2oo x 90;5IO x 100 = 133%
I. Install the scaleplate matching the Port I sensor on the
Controller if not already done.
2. Disconnect the sensors to Ports 1 and 2 and connect an
adjustable restriction to each port. If an external main
has been connected to the sensor line, install the
adjustable restriction on the sensor side of the tee.
3.
Select the low end of the Sensor 1 schedule or 90F
(32C). Adjust the No. 1 restriction until the Sensor 1
gage reads that temperature. Also set Sensor 2 at "low
end" of reset schedule, in this example 65F (18C).
4 Set the authority and proportional band into the Con-
troller. Adjust the setpoint knob until the branchline
gage reads 12 psi (83 kPa), or until it reads the "valve
closed" pressure if the valve has other than a standard
4 to 11 psi (28 to 76 kPa) spring range.
5 Move the scaleplate until the desired setpoint lines up
with the setpoint indicator and tighten. The Control-
ler is now in calibration.
REPLACEMENT PAGE FOR:
Service Data
75-5531 6-84 II. C. 6.
8-88