Honeywell RP908A 1039, RP908B Series, RP908A 1047, RP908A 1013, RP908A 1054 Service Data

GENERAL

DESCRIPTION

The RP908A and B are force balance pneumatic
amplifiers, with adjustable proportional band, for use as
Controllers in HVAC systems,

The RP908 is no longer being manufactured. There is
an RP920 model available for all RP908A and B applica­tions. Refer to Application Replacement Listing in the
REPAIR section.

APPLICATION

RP908A

RP908A &

B

PNEUMATIC CONTROLLERS

The RP908A, in conjunction with a remote sensor,

provides direct or limit control in air conditioning con­trol systems, with or without remote control point
adjustment (CPA). It may be adjusted for either direct or
reverse acting operation. Refer to Figures 1 and 2 for
these applications.

5737-2

Fig. 1. Typical RP908A Primary Control of Normally

Open Valve.

PRIMARY

CONTROLLER

LIMIT

LOW

CONTROLLER

5759-2

Fig. 2. Typical RP908A Low Limit Control.

RP908B

The RP908B dual input Controller is applied to com­pensating systems by using an additional compensating
signal to change the control point of the Controller (Fig.

3). Temperature, humidity, pressure, or dewpoint may
be controlled by using the appropriate sensors,

Rev. 6-84

Form No

75-553 1

Commercial Bldp

MLFTAB:

Group

111

C. 6.

1

DISCHARGE

hl

Q

I

RP908B

I

I

I+

SENSOR 2

D.A.

J

5760-2

SUPPLY AIR PRESSURE: 18 psi (124
MAXIMUM SAFE AIR PRESSURE: 25 psi (172

kPa)

nominal.

kPa).

AMBIENT TEMPERATURE LIMITS: 40 to 120F (4 to

9OC).

PROPORTIONAL BAND:

2-1/2

to 40 percent of pri-

mary sensor span, field adjustable.

AUTHORITY: 10 to 200 percent of primary sensor span,

field adjustable.

REMOTE CPA:

-t

10 percent of sensor span.

OPERATION

RP908A

Fig. 3. Typical RP908B Application.

SPECIFICATIONS

MODELS: See Table

Table 1. RP908A and B Model Listing.

Single

Model

RP908A

1005 X

RP908A

1013 X

RP908A

1021 X

RP908A

1039 X

RP908A

1047

RP908A

1054 X

RP908A

1042

RP908B

1003

RP908B

1011

RP908B

1029

RP908B

1037

Input Limit

1
I
I
I
I

1.

Single

Input

IXI

I
I

11x1

I

Dual With

Input Cover

X

I I I

lxlxl

Ixlxlxl

I

1x1

X
X

X
X

With
CPA

X

X

Ixlx

With

Calibration

Knob

X
X
X
X
X
X

Ix

x

Ix

When used in a heating system with the Controller set

for direct action, a drop in temperature at the sensor
lowers the branchline pressure, opening the valve to
increase the flow of heating medium to the coil (Fig. 1). If
the CPA model is used, an increase in air pressure on the

CPA port raises the setpoint of the Controller and a

decrease lowers it.

Used as a low limit Controller, the

RP908B

reduces

branchline pressure from the primary Controller, open­ing a heating valve to maintain discharge temperature at
setpoint (Fig. 2).

RP908B

Operation is similar to RP908A. However,

an

increase
in outdoor temperature causes the compensating (out­door) sensor to reset the control point of the Controller
to raise the branch line pressure (Fig. 3). The resulting
change in branch line is dependent on the authority set­ting on the Controller.

MAINTENANCE

EQUIPMENT REQUIRED

-

Plastilube 311057 for O-rings, screw threads, etc.

-

Molycote or similar commercial powdered lubri-

cant for pivots and plungers in units with CPA.

-

0 to 30 psi test gage (if gages have not been installed

in system).

GENERAL

The only preventive maintenance necessary is an

annual visual check for leaks, loose fittings, etc, and an

operational and calibration check.

75-553 1

OPERATIONAL CHECK

Check main air pressure for 18 psi (124 kPa) at Con­troller. For models with manual CPA adjustment, move
CPA switch up and down and observe the action of the
Controller. If the system’ does not have CPA, the sen-

sor(s) may be disconnected and the port plugged so that
pressure may be slowly built up, causing the Controller to
operate from one extreme position to the other. Bleeding
this pressure slowly causes the Controller to operate in
the opposite direction. The main lever should move
smoothly, and the branchline pressure should change

gradually in the proper direction.

2

CALIBRATION CHECK

RP908A

Change the

sensor temperature if the two do not already match. The
branchline pressure should be at its calibration pressure
of 8 + 1 psi (55
drawings. If recalibration is necessary, see CALIBRA­TION,

RP908B

Determine the existing calibration temperature from
the reset schedule on the job drawings. Draw a straight
line on graph as shown in Figure 4. Measure the tempera­ture at the Port 2
sor temperature (Fig. 5). Measure temperature at Port 1
sensor location. This temperature must agree within two

100

(38)

setpoint

-t-

7

kPa)

sensor.

of the Controller to the existing

unless otherwise noted on job

Find corresponding Port 1 sen-

percent of the sensor range span and the branchline pres­sure must be within the operating range of the controlled
device. If recalibration is necessary, see CALIBRA­TION.

CHANGE FROM DIRECT TO REVERSE
ACTING

The change from direct to reverse acting is made by

following the three steps listed in Figure 6.

OUTDOOR AIR TEMPERATURE IN F(C)

20884

Fig. 4, Typical Discharge Air Reset Schedule.

21c

(99:

T

??
:

19c

z

(88

2

17c

2

(77:

;:

w 150

ii

(66)

?
[L

130

p

(54)

s

110

g

(43)

I

(Zi,

40 30

OUTDOOR AIR TEMPERATURE IN F (C)

(4)

(-1)

(770)

(-:“z,

(-:8) (::i)

Fig. 5. Typical Hot Water Reset Schedule for

Radiation Systems.

20885

1.

RENlOVE PIVOT

SCREWS (2)

A

DISCONNECT WHEN PROPORTIONAL BAND (PB) IS

LESS THAN 15 PERCENT, REGARDLESS OF ACTION.

3.

CbNNECT

TO NOTCHES (2)

RA SPRINGS (2)

4707-l

Fig, 6. Set-Up for Reverse-Acting Operation.

ADJUSTMENTS

PROPORTIONAL BAND

If the original setting shown on the job drawings must

be changed:

1. Loosen the Proportional Band Adjustment Knob
(Fig. 7) and slide the Proportional Band Indicator
to the desired setting.
NOTE: When a proportional band of more than

I5 percent is used, fasten the Bias Spring
to the Lever Assembly (Fig. 6). This spring
is left unconnected if proportional band is

15 percent or less.

I

2. The Controller must be recalib
proportional band is changed.

ra

.ted

whenever the

75-553 1

REMOTE CONTROL POINT

A pressure change from 3 to 13 psi (21 to 90

kPa)

in the
remote CPA port changes the control point proportion­ately by 20 percent of the sensor span.

1 (and Port 2 if RP908B is used). If the reset signal on an
RP908B is produced by the output of a room thermostat
or pressure control, a
Port 2. Otherwise, the scale range on the

30-psi

gage must be installed in

gage should

match the range of the sensor for Ports 1 and 2.

SETPOINT

The

setpoint

is raised by turning the

Setpoint

Knob
(Fig. 7) counterclockwise. This increases the force from
the

setpoint

spring on the lever assembly so that a greater

force from the sensor chamber is required to cause an

increase in branchline pressure.

The

setpoint

It has 10 marked divisions, each equaling

screw has a vernier for fine adjustment.

l/2

percent of
the sensor span (1 degree F per division for a 200-degree
sensor span, or 20 turns for the whole span).

SETPOINT

SCREW WITH 10 MARKED

DiVISIONS

CONTROLLER WITHOUT CPA

ADJUSTMENT

SETPOINT

KNOB

RP908A

There are two methods of calibrating the RP908A

when 18 psi (124

kPa)

main air is supplied to the Control-

ler and the internal restriction is not blocked.

CALIBRATION USING AN ADJUSTABLE
RESTRICTION

1,

Install the proper scaleplate on the Controller if not
already installed.

2.

Disconnect the Port 1 sensor and connect an adjust­able restriction to Port 1.
Choose a

3.

setpoint

and adjust the restriction until the
gage in Port 1 shows this setpoint.
Turn the

4.
reads 8 psi (55

setpoint

knob until the branchline pressure

kPa).

This is standard calibration. On
some applications, the calibration branchline pressure
may be set at a different point that relates to the spring
range of the device being operated.
Move the scaleplate until the

5.

setpoint

indicator agrees

with the chosen setpoint, and tighten.

6.

Disconnect the adjustable restriction and reconnect

the sensor.

LPROPORTIONAL

CONTROLLER WITH CPA

BAND INDICATOR

.

20887

Fig. 7. RP908 Adjustment Locations.

CALIBRATION

GENERAL

Always calibrate with 18 psi (124
sure. On CPA models, set CPA to 8 psi (5.5
30-psi gage must be installed in the branchline port
during calibration and a temperature, humidity, or pres­sure gage matching the sensor should be installed in Port

kPa)

main air pres-

kPa).

A 0 to

CALIBRATION WITH GAGES

1. Install

30-psi

match Port

gages in Ports 1 and B. Port I gage must

B.
Install proper scaleplate on the Controller if not
already done.
Turn the
psi (55

setpoint

kPa)

knob until branchline pressure is 8

or other selected calibration pressure (see
Step 4 of CALIBRATION USING AN ADJUST­ABLE RESTRICTION).
Move the scaleplate until the

setpoint

indicator is at
the reading shown on the Port 1 gage and tighten.
Adjust the

setpoint

to the desired control setting.

RP908B

The only method of calibrating the RP908B is to use

an adjustable restriction. If the settings are known, pro­ceed to Step 1 of RP908B CALIBRATION PROCE­DURE. If the settings ‘must be calculated, read the

following and calculate as shown. Authority Calculator
No. 813 may be used to simplify these calculations.
Although these examples are shown for a hot water reset
system, the formulae also apply to other systems-

75-553 I

4

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 out­door air temperature at which designed maximum heat­ing 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 tem­perature at the sensor to cause the valve to move from
fully open to fully closed (full heat to no heat). It is desir­able to have a throttling range as narrow as possible for
efficient control and yet wide enough to produce stable
operation. A throttling range of
mended trial setting. If the system will not stabilize after
being in operation for a short period of time, the throt­tling range should be increased (this requires recalcula­tion 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.

a

10F (5C)

is a recom-

Where:

TR = Throttling Range

AT1= Change in Hot Water Temperature
AT2

= Change in Outdoor Air Temperature

EXAMPLE:

Determine settings for a system where hot water should
be controlled at 180F (82C) when outdoor air
temperature is -lOF
when outdoor air is

(5C). Hot Water Sensor

Sensor

-40/160F

AT2
ATI

TR =

PB = &

(-23C),

65F

(18C). Assume a TR of

40/24OF

(Span =

Reset Schedule

=

65-(-

10) = 75

=

180-90

=

10F

x100=5%

and reduced to

(Span =

2OOF).

go

9OF

200F);

(32C)

0-A.

IOF

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 deter­mined by using the following formula:

TR

Sensor Span

EXAMPLE:

When a sensor having a 200-degree span is used and a

IO-degree TR has been selected:

PB =

200

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
and Sensor 2. The complete formula is:

Authority

=

Span S,

Span SI

x 100 = PB%

10

-

x 100 = 5%

the

spans of Sensor

A~l+m xloQ

X

Al-2

1

Authority =

RP908B

I. Install the scaleplate matching the Port I sensor on the

2.

3.

4

5

CALIBRATION PROCEDURE

Controller if not already done.
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.
Select the low end of the Sensor 1 schedule or
(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
Set the authority and proportional band into the Con-

troller. Adjust the
gage reads 12 psi (83
closed” pressure if the valve has other than a standard
4 to 11 psi (28 to 76

.

Move the scaleplate until the desired
with the
ler is now in calibration.

REPLACEMENT PAGE FOR:

RP908A&B

Service Data

75-5531 6-84 II. C. 6.

setpoint

200

2oo

indicator and tighten. The Control-

Pneumatic Controllers

90;5IO

x

setpoint

kPa),

kPa) spring range.

x 100 = 133%

knob until the branchline

or until it reads the “valve

65F

(18C).

setpoint

8-88

90F

lines up

75-553 1