Field Controls B794594 User Manual

COMBUSTION AIR SYSTEM

MODEL: CAS-4

This product is designed for use with any natural gas or LP burning furnace, water heater, or boiler with a
24 VAC control system. It may be used with a millivolt powered system with additional hardware. It may
also be used with more than one appliance. The CAS unit mechanically draws air into a structure and
disperses it near the combustion air intake of an appliance. If an optional Vacuum Relief Valve (VRV) is
used, the incoming air is tempered before entering the structure’s airspace. Refer to Diagrams A and B for
guidance in setting up the CAS system based on the size and length of the connecting ductwork and the
input rating of the appliance.

ITEMS INCLUDED IN KIT:

1- Motorized Blower
1- 4" galvanized intake air Vent Hood
2- Mounting brackets to secure the CAS to a wall
2- Wire/conduit connectors
1- 4" x 6" Pipe Increaser Fitting
1- 6" Orifice Ring
1- Instruction Sheet
1- High/Low Motor Speed Switch

READ THESE INSTRUCTIONS CAREFULLY AND COMPLETELY BEFORE PROCEEDING WITH THE INSTALLATION.

This device MUST be installed by a qualified agency in accordance with the manufacturer's installation instructions. The definition of
a qualified agency is: any individual, firm, corporation or company which either in person or through a representative is engaged
in, and is responsible for, the installation and operation of HVAC appliances, who is experienced in such work, familiar with all the
precautions required, and has complied with all the requirements of the authority having jurisdiction.

Please retain these instructions after installation.

Installed By: Phone:

Installation Date:

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GENERAL SYSTEM OPERATION

The thermostat (wall thermostat, or aquastat) calls for heat and energizes a relay which activates the 1.
CAS unit. After the CAS fan has come up to speed, an internal air pressure switch closes and completes
the circuit to allow the burner to fire. If the appliance is power vented, the venter and CAS activate
simultaneously. After the CAS comes up to speed, a pressure switch in the unit closes and allows the
appliance to fire.

After the heating requirement has been satisfied, the thermostat circuit will open and deactivate the 2.
burner and CAS unit.

For power vented systems with a post purge device, the power venter and CAS operate for a period of 3.
time after the burner has shut off to purge remaining flue gases from the vent system.

INSTALLATION SAFETY INSTRUCTIONS
CAUTION: This device must be installed by a qualified installer in accordance with the manufacturer’s

installation instructions.

This combustion air system must be installed by a qualified installer. “Qualified Installer” shall mean an 1.
individual who has been properly trained or a licensed installer.

Plan the system layout before installation to avoid the possibility of accidental contact with concealed 2.
wiring or plumbing inside walls.

Disconnect power supply before making wiring connections to prevent electrical shock and 3.
equipment damage.

SIZING AND SETUP

Diagrams A and B and Table 1 show the maximum equivalent length and size of duct pipe that should be
used when installing the CAS system. Using these charts will help ensure that the proper amount of air is
drawn into the structure as needed by the appliance. The defined regions shown correspond to the CAS's
airflow characteristics when using 4" and 6" diameter sheet metal duct pipe and the included orifice ring.
Follow the guidelines below to properly size and setup the CAS.

Determine the maximum input firing rate of the appliance, or the maximum total firing rate of multiple 1.
appliances that will be used.

Position the motorized CAS unit according to the guidelines in the Installation section.2.
Determine where the intake air vent hood will be located based on the recommendations in the 3.

Installation section.
The CAS is equipped with a High/Low motor speed control switch. When calculating the maximum 4.

equivalent feet, determine which speed is appropriate for the application. (Refer to Diagram A or B or
Table 1)

Calculate the total equivalent length of duct pipe including elbows and fittings needed to connect the 5.
CAS unit to the vent hood. (Refer to Diagram A or B or Table 1)

On Diagram A or B, locate the point that corresponds to the firing rate along the horizontal axis and the 6.
equivalent length of the duct pipe along the vertical axis, if using for make up air.

The point should fall within one of the three regions on the table. Each region corresponds to a duct pipe 7.
diameter of 4", 4" using the orifice ring, and 6" respectively. If the point falls near the rightward border
of a particular region then the VRV is not necessary. If the point falls away from the rightward border,
then a VRV is needed to reduce the amount of air drawn in.

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Draw a horizontal line on Diagram A or B that passes through the point located in step 5. The position of 8.
the point along this line relative to the left and right borders of the region it falls into indicates the relative
position that the balance weight of the VRV should be adjusted to. If the point falls near the leftward
border of a region, then the VRV balance weight should be adjusted to its minimum position. This
corresponds to turning the balance weight screw as far counterclockwise as possible.

EXAMPLE: A gas fired appliance firing at 100,000 BTU/hr. where the CAS unit needs to be placed 30
equivalent feet from the intake hood.

From Diagram A or B, the point at 100,000 BTU/hr. on the Gas Firing Rate scale and 30 equivalent feet

2

falls in the 4" Duct, Hood, and Orifice Ring region. The point is approximately

⁄3 of the horizontal distance
between the left and right boundary of the region. The left boundary is the edge of the graph, the right
boundary is the diagonal line that says 4" Duct, Hood & Orifice Ring. Therefore, place the Orifice Ring into
the inlet of the CAS so that it sits on the ledge above the fan. It does not matter which way the Orifice Ring is
turned as long as it is pushed down against the ledge completely. Use 4" diameter pipe to connect the vent
hood and the CAS unit. Install a VRV and the included 4"x 6" increaser on top of the CAS unit and adjust the

2

balance weight to

⁄3 of the distance of its full adjustment range from its minimum setting. The minimum setting

is with the balance weight turned all of the way counterclockwise.

Table 1

TOTAL INPUT OF APPLIANCE MAXIMUM EQUIVALENT FEET OF INSTALLATION

CAS-4, 4mV Gas

4" Duct And Hood w/

Restrictor

4" Duct and 4"

Intake Air Hood

6" Duct and 6"

Intake Air Hood

(BTU/hr.)

HI LOW HI LOW HI LOW

50,000 300 300 300 300 300 300

75,000 300 300 300 300 300 300

100,000 300 300 300 300 300 300

125,000 300 220 300 220 300 300

150,000 174 108 232 118 300 300

175,000 99 48 152 63 300 300

200,000 52 14 102 32 300 300

225,000 20 NA 68 13 300 239

250,000 NA NA 45 NA 300 150

300,000 NA NA 16 NA 300 53

350,000 NA NA NA NA 193 8

400,000 NA NA NA NA 109 NA

450,000 NA NA NA NA 56 NA

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LOW SPEED SIZING CHART

0

50

100

150

200

250

300

50 100 150 200 250 300 350 400

GAS INPUT FIRING RATE (1000 BTU/hr.)

MAX. EQUIVALENT DUCT LENGTH (ft.)

0.0 0.50 1.00 1.50 2.00 2.50

OIL INPUT FIRING RATE (gallons per hour)

(1 gallon oil/hr = 140,000 BTU/hr.)

6" Duct &Hood

4" Duct &Hood

4" Duct &Hood w/ Orifice

Diagram A

Diagram B

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