System Sensor 7251DH User Manual

Page 3

SS-400-000

I56-3065-000R

Step 5. Install the Inlet Tube

The inlet tube (shown in Figure 4) is identified by a series of air
inlet holes on the tube. This tube must be purchased separately.
Order the correct length, as specified in Table 1, for the width of
the duct where it will be installed. The exhaust tube is molded
into the base of the duct housing.
The inlet tube is always installed in the right house bushing, with
the air inlet holes facing into the air flow. To assure proper instal-
lation, the tube mounting flange is marked with arrows. Mount
the inlet tube so that the arrows point into the air flow. Figure 5
shows the various combinations of tube mounting configurations
with respect to air flow.

Figure 4. Air duct detector inlet sampling tube:

INLET

TUBE

END

PLUG

AIR HOLES

ARROWS

MUST FACE

INTO AIR FLOW

AIR FLOW DIRECTION

FLANGE

H0108-00

A. Installation for Ducts Less Than 8 Feet Wide

1. Any inlet tube over 3 feet long must be supported on the op-

posite side of the duct detector housing.

2. If the inlet tube is longer than the width of the air duct, drill a

˝ hole in the duct directly opposite the hole already cut for

the inlet tube. If the inlet tube is shorter than the width of the
air duct, install the end cap into the inlet tube (see Figure 4).

3. Slide the inlet tube into the right housing bushing. Position

the tube so that the arrows point into the air flow.

4. Secure the tube flange to the housing bushing with the two

#6 self-tapping screws.

5. For tubes longer than the width of the air duct, the tube

should extend out of the opposite side of the duct. If there are
more than 2 holes in the section of the tube extending out of
the duct, select a different tube length using Table 1. Other-
wise, trim the end of the tube protruding through the duct so
that 1 to 2 inches of the tube extends outside the duct. Plug
this end with the tube end plug and tape closed any holes in
the protruding section of the tube. Be sure to seal the duct
when the tube protrudes.

WARNING

In no case should more than 2 air inlet holes be cut off the tube.
There must be a minimum of 10 holes in the tube exposed to the
air stream.

HOLE B

HOLE A

DUCT

WALL

SPEED

NUT

SCREW HOLES FOR ATTACHING

DETECTOR HOUSING TO DUCT

FOAM GASKETS

EXHAUST TUBE

(EXTENSION BUSHING)

DUCT DETECTOR

HOUSING

O-RINGS

MOUNTING

SCREWS

INLET SAMPLING

TUBE BUSHING

Figure 3A. Speed nut mounting location:

H0116-00

H0238-00

Figure 3B. Installation of foam gaskets over sam-
pling tube bushings:

B. Installation for Ducts More Than 8 Feet Wide

NOTE: To install inlet tubes in ducts more than 8 feet wide, work
must be performed inside the air duct. Sampling of air in ducts
wider than 8 feet is accomplished by using the ST-10 inlet sam-
pling tube.
Install the inlet tube as follows:
1. Any tube (over 3 feet long) that doesn’t protrude through the

duct (on the side opposite the housing) must be supported by
other means.

2. Drill a

˝ hole in the duct directly opposite the hole already

drilled for the inlet tube.

3. Slide the inlet tube with the flange into the right housing

bushing. Position the tube so that the arrows point into the
air flow. Secure the tube flange to the housing bushing with
the two #6 self-tapping screws.

4. From inside the duct, couple the other section of the inlet

tube to the section already installed using the

˝ conduit

fitting supplied. Make sure the holes on both of the air inlet
tubes are lined up facing the air flow.

5. Trim the end of the tube protruding through the duct so that

1 to 2 inches of the tube extends outside the duct. Plug this
end with the tube end cap and tape closed any holes in the
protruding section of the tube. Be sure to seal the duct when
the tube protrudes.

NOTE: An alternate method to using the ST-10 is to use two ST-5
inlet tubes. Remove the flange from one of the tubes and install
as described above. After the installation, use electrician’s tape to
close off some of the sampling holes so that there are a total of 12
holes spaced as evenly as possible across the width of the duct.
NOTE: Air currents inside the duct may cause excessive vibration.
This vibration can slowly open the seal around the tube and per-
mit air to escape. To prevent this from occurring, a 3˝ floor flange,
available at most plumbing supply houses, may be used. This
flange/connector mounting technique makes the initial installa-
tion easier because a 1˝ to 1

˝ hole may be drilled where the

flange/connector will be used. It is easier to push the inlet tube
through the larger hole.

Modification of Inlet Sampling Tubes

There may be applications where duct widths are not what is
specified for the installation. In such cases, it is permissible to
modify an inlet sampling tube that is longer than necessary to
span the duct width.
Use a 0.193-inch diameter (#11) drill and add the appropriate
number of holes so that the total number of holes exposed to
the air flow in the duct is 10 to 12. Space the additional holes as
evenly as possible over the length of the tube.