System Sensor 7251DH User Manual
INSTALLATION AND MAINTENANCE INSTRUCTIONS
I56-3065-000R
7251DH Intelligent
Air Duct Smoke Detector
3825 Ohio Avenue, St. Charles, Illinois 60174
1-800-SENSOR2, FAX: 630-377-6495
Specifications
Length: 14.5 inches (36.7 cm)
Width: 5 inches (12.7 cm)
Depth: 3.5 inches (8.9 cm)
Weight: 2.8 lbs. (1.1 kg)
Operating Temperature Range: 32° to 100°F (0° to 38°C)
Operating Humidity Range: 10% to 93% Relative Humidity
Duct Air Velocity: 300 – 4000 ft./min.
Voltage Range: 15 – 32 VDC Peak
Standby Current (nominal): 230uA at 24 VDC (no communication)
330uA at 24 VDC (one communication every 5 sec. with LED enabled)
LED current (nominal): 6.5mA at 24 VDC
Before Installing
Please thoroughly read System Sensor’s Guide for Proper Use of
Smoke Detectors in Duct Applications (I56-473-XX), which pro-
vides detailed information on detector spacing, placement, zoning, wiring, and special applications. Copies of this manual are
available from System Sensor. NFPA Standards 72, 76, and 90A
should also be referenced for detailed information.
NOTICE: This manual should be left with the owner/user of
this equipment.
All sensors must be tested after installation and periodically thereafter. Testing methods must satisfy the Authority Having Jurisdiction (AHJ). Sensors offer maximum performance when tested and
maintained in compliance with NFPA 72.
General Description
An HVAC system supplies conditioned air to virtually every area
of a building. Smoke introduced into this air duct system will be
distributed to the entire building. Smoke detectors designed for
use in air duct systems are used to sense the presence of smoke
in the duct. In facilities with large air changes per hour, airflow
can carry smoke particles directly to the HVAC system. Per NFPA
76 Very Early Warning Fire Detection (VEWFD) sensors shall be
installed to monitor return air from the space. The 7251DH can be
installed to monitor the return air from the space in order to meet
NFPA 76 requirements.
The 7251DH Air Duct Smoke Detector uses a 7251 laser smoke
sensor. This smoke detection method combined with an efficient
housing design that samples air passing through a duct provides
early detection of a developing hazardous condition. When sufficient smoke is sensed, an alarm signal is initiated at the fire
control panel monitoring the detector, and appropriate action can
be taken to shut off fans and blowers, change over air handling
systems, etc. This can prevent the distribution or isolation of
toxic smoke and fire gases throughout the areas served by the
duct system as well as provide an alert of a developing hazardous
condition in a facility.
Two LEDs on each detector latch ON to provide a local alarm
indication. There is also a remote alarm output for use with auxiliary devices. The 7251DH has remote test capability with the
RTS451/RTS451KEY Remote Test Station.
Contents of the Duct Detector Housing Kit
The 7251DH consists of the following items: (See Figure 1).
See Table 1 on Page 2 for inlet sampling tube and detector supply information.
1. Complete laser duct assembly.
2. Two #10 sheet metal mounting screws.
3. Two sampling tube filters.
4. One test magnet.
5. Drilling template.
6. Two
7. Two rubber tube bushings seals.
8. Four #6 self-tapping mounting screws for the sampling tube.
9. One filter adapter.
10. One inlet tube end plug.
11. Four #6x
12. Two #10 speed nuts.
13. One test coil and parts bag.
NOTE: The inlet sampling tube must be ordered separately. It
must be the correct length for the width of the duct where it will
be installed. See Table 1 to determine the inlet tube required for
different duct widths.
5
/16˝ O-rings.
7
/10 cover mounting screws.
Installation Sequence Page
Step 1. Determine duct mounting application ............ 2
Step 2. Verify duct air flow direction and velocity . . . . . . . . . 2
Step 3. Drill the mounting holes ...................... 2
Step 4. Secure the housing base to the duct . . . . . . . . . . . . . 2
Step 5. Install the inlet sampling tube .................. 2
Step 6. Field wiring ............................... 4
Step 7. Install the filters and check pressure differential . . . . 4
Step 8. Perform detector check ....................... 4
Step 9. Install the cover ............................ 5
Step 10. Perform the final system check (Testing) .......... 7
Step 1. Verify Duct Air Flow Direction and Velocity
For “Very Early Warning Fire Protection” (VEWFP) NFPA 76 calls
for sensors to be mounted at the return air location for a space
(section 8.5.3.1.2.3). (A) “Where stand-alone packed HVAC units
are installed, sensors or ports shall be installed where return air is
brought back to the unit.” (B) “Sensors or ports shall be installed
such that each covers no greater than 0.4sq.m (4sq.ft.) of the
return air opening.” The System Sensor duct smoke detector is
designed to satisfy NFPA 76 for VEWFD.
www.systemsensor.com
SS-400-000 1 I56-3065-000R
3ft
1
2ft
CENTER
7ft
2ft
2
1
2
3
“LENGTH”
TYPIC
SAMPLING TUBE
FILTERS
CONDUIT HOLES
DUCT DETECTOR
HOUSING
TERMINAL STRIP
DUCT DETECTOR
COVER
SAMPLING TUBE
MOUNTING
SCREWS
DETECTOR HEAD
(SUPPLIED SEPARATELY)
EXHAUST
FILTER ADAPTER
DETECTOR BASE
INLET SAMPLING TUBE
(SUPPLIED SEPARATEL Y)
O-RINGS
FOAM
GASKETS
HOUSING
MOUNTING SCREWS
TEST MAGNET
TUBE
END
PLUG
Figure 1:
CAUTION
Each 7251DH duct smoke detector should be mounted as follows:
1. Location: Mount detector on the room side (up stream) of any
air filter. NOTICE: Mounting detector after filter may filter out
smoke particles and is not recommended.
2. Coverage: Each detector pipe length should be mounted
along the longer direction of the return air opening (length
direction). One detector and one standard, pre-drilled pipe
length should be installed for every 2ft of return air opening width. When more than one detector is required (widths
greater than 2ft.), detector housings should be staggered on
opposite ends of the opening length. (refer to illustration).
3. Hole Direction: holes facing directly into flow for positive
pressure effect
4. Insure adequate pressure differential across the detector housing according to step 8.1 of this document.
Figure 2. APPLICATION EXAMPLES:
Step 2. Verify Duct Air Flow Direction and Velocity
The 7251DH is designed to be used in air handling systems having
air velocities of 300 to 4000 feet per minute. Be sure to check engineering specifications to ensure that the air velocity in the duct
falls within these parameters. If necessary, an Alnor Model 6000P velocity meter, Dwyer 460 differential pressure gauge, or their
equivalent, may be used to check the air velocity in the duct.
Step 3. Drill the Mounting Holes
Remove the paper backing from the top and bottom of the template. Affix the template to the air duct at the desired mounting
location. Make sure the template lies flat and smooth on the air
duct. Center punch hole targets and remove the template. Drill the
holes as indicated on the template. Slide the two speed nuts over
the two small holes (Hole A) next to the sampling tube bushing
holes (Hole B) previously drilled in the duct (See Figure 3A).
Step 4. Mount Duct Housing
Remove the duct housing cover. Slide the foam gaskets over the
tube bushings as shown in Figure 3B. Make sure the two small
holes in the gaskets line up with the two base mounting holes. Put
5
one
/16˝ O-ring over each of the two #10 sheet metal screws. Use
the two sheet metal screws to secure the duct housing to the duct.
SS-400-000 2 I56-3065-000R
C0931-00
Do not overtighten the screws.
Table 1. Sampling (Inlet) Tubes
Tube Outside Duct Width
ST-1.5 1 to 2 ft. (0.3 to 0.6 m)
ST-3 2 to 4 ft. (0.6 to 1.2 m)
ST-5 4 to 8 ft. (1.2 to 2.4 m)
ST-10 8 to 12 ft. (2.4 to 3.7 m)
H0545-00
INLE
T
TUBE
END
PLUG
AIR HOLES
ARROWS
MUST FACE
INTO AIR FLOW
AIR FLOW DIRECTION
FLANGE
WARNI NG
HOLE B
HOLE A
DUCT
WALL
SPEED
NUT
SCREW HOLES FOR AT TACHING
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:
Figure 3B. Installation of foam gaskets over sampling tube bushings:
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 installation, 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:
A. Installation for Ducts Less Than 8 Feet Wide
1. Any inlet tube over 3 feet long must be supported on the opposite side of the duct detector housing.
2. If the inlet tube is longer than the width of the air duct, drill a
3
/4˝ 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. Otherwise, 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.
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.
H0116-00
H0108-00
H0238-00
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 sampling 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
3
/4˝ 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
1
/2˝ 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 permit 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 installation easier because a 1˝ to 1
1
/4˝ 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.
SS-400-000 3 I56-3065-000R
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