IMC 2009 requires smoke detectors on fan systems with capacity greater than 2,000 CFM on return duct or plenum upstream of any filters, exhaust air connections, outdoor air connections, or decontamination equipment and appliances. There are several exceptions. See below.

NFPA 90A requires smoke detectors Downstream of the air filters and ahead of any branch connections in air supply systems having a capacity greater than 944 L/sec (2000 ft3/min)

See below for all exceptions:

From IMC 2009 “606.2 Where required. Smoke detectors shall be installed where indicated in Sections 606.2.1 through 606.2.3. 

Exception: Smoke detectors shall not be required where air distribution systems are incapable of spreading smoke beyond the enclosing walls, floors and ceilings of the room or space in which the smoke is generated.

606.2.1 Return air systems. Smoke detectors shall be installed in return air systems with a design capacity greater than 2,000 cfm (0.9 m³/s), in the return air duct or plenum upstream of any filters, exhaust air connections, outdoor air connections, or decontamination equipment and appliances.

Exception: Smoke detectors are not required in the return air system where all portions of the building served by the air distribution system are protected by area smoke detectors connected to a fire alarm system in accordance with the International Fire Code. The area smoke detection system shall comply with Section 606.4.

606.2.2 Common supply and return air systems. Where multiple air-handling systems share common supply or return air ducts or plenums with a combined design capacity greater than 2,000 cfm (0.9 m³/s), the return air system shall be provided with smoke detectors in accordance with Section 606.2.1. Exception: Individual smoke detectors shall not be required for each fan-powered terminal unit, provided that such units do not have an individual design capacity greater than 2,000 cfm (0.9 m³/s) and will be shut down by activation of one of the following:

1. Smoke detectors required by Sections 606.2.1 and 606.2.3.

2. An approved area smoke detector system located in the return air plenum serving such units.

3. An area smoke detector system as prescribed in the exception to Section 606.2.1. In all cases, the smoke detectors shall comply with Sections 606.4 and 606.4.1.

606.2.3 Return air risers. Where return air risers serve two or more stories and serve any portion of a return air system having a design capacity greater than 15,000 cfm (7.1 m³/s), smoke detectors shall be installed at each story. Such smoke detectors shall be located upstream of the connection between the return air riser and any air ducts or plenums.

From NFPA 90A: 6.4.2.1: Smoke detectors listed for use in air distribution systems shall be located as follows:

• Downstream of the air filters and ahead of any branch connections in air supply systems having a capacity greater than 944 L/sec (2000 ft³/min)

The purpose of these smoke detectors is to shut down the air handler (fan) of the single unit it is serving in the event of a fire or smoke from the motor, filter, belts, and so on, of that unit. The fire load from those items is small and will likely burn out long before first responders can arrive. In addition, the amount of smoke generated will likely not migrate out of the system once the fan is shut down. Hence, there is no requirement to sound an evacuation alarm because an evacuation is more likely to cause problems than the small amount of smoke.

• At each story prior to the connection to a common return and prior to any recirculation or fresh air inlet connection in air return systems having a capacity greater than 7080 L/sec (15,000 ft³/min) and serving more than one story

6.4.2.2 Return system smoke detectors shall not be required where the entire space served by the air distribution system is protected by a system of area smoke detectors.

There are two purposes for smoke detectors on returns. The first is to shut down the air handler (fan) of the single unit it is serving in the event of a fire or smoke from the motor, filter, belts, and so on, of that unit and to close the smoke dampers required by 4.3.9.2 to contain smoke from a fire in the unit. The second is to provide shutdown where smoke might migrate from other floors through a common return prior to shutdown by the supply side detector. The committee considers area detection, and therefore a building alarm, sufficient for occupant safety so that the return side detector is not required where there is area detection.

6.4.2.3 Smoke detectors shall not be required for fan units whose sole function is to remove air from the inside of the building to the outside of the building.

There are two purposes for smoke detectors on returns. The first is to shut down the air handler (fan) of the single unit it is serving in the event of a fire or smoke from the motor, filter, belts, and so on, of that unit and to close the smoke dampers required by 4.3.9.2 to contain smoke from a fire in the unit. The second is to provide shutdown where smoke might migrate from other floors through a common return prior to shutdown by the supply side detector. The committee considers area detection, and therefore a building alarm, sufficient for occupant safety so that the return side detector is not required where there is area detection.