Whenever there is a conversation where “code” is involved, it's important to mention that codes can vary depending on the AHJ or authority having jurisdiction. It's becoming more common for governments to lean heavily on the ICC (International Code Council). In the case of HVAC/R, that is the IMC (International Mechanical Code), and in the case of fire protection and electrical codes, the NFPA (National Fire Protection Association). Those have become the authority for codes and standards in the US.

So, what is the purpose of duct smoke detectors?

NFPA 90A, 2012, A.6.4 makes this pretty clear by stating:

Protection provided by the installation of smoke detectors and other related requirements is intended to prevent the distribution of smoke through the supply air duct system and, preferably, to exhaust a significant quantity of smoke to the outside. Neither function, however, will guarantee either early detection of fire or the detection of smoke concentrations before dangerous smoke conditions if the smoke movement is other than through the supply air system.

In other words, duct smoke detectors keep units from circulating smoke in the space and, when possible, send it outside. They aren't there as a replacement for space smoke detectors.

When do they need to be installed? 

Both NFPA 90 and IMC 606.2.1 state similar things that can be summarized and paraphrased as, “If the duct system is designed for more than 2000 CFM, the system must have a duct smoke detector installed,” and, “If the duct system is designed for  more than 15,000 CFM, one in the return and supply is required.”

NFPA 90 states that the smoke detector should be installed in the SUPPLY after 2000 CFM, and IMC 606.2.1 says the RETURN. This means that it up to the AHJ to decide which standard they follow.

NFPA 90A also states: “Where an approved fire alarm system is installed in a building, the duct smoke detectors shall be connected to the fire alarm system.”

Now, these are summaries of more complicated texts with exceptions and lots of extras, so if you want to know all the details, I would suggest you read the code for yourself, but in general:

• A duct smoke detector should shut off a typical blower and fresh air and turn on the exhaust.
• Duct detectors aren't a replacement for room sensors.
• If the duct system is designed to carry more than 2000 CFM (5 tons nominal) of air, you need one in the return if IMC is being followed and the supply if NFPA is being followed.
• If the duct system is designed to carry more than 15000 CFM of air, you need one in the return and one in the supply.
• If a central fire monitoring system is in place and a duct smoke detector is in use, it must be connected to the fire monitoring system.

—Bryan