Duct Smoke Detectors. Install in return, supply, or both return and supply air ducts?

Duct Smoke Detectors.

Install in return, supply, or both return and supply air ducts of an air handling unit?

Let’s first establish the goal of this exercise.  Why do we need duct smoke detectors?  The goal is not to fan the fire.  In case there is a fire, all air movement should be stopped so that the fire is not aided by the air provided by an air handling unit, a rooftop unit, a forced-air furnace, a supply air fan, or any other HVAC equipment.  A duct smoke detector is installed to sense smoke (a byproduct of fire), and in case there is a fire send a signal to an alarm panel so that the respective HVAC unit is shut down.

Now let’s imagine an HVAC system comprised of a rooftop unit. [The logic presented below applies to other HVAC systems comprising of supply and return air ducts.]

Scenario 1:  Duct smoke detector in the supply air duct.  Rooftop unit WITHOUT an economizer.

A)  If there is a fire in the space, the smoke will go in the return air duct, and ultimately end up in the supply air duct.  Once the duct smoke detector installed in the supply air duct senses the smoke, the signal will be sent to an alarm panel that will work towards shutting down the rooftop unit.

B)  If there is a fire in the rooftop unit, the duct smoke detector installed in the supply air duct will sense the smoke and other processes will follow to shut down the unit.

Conclusion: For units without an economizer a single duct smoke detector installed in the supply air duct will work fine.

Scenario 2:  Duct smoke detector in the return air duct.  Rooftop unit WITHOUT an economizer.

A)  If there is a fire in the space, the smoke will go in the return air duct where the detector will sense the smoke, the signal will be sent to an alarm panel that will work towards shutting down the rooftop unit.

B)  If there is a fire in the rooftop unit, the supply air will first go to the space, and eventually come back to the return air duct where the installed smoke detector will sense the smoke and other process will follow to shut down the unit.

Conclusion: For units without an economizer a single duct smoke detector installed in the return air duct will work fine too, but will expose the people to smoke in case there is a fire in the unit.

Scenario 3:  Duct smoke detector in the supply air duct.  Rooftop unit WITH an economizer.

A)  If there is a fire in the space, the smoke will go in the return air duct; if the unit is not running in the economizer mode it will ultimately end up in the supply air duct.  Once the duct smoke detector installed in the supply air duct senses the smoke, the signal will be sent to an alarm panel that will work towards shutting down of the rooftop unit.  BUT if the unit is running in the economizer mode, all return air will be exhausted, and the duct smoke detector will not have a knowledge of the smoke in the space.

B)  If there is a fire in the rooftop unit, the duct smoke detector installed in the supply air duct will sense the smoke and other process will follow to shut down the unit.

Conclusion: For units with an economizer a single duct smoke detector installed in the supply air duct will NOT work.

Scenario 4:  Duct smoke detector in the return air duct.  Rooftop unit WITH an economizer.

A)  If there is a fire in the space, the smoke will go in the return air duct where the detector will sense the smoke, the signal will be sent to an alarm panel that will work towards shutting down the rooftop unit.

B)  If there is a fire in the rooftop unit, the supply air will first go to the space, and eventually come back to the return air duct where the installed smoke detector will sense the smoke and other process will follow to shut down the unit.  But before this happens, people in the air conditioned space will be exposed to a lot of smoke.

Conclusion: For units with an economizer a single duct smoke detector installed in the return air duct will work, but it is better to have detectors in both supply and return air ducts to avoid exposing people to smoke.

 Photo courtesy: System Sensor