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
5 Comments:
Such an amazing article. I am really motivated by this article.
Heating and Cooling Concord
Thanks for sharing about Smoke duct detectors. That article is great.
Heating and Air Conditioning New Market
I wanted to thank you for this great read!!
Heating and Cooling Concord
Thank you for sharing this.
HVAC Clean Room
Thanks for sharing the useful information.
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