When are duct detectors not required on return air ducts?

Duct detectors are used in HVAC systems to detect smoke in the air ducts and are crucial for ensuring safety in occupied spaces. They are often required on return air ducts to ensure that any smoke is detected before it has the chance to circulate throughout the building. However, one condition under which duct detectors may not be required is when there are other smoke detection systems in place that can perform the shutdown function.

When system smoke detectors are installed and are capable of effectively shutting down the HVAC system upon detection of smoke, the need for additional duct detectors may be reduced or deemed unnecessary. This is because the existing smoke detectors would already provide a means to mitigate the risk of smoke spreading through the ductwork, ensuring that the primary function of smoke detection and response is adequately addressed.

The other choices do not meet the criteria under which duct detectors might not be necessary. Factors such as whether the building has a fire alarm system or the height of the installation do not directly negate the requirement for duct detectors. Additionally, the normal functioning of HVAC systems does not influence the necessity for smoke detection in ducts, as it is primarily concerned with smoke presence rather than operational status.