Duct Smoke Detector Holding the RTU or Furnace Off: Terminals and Bypass

The Facts

Duct Detector Holding Unit Off

What It Usually Is

Detector in alarm from dust, insect debris, or a dirty sensing chamber: the most common cause.
Detector lost its 24 V supply, so its relays dropped and the normally-closed contacts opened.
Sampling tube gaskets or the housing cover loose, changing airflow through the chamber.
Sensor head at end of life or a real smoke event that was never reset.

Check This First

Look at the detector's LEDs: power, alarm, trouble. A green power LED and a red alarm LED tell different stories.
Meter the 24 V supply at the detector's power terminals.
Meter the interlock contacts in the unit's control circuit with power off: open means the detector is holding the unit.
On the APC SL-2000 series, the fan-shutdown interlock is the AUX A normally-closed contact, terminals 16 and 6, wired to break the 24 V power wire to the thermostat.

When To Call

The unit will not run and the detector shows alarm or trouble.
You have reset the detector and it alarms again within minutes.
Any time a detector needs to be bypassed to keep a building conditioned.

What the Repair Usually Costs

Cleaning and resetting a detector is a service-call-level job. A new sensor head or a new detector is a mid-range HVAC controls repair. We quote the number on site.

The diagnostic visit turns that range into a number for your unit, and you approve the number before any repair happens.

Dirty Works technician servicing a rooftop unit

Controls

Need the Documents Today?

Text 601-337-1069 with the certificate holder's legal name and address, or email the vendor form. The W-9 and certificate of insurance come back the same business day.

Text the Request

Common Questions

Duct Detector Holding Unit Off FAQ

Can I just jumper the detector to get the AC running?

A temporary bypass with the building notified and the detector tagged is sometimes necessary to keep a business open for a day. A permanent jumper defeats a life-safety device and is never the fix. We clean, test, or replace the detector and remove the bypass the same visit.

Why does it keep alarming?

A dirty sensing chamber is the usual reason. Kitchens and dusty returns foul them faster than the manufacturer's cleaning interval.

Do you carry replacement heads?

For the common models, yes, along with the installation documentation for the terminal layouts.

Duct Smoke Detector Won't Let the Unit Run