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Duct Smoke Detectors and Coil Freeze Protection for Commercial Rooftop Units

This category covers two kinds of unit-level protection that commercial packaged units often need and that are easy to leave off an order: duct smoke detection, which shuts the unit's fan down so it does not spread smoke through the building, and indoor coil freeze protection, which keeps a lightly loaded unit from icing its own coil. Both are life-safety or equipment-protection items, both are inspected or noticed when they are missing, and both are much easier to install before the unit is in service.

What Is in This Category

PartFunctionUnit fit
Rheem RXRX-BS01Return air duct smoke detector, field kitC cabinet: RACCYB 3-5 ton, RGECYB and RHPCYB 3-6 ton
Rheem RXRX-BS02Return and supply air duct smoke detector, field kitC cabinet, 3-6 ton
Rheem RXRX-BS03Communicating return air smoke detector kitD cabinet: RACDYB and RGEDYB 7.5-12.5 ton, RHPDYB 7.5-10 ton
Rheem RXRX-BS04Return and supply air smoke detector kitD cabinet, same units as above
Daikin FSK02AFreeze protection thermostat kitDaikin packaged rooftop units listed on the product page
Daikin FSK03AFreeze protection thermostat kitDaikin packaged rooftop units listed on the product page

Other controls live with the equipment they serve. Economizer controllers, enthalpy sensors and the Daikin CO2 sensor are under Rooftop Unit Economizers. The FSK04A freeze protection kit for 7.5 to 10 ton Daikin units is under Blower, Vent and Conversion Kits. DDC versions of dampers and power exhaust are in their own categories.

When a Rooftop Unit Needs a Duct Smoke Detector

Most US jurisdictions adopt the International Mechanical Code, which requires smoke detectors in the return air of air distribution systems above 2,000 CFM, with listed exceptions and additional requirements for some configurations. Many jurisdictions amend the model code, so the authority having jurisdiction and the project drawings have the final word. In practical terms:

  • Know the design airflow, not just the tonnage. At a typical 350 to 400 CFM per ton, a 5 ton unit sits right around 2,000 CFM and a 6 ton unit is usually above it. Units running higher airflow per ton, or with a high static kit, can cross the threshold sooner. Use the scheduled airflow.
  • Everything from 7.5 tons up should be assumed to need return air smoke detection unless the design shows an exception applies.
  • Supply-side detection is called for in some arrangements, such as multiple units sharing a common supply or return, or where the designer or code official requires it. That is where the return and supply kits (RXRX-BS02 and RXRX-BS04) come in.
  • Buildings with a fire alarm system usually require duct detectors to report to it. Coordinate with the fire alarm contractor on monitoring and the supervisory or alarm signal the local code requires.

Choosing Between the Rheem Kits

Start with the cabinet. The BS01 and BS02 kits fit the C cabinet; BS03 and BS04 fit the D cabinet. Then decide return-only or return-and-supply based on the code analysis. For D-cabinet units, the BS03 is listed as a communicating return detector kit, intended for units using Rheem's communicating controls; confirm with the unit's control documentation that it matches your board before ordering. These unit-mounted kits install in the unit's return and, where applicable, supply sections, which saves field duct work and keeps the detectors where the service technician can reach them.

Installing and Wiring a Duct Smoke Detector

  1. Confirm the kit matches the unit cabinet and the code requirement (return only, or return and supply).
  2. With power locked out, mount the detector and its sampling components in the locations the kit instructions specify. Location matters: the detector must see representative airflow.
  3. Wire the detector so that an alarm shuts down the unit's supply fan, using the terminals and method the unit wiring diagram shows. Remove only the factory jumpers the instructions tell you to remove.
  4. If the building has a fire alarm system, provide the auxiliary contacts or connections the fire alarm contractor needs. Who makes that connection is usually set by the project scope and local licensing.
  5. Power up and test: trigger the detector with the test method the manufacturer allows, confirm the fan shuts down and stays down until reset, and confirm the fire alarm receives the signal where applicable.
  6. Record the test in the start-up report.

Testing and Maintenance

Duct detectors are inspected and tested periodically under the fire code and the detector manufacturer's instructions. Dust buildup in the detector or its sampling path causes nuisance alarms, and a failed detector is a code violation found at the next fire inspection. Include the detectors in the unit's planned maintenance, keep the test records with the building's fire and life safety documentation, and never leave a detector bypassed after service.

Why Indoor Coils Freeze on Commercial Units

An indoor coil freezes when its surface temperature stays below 32 degrees long enough for condensate to turn to ice. On commercial packaged units, the usual causes are:

  • Low airflow from dirty filters, slipping belts, closed dampers or excess duct static.
  • Low load, such as a unit cooling a lightly occupied space at night or on a mild day.
  • Cool outdoor air mixed into the return through an economizer or fresh air damper, lowering entering air temperature.
  • Low refrigerant charge or a metering problem.

Ice blocks airflow, cuts capacity, and when it melts it can overflow the drain pan into the ceiling below. Repeated freezing also risks liquid refrigerant reaching the compressor.

Daikin Freeze Protection Thermostat Kits

The FSK02A and FSK03A kits protect the indoor coil from freezing during low-load cooling operation on the Daikin packaged units listed on each product page. A freeze protection thermostat of this kind senses coil temperature and interrupts cooling when the coil gets too cold, letting it recover before cooling resumes. Check the product page and the unit accessory table to pick the kit listed for your unit. For 7.5 to 10 ton units, the FSK04A kit is listed under Blower, Vent and Conversion Kits.

A freeze protection kit is protection, not a cure. If a unit trips it regularly, find the cause: check airflow, filters, belts, refrigerant charge, economizer minimum position and the control sequence.

Smoke Detection on Units With Economizers

On a unit with an economizer, smoke detector shutdown should also leave the outdoor air and relief dampers in the position the code and fire protection design call for, which on most packaged units means the dampers return to their closed or minimum position when the fan stops. Confirm the sequence during the detector test.

Ordering Tips

  • For Rheem smoke detectors, send the unit model number and confirm C or D cabinet, and whether the code analysis calls for return only or return and supply detection.
  • For D-cabinet units with communicating controls, ask us to confirm whether the communicating BS03 is the right match.
  • For Daikin freeze protection, send the unit model number so we can match FSK02A, FSK03A or FSK04A.
  • Order these with the unit. Inspectors will look for smoke detection at final, and adding it later means another roof trip.

Frequently Asked Questions

Does a 5 ton rooftop unit need a duct smoke detector?

It depends on its design airflow and local code. Under the International Mechanical Code threshold of 2,000 CFM, a 5 ton unit is borderline. Use the scheduled airflow and check with the code official.

Return only, or return and supply?

Return air detection is the common requirement. Supply detection is added for certain configurations or when the designer or code official calls for it.

Can a duct smoke detector be tied to the building fire alarm?

Yes, and where a fire alarm system exists it is usually required. Coordinate the connection with the fire alarm contractor.

What happens when the detector trips?

It shuts down the unit's supply fan so the system does not distribute smoke. The unit stays off until the detector is reset.

My unit keeps tripping its freeze protection. Is the kit faulty?

Usually not. Repeated trips point to low airflow, low charge, low load or too much cold outdoor air. Diagnose the unit before replacing the kit.

Can I install a smoke detector kit on a unit already in service?

Yes, these are field-installed kits. Schedule a test with the fire alarm contractor if the building has a monitored system.

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