In this short podcast, Bryan answers a listener-submitted question about duct smoke detector facts and wiring. He focuses on Honeywell duct smoke detectors (namely the D4120W), but there are some general best practices that can be applied to most duct smoke detectors.

The power supply may be 24v or 120v AC or DC and may have multiple inputs. You should not break that constant power source. There are also fan shutdown contacts, which may be normally open (alarm condition) or normally closed (non-alarm condition). Supervisory contacts open to indicate a trouble condition to the panel when something in the circuit isn't working properly (NOT an alarm condition). There is often a resistor at the end of the loop, and the alarm panel recognizes a specific resistance. Alarm initiation contacts close and create a shorted condition when there is an alarm condition (the supervisory contacts open if there is a trouble condition).

Smoke detectors in the ducts are different from the ones on ceilings; they usually shut down blowers to prevent the distribution of smoke through the ductwork (and possibly exhaust it to the outside). They are required in duct systems designed for more than (or at least) 2000 CFM per NFPA90 and IMC 606.2.1, though those codes conflict. NFPA90 requires installation in the supply duct, but IMC 606.2.1 requires installation in the return; the AHJ may interpret the code either way. (Smoke detectors in systems >15000 CFM must be in both the return and the supply.)

Overall, remember that different detectors are suitable for different velocities. You will want to keep them downstream of bends for at least 18 inches, test them according to the AHJ, and ensure that they are mounted in the right configuration as recommended by the manufacturer.

Have a question that you want us to answer on the podcast? Submit your questions at https://www.speakpipe.com/hvacschool.

Purchase your tickets or learn more about the 6th Annual HVACR Training Symposium at https://hvacrschool.com/symposium.

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In this short podcast episode, Bryan dives into soft start & solar technology, which often work together. He also talks about how we can install ductless systems in homes that rely on solar power.

Soft starts are NOT hard starts. Hard starts add more current to the start (auxiliary) winding, which creates a stronger phase shift. Instead, soft starts control the current on the start and run windings and provide just enough of a phase shift to ramp up the motor slowly; it provides a regulated power supply, which means that a slow startup doesn't necessarily cause overheating because there is no locked rotor condition.

We can use soft starts on two-stage compressors but need to make sure the compressor goes up to high speed during the starting sequence. Soft starts "learn" the compressor's cycles, so we need to make sure they are trained to reach the high stage, not the low stage. In systems with multiple compressors, we need to have a separate soft start for each compressor. We don't need to use soft starts on inverter-driven or variable frequency drive (VFD) systems.

Soft starts also make sense in systems that rely on solar technology. Solar technology may be on-grid (where we sell back energy during peak times) or off-grid (where energy is banked). Ductless systems can work with solar energy, but they must be connected to the grid or the bank. Banked solar energy doesn't need to be rectified in the same way that AC power from the electrical company does, which may be advantageous for ductless systems that rely on DC-driven technology.

Have a question that you want us to answer on the podcast? Submit your questions at https://www.speakpipe.com/hvacschool.

Purchase your tickets or learn more about the 6th Annual HVACR Training Symposium at https://hvacrschool.com/symposium.

Subscribe to our podcast on your iPhone or Android.

Subscribe to our YouTube channel.

Check out our handy calculators here or on the HVAC School Mobile App for Apple and Android.