Dryer Booster Fan Failure Detection System

One of my clients has a multi unit residential building where the dryers are serviced by riser ducts.

I.e. Units 101, 201, 301, and 401’s dryer exhausts are all serviced by the same riser duct.

The fire inspector has asked him to install a booster fan at the top of each duct, and that’s simple enough.

However he also wants a sensor in each riser duct to detect fan failure, and in the event of a failure, to disable the separate 120v circuits that power the dryers serviced by that duct and fan.

I’ve picked out contactor relays for the 120V circuits on one end, and sensors for the ducts on the other end.

In between, I’m looking for a PSU that’s rated for fire from Altronix. I’m not sure if it needs to be rated for fire, but just in case.

The problem I have is that, per the vendor, I shouldn’t wire the sensors in-line with the power source. The in-rush current may, with time, burn out the sensor, especially if we get into a situation where the signal is “fluttering”, causing the relays to rapidly toggle.

To solve that, I found a second type of relay to put in between the sensor (input) side and the 120V contactor relay (output) side.

This got me wondering: should that relay be ul listed for fire? If so, what about the downstream contactor relays for the 120V dryer circuits?

Any help is appreciated.



I don’t think any of this setup would be under the umbrella of a fire alarm system. It would more likely be under the HVAC /ASHRAE codes. Any UL rated components should be fine. I would consider using a transformer and wiring the airflow sensors with 24vac which in turn activate 24vac relays or contactors with 120vac contacts rated to handle the dryer load.