*TL;DR: a flow sensor on a kitchen exhaust duct fails to work reliably in its current location. Will moving it down two floors closer to the upstream exhaust hoods help or be a waste of effort?

We have a four floor, two unit townhouse with one kitchen on the first floor and another on the 2nd floor. We recently completed a renovation of the 2nd, 3rd and 4th floors. This included redoing the kitchen exhaust ducting from stove hoods in both kitchens.

The mechanical engineer designed a system where the exhaust ducts from the 1st and 2nd floor kitchen join at the 3rd floor level, then continue through the 4th floor to the flat roof of the townhouse, where they connect to another exhaust fan. This roof-mounted exhaust fan is operated by a flow sensor. The flow sensor was installed inside the roof-mounted fan. The intention is that the roof-mounted fan is triggered by flow from the stove hoods and provides a boost that prevents cooking smells from one kitchen flowing back to the other.

Unfortunately the system does not work reliably. Sometimes the roof-mounted fan will turn on if the kitchen hood is turned to full, but often it won’t.

The HVAC contractor has suggested moving the flow sensor to the 3rd floor where the ducting from the two kitchens join. It’s a convenient location next to the main electrical panel and with good access to the ventilation duct. I have asked the mechanical engineer to assess this suggestion, but given the initial design didn’t work I don’t trust his judgement.

The relocation of the sensor will be expensive, but the only other alternative I see is to have the roof fan run permanently. It’s a 1/7 horsepower fan so I expect ~$200 a year in electricity plus additional heating/cooling costs. Does the relocation proposal make sense, or is the whole flow sensor setup unlikely to work in my situation?

1 Answer 1


Flow sensor was/is a stupid way to do this, and the engineer is an idiot for specifying it. Perhaps a malpractice level idiot if they are a professional engineer. If you paid a licensed mechanical engineer to design a system and it does not work as designed, the cost of making it do so should be on them, not you.

A relay connected to each exhaust fan is simple and reliable.

If either exhaust fan is on, the roof fan is on. If both exhaust fans are on, the roof fan is on. If both exhaust fans are off, the roof fan is off. No sensor needed. No modes where a sensor fails to notice the circulation from one fan to the other, or where the roof fan itself keeps the sensor triggered.

An even better design would have a roof fan doing the whole job, rather than using 3 separate fans for this task. The fan switches below would then just turn on their relay, rather than their local fan and their relay.

  • Thank you for this. The mechanical engineer who designed this is a professional but he has now left the firm we were working with. We are working with his successor. The architect said that the original engineer rejected the idea of using a tray because the cooker hood does not have a terminal for a relay. What should be done here - should we use an contactless current sensor on the load wire for the cooker hood?
    – jweob
    Commented Jan 26, 2022 at 1:21
  • Few things have a extra relay terminal built-in. A junction box and wire is generally preferable to overcomplicating this with pricy sensors.
    – Ecnerwal
    Commented Jan 26, 2022 at 1:35

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