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Community and Q&A

low voltage motion switch for hrv in bathrooms

nrosdal | Posted in General Questions on

I am building a home and we are using the Fantech Hero 200h to bring in fresh and remove stale air (ducted to the 3 bathrooms for air removal).  I currently have low voltage wire ran to each bathroom for a boost button.  But i would like to figure out how i can use something like i have now running bath fans in my current home (lutron motion switch that will trip and turn it on automatically for the set amount of time since i doubt my wife or guests will reliably push the boost button if that is the way we go).

does anyone know of a low voltage motion switch that would be compatible?  or is there a way to make a line voltage motion switch send the right signal to enable turning the hrv up when tripped?

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  1. Expert Member


    I've never understood how that works in practice. Maybe I'm odd, but I sometimes wander into the bathroom, look at myself briefly in the mirror and wander out again. I also go in just to wash my hands, urinate, or brush my teeth, get a pain pill after a long day, or simply look out the window. None of those necessitate activating an exhaust fan.

    1. cs55 | | #4

      ideally you don't rely on a single sensor. because you're right, theres no need to boost the cfm or turn on a fan if just piddling.

      a humidity sensor is ideal for the shower. but for toilet purposes thats more difficult. an mmwave occupancy sensor can work to some extent. depending on the layout of the room it would be possible to differentiate someone sitting on the toilet and someone looking out the window.

      personally, i have a heated bidet that is plugged into a smart outlet that monitors energy usage. the bidet will go between 800 and 1500 watts when the water is being heated. so i am able to turn on the bathroom fan based on that data.

      however, that relies on the bathroom user to use the bidet.

      then the fan will stay on as long as someone is in the bathroom, and turn off after 15 minutes of no activity.

  2. user-5946022 | | #2

    IANAE (I am not an Electrician) so double check on this, but....
    You need a device that will switch your wiring and that has a motion detector. The challenge is most switches/buttons with a motion detector power it from line voltage. So you need a device with a powered motion detector, and a SEPARATE dry contact switch for your fan wiring. One strategy is to increase the size of the existing box for your bath lights by one switch space, and run 14 GA control wiring for the Fantech to that box. Then you use the 14 ga wire for the switch, and a 120 jumper from the adjacent light switch circuit to power the motion detector. You could use something like the Swidget Aux control switch, with the Temp/Humidity/Motion insert, and wire is up as a dry contact.
    The advantage of this method is that is uses generally understood technology; the disadvantage is that most residential electricians have no clue what a dry contact is...and why you can't power it...

    Another approach would be to implement a smart home system. Install either a zwave or zigbee low voltage momentary switch, or a regular momentary switch with a low voltage zwave or zigbee relay behind the switch in the box. This keeps it all low voltage. Then install a multisensor somewhere - something like the Zooz Motion/Temp/Humidity/Lux sensor. They connect to a Hub such as Hubitat, and you set up the hub to turn the switch on for x minutes whenever the motion triggers, or to turn it on when the humidity is > y. The great thing about this solution is you can include conditions, so for example you can exclude motion from triggering the switch 3 am when someone just makes a quick trip into the bathroom.

    Another smart home system would use the wiring setup in the first example, but install just a momentary switch to the control wiring. Install a Zooz51 behind the switch and connect the dry contact part to the switch control, and the balance to the line voltage at the light switch adjacent to it in the box. Then have the rest of it be the same as the MultiSensor described above.

    There are ALOT of ways to do this, and the technology changes. My recommendation is to give yourself multiple options by maybe even wiring it up for both. It is much easier to put a blank on the end of a box now (or even a switch that does nothing) than to try to fish in wire later.

  3. Expert Member
    BILL WICHERS | | #3

    Just use one of two things: alarm system motion detectors, which typically run on 12v DC systems, or borrow a nifty gizmo from the commercial access control world: the REX (Request to EXit) motion detectors, which are more configurable. This is a link to the classic REX motion sensor:

    These can normally run on 12v or 24v DC, and they have a set of dry contacts along with some control logic that can be set to hold the contacts closed for various amounts of time after motion is detected. These can probably be setup to do exactly what you want to do.

    You can get basic DC power supplies from many sources, but if you want something permanently installed, you might want to go with one of the smaller units Altronix makes -- they are a big manufacturer of power supplies for security systems. If you need the low voltage wiring to control a line voltage gizmo somewhere, get a relay that can be controlled by the same 12v or 24v DC that's powering the motion sensors, and use the contacts in that relay to switch line voltage. I recommend octal base relays and DIN rail sockets to make installation easy. Grainger is a good source for these. You'll probably need either an SPDT relay (which can act as a three way or a regular switch), or a DPDT relay, which can be wired to act as a 4 way switch. There are lots of ways to do the wiring itself, but you'll probably want all the motion sensor contacts in parallel, so that any one of the motion sensors can trigger the boost function.


  4. nrosdal | | #5

    appreciate all the replies, It seems a bit more complicated than my electrical skillset is up for. But i will forward the link to this thread to my electrician and see if he can make sense of it and make it work.

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