Would you want the air exchanger turned on and off similar to a fan? What conditions would you like to control the exchanger?
I use separate, inexpensive, temperature, motion, and humidity sensors. No need to mount them near the switches.
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Our exhaust fan is in a separate potty closet off the master bathroom. When the humidity above the shower is greater than the humidity in the potty closet it turns on the exhaust fan. When there is motion in the potty closet it turns on the light and the exhaust fan.
You can probably replace the exchanger switch with a Zooz or other brand Z-Wave or Zigbee switch. These usually have the option to enable or disable local control. Hubitat can handle the timing function. Hope this helps. BTW, I am not an expert user. There may be other opinions!
Two possibilities, something like :
#1 if humidity is greater than X, the fan turns on X min.
#2 if motion is detected for 5 min then the fan turns on X min.
The location near the switch is optimal in my case. The line of sight is perfect. Plus, I've got power there.
Your Venmar boost switch puts the HRV/ERV into high speed when activated. If the HRV is off, the boost switch toggles it on and to high speed for 20/40/60 minutes. It's just a dry contact control, so you can use the Zooz Zen 51, which should fit into that box (same as your boost controller), but you'll also need power for the relay (120V). This can be triggered with RM rules and any of the sensors you see in this thread that report humidity. The light on your boost switch will not illuminate though as that power would come to the boost switch control panel via the circuit board in the HRV, but you can likely do that by connecting the OC to I terminal. The Zen 51 dry contact connections go to the OL and OC terminals on the HRV remote boost switch. Close the circuit between OC and OL and the HRV will go into boost mode. The I terminal is power for the LED/circuit board in the control, so if that light operation is important to you, you'd need a second Zen51.
You'll likely want the HRV in boost mode based on motion (stink situation) and/or humidity.
If you are looking for a reliable wired in humidity sensor, you might want to to look at the Fibaro Smart Implant and an inexpensive DHT22 sensor connected to it. The sensor can be mounted remote to the implant, so could be hidden in your existing fan housing. As far as the smart switches, lots out there but I like the Zooz Zen72s. My favourite wireless motion sensor for both reliability and battery life is the Ecolink ZWave Plus Pet Immune. They are rock solid.
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AFAIK, an air exchanger consists of 2 fans, the 1st to exhaust air from the house and a 2nd one to bring in fresh air. If you have only an exhaust fan, the fan will create slight negative pressure in the house and air will come in through any leakage points around windows, doors, air ducts, ceiling, foundation, etc. No home is sealed completely so the pressure will equilibrate eventually. The advantage to the air exchanger is that you can filter the air coming into the home.
For this reason I prefer installing the relay at the HRV screw terminal in parallel with the existing switch, this way the manual switch still works (and illuminates when HRV is running full-speed). A 5V relay (e.g. MHCOZY) would also work and may be easier to supply power to than the Zooz.
They both use usb power so it really shouldn't matter. Though the zooz is more flexable with it's voltage input. Both are good choices.
The Zen51 was suggested in the post I replied to, and that one requires line power, thus my suggestion. You might be thinking of a Zen17, which, like the MHCozy, accepts a range of DC supply voltages.
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I was
If you go to OC and OL at the wall control, it's in parallel there, and the wall switch would also work. That's how mine is setup. The main HRV wall control lights up (to indicate the HRV is running) but I'm pretty sure the timer/remote boost switches are powered and operate independently. They are only powered and flashing to indicate operation at each individual boost switch. I just sorted a wiring mess in a home with 5 of those remote/boost controls. I'm about 99% sure the other 4 remote switches don't operate/indicate when one of them has been activated, but the HRV main wall control does indicate ventilation/operation if the unit was off.
@hubitrep , are you seeing something different? I have never tested jumpered OC/OL terminals at the HRV to see if the remote boost switches indicated anything. My personal setup used a Fibaro smart implant on OC/OL at the HRV itself, no remote boost switches so I can't test this. I've also removed the Venmar fan and am using external EC fans x 2 controlled with 0-10v.
The older/less efficient units will have 1 motor, which drives both supply and return fans. Newer will have two EC fans, often auto balancing. Either way, you balance them via differential motor speed (2 motor setup) or balancing dampers (single motor) so that the house pressure remains neutral.
The advantage to the air exchanger is more to do with sensible/enthalpy (HRV/ERV) energy exchange, although yes, you can filter up to HEPA levels if desired. All new construction here requires an HRV or ERV by code.
Right, doesn't make any difference, electrically speaking. There was more space & power supply options to work with where the HRV unit is located than in a junction box however, so I found it more convenient to hook up the relay there.
Couldn't really say, I only have one bathroom control, and it does light up when the relay is triggered for high speed.
@hubitrep , ah, well for the OP, that should work great. I like the main wall control LED on specifically (when OC/OL are connected) to monitor the automation at a quick glance. It’s good to know that remote boost controls will also light up.
In my case, with the internal fan removed, I just use the OC/OL dry contacts to open and close the HRV dampers for the defrost programming via Rule Machine and temp sensor inputs.
Found this:
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Yep. Explains the remote wiring quite nicely 
Thank you everyone for your reply. I have many options to analyse now!
Be sure to let us know which option you choose and how the project turns out. Good luck! 
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