DIY HVAC Air Flow Sensor - Quicker than Thermostat Reports - Uses MCT-340 & Paddle Switch

[ronmansfield3]

I have a rule that tells me if doors or windows are open when the HVAC runs. 'Trouble is the thermostat takes forever to report that it's running the AC or heat. So I placed a "paddle switch" and a modified MCT-340 contact switch in my cold air return. Now, when air moves, the switch closes instantly and the rule quickly gives me a heads-up regarding open doors or windows. As a bonus, I now know the return air temperature too.

I used the same technique I wrote about earlier to modify the contact sensor:

DIY Wired Contact Hack Using Visonic MCT-340 E

After looking, unsuccessfully, for an off-the-shelf Zigbee device that would sense and report the state of mechanical contacts---relay contacts, simple pushbuttons, etc. I decided to make my own from a Visonic MCT-340 E. It's a "supported,'" relatively low cost magnet sensor used for doors, windows and the like. 'Turns out it is also quite easy to hack. (See photos.) [sensor%20hack%20small] [sensor%20hack%20PCB%20Small] Remove the battery cover. Pry off the rest of the rear cover. (Use a …

I needed to add a larger plastic plate to the switch's paddle to increase its sensitivity to the relatively low air flow in the return.

[aaiyar]

Nice! I use a current transducer switch connected to a contact sensor for the same purpose. The neutral of the air-handler blower is passed through the current transducer. The contact closes when the blower comes on.

I use contact sensor status to control the volume on my TV (making it louder when the blower is on, and quieter when it is off).