Air flow sensor ideas?

I’m on the verge of purchasing a Nest thermostat for my upstairs zone - which is not heating, it is air conditioning only. The reason I’m doing this is to configure the Ecobee4, which runs the downstairs zone (both AC and heat), to prevent it from putting heat into the house if the upstairs AC is running.

But that is expensive.
And I do not wish to put power flow sensors on the 220 line that feeds the air handler.

Sooo… I’m thinking about some sort of airflow sensor. Looking for ideas.
My first thought is a very lightweight flap in an air duct, attached to a very lightweight spring. When the air begins to flow from the duct into the room, the flap gets pushed down by the air. This disconnects a circuit attached to a sensor, and that sensor then reports the event.

As long as the circuit is disconnected, the ecobee is forced into a mode where the heat start point is so low that the heat never turns on.

Of course, in the long term there will be battery costs - both for the sensor, and for my existing thermostat. The Nest running off the AC common, plus its learning capability, might result in it paying for itself soon enough… so I have to think about that.

In the meantime, anyone have thoughts on such an airflow sensor? Pitfalls? Better ideas? As one possible pitfall, I’m imagining it might create whistling noises in the duct…

You could install a backdraft damper, something like this, in the duct and then rig a magnet to the exposed portion of the pin to trigger a contact sensor.

Wouldn’t it be easier to just query the upstairs thermostat, and if it’s running, then have the downstairs thermostat act accordingly? Unless I’m missing the point, checking for air flow seems like an overly complicated way to see if the upstairs A/C is running.

I’m not familiar with what a Nest thermostat exposes, but a Honeywell Total Connect exposes the operating state. I haven’t used this item specifically, but it seems to indicate if the thermostat is running. The possible states for my thermostat are cooling, fan only, heating, idle, pending cool, pending heat and vent economizer.

Using webCore, I’d use a change in that status to throw a virtual switch and then condition the downstairs thermostat to refer to that virtual switch’s state and act accordingly.

Sorry, I should have been clearer: because the Nest is expensive, I am seeking a way to determine whether the upstairs AC is running WITHOUT buying the Nest.

My current thermostat is not a smart device.
Hence the idea of some other type of sensor to detect air flow.

Might be easier just to put a smart temp sensor upstairs on query that then? I’ve seen 4-in-1 sensors for under $40 pretty regularly.

Done one way: This could easily be problematic on cold days. A sensor’s temperature could be dropping upstairs not because the upstairs AC is on, but because the overall temperature is actually dropping. In which case, having a dropping temperature on that sensor prevent the downstairs heat from going on would be counterproductive.

Done another way: if things are set such that an upstairs sensor is programmed in webcore to mirror the upstairs thermostat schedule, then you don’t account for all the instances where someone manually changes the thermostat.


I’ve seen some suggestions to use a vibration sensor. IIRC, people received mixed results using one.

Are you looking to do DIY sensor?

If yes, there are many flow sensors like here. It is basically a hall effect sensor with small turbine. It generate pulse when the turbine is rotated. You can count this rotation with arduino or any of your favorite MCU to indicate whether your AC is on or off. I have open one up. The whole sensor part is much smaller. You can replace the turbine with this kid toys and magnet.

If you are looking for off the self solution, it would interesting if you could explore doing this with two temperature sensors. I notice that when I am standing in front of an AC, it will always be cooler compared to the corner of the room. This is not perfect solution in extreme case. The detection period could also be slow. However, in reasonable ambient setting, the delta in temperature can be significant enough to tell whether your AC is on or off. You do have to install one sensor close to the AC( in front of the vent if possible). The other one is in some corner of a room. They will have some delta. I hope the delta temperature is enough to tell whether the AC is on or off.

I prefer to avoid moving parts or impeding flow so I would use a current status sensor like Veris H300 or H608 , with a dry-contact transmitter like Aeotec.

If you are conditioning air, not just moving air, then it seems easiest to drop a wireless temperature sensor into a duct and compare the recent max to the present temp, to see that cooling airflow is occurring (comparison in Webcore).

@Glen_King do you have the Ecobee remote sensors? I had a similar issue where downstairs heat was causing upstairs to turn on AC. I ended up putting Ecobee sensors in my bedrooms and disabled the thermostat sensor and just use the bedroom sensors as primary. You could do something similar. I have always thought it was stupid for contractors to install thermostats in hallways for this reason. The ecobee sensors has been my solution.

I do have a remote sensor, but not where it has that effect.
It’s not downstairs heat causing the upstairs AC to come on. The issue is the opposite. Upstairs gets warm quickly from the sun on the roof, while downstairs remains cool. So the upstairs AC might kick in to cool that area while the downstairs remains cool enough for the heat to come on, especially if my wife turns the upstairs thermostat AC point down while she’s sleepwalking.

I just want to ensure that the upstairs AC and downstairs heat are never going simultaneously.

But something happened today that kinda sent me in the direction of just biting the bullet, and installing a smart thermostat upstairs. I’m just not sure which one. Seems the Nest is far from perfect.