detecting airflow in a HVAC duct

Hello,

I am new to the arduino world and embarking on a new project. My goals is to detect airflow in my home's HVAC duct in a cheap way so I know reliably that the heating/cooling is currently running.

I have looked online but they all talk about expensive anemometers that actually "measure" volume of air moving. I DO NOT need to measure any thing. I just need to detect when significant air is flowing for a certain period of time. (several seconds)

My idea is to place a cheap CPU fan inside the duct and detect when it is turning (which tells me that the air is flowing).

Is that the right approach? How do I detect the fan turning using arduino. Any ideas are welcome.

Thanks

Fike

A pressure sensor inside the duct should measure a significant increase when the circulator fan is on. This one is very sensitive and easy to use, but may need to be temperature compensated.

A thermistor would detect whether warmer or colder air is present, and would be much simpler and cheaper.

Why not just detect when power is applied to the blower motor?

Connect this to the blower motor, with the output to an Arduino with input pullup resistor.
Output goes low when AC power is present.
AC_loss.jpg

yes, I had read about tapping into the blower motor earlier in my research.

But I want to keep this assembly close to a window in my house and not where the furnace is. There is a vent opening right above the window. The idea is that whenever airflow is detected in/near that vent (or inside the duct where the vent is),

"AND"

the window is open (which I will detect with a reed switch), the system will send me a text message alert.

This will allow me to turn off the heating/cooling remotely with my phone. (I have a smart thermostat installed)

It has happened to me many times when I leave the house for work in the morning with the window open and come back later in the evening with heating/cooling running full force and that window open. >:(

Thanks

Fike

You could do it like a MAF meter in a car - a filament is heated to a costant temperature ( detected by it's resistance) , and the current required to maintain it is proportional to the mass air flow...

you need a reference ambient temperature, of course..

regards

Allan

I would think a "smart thermostat" would know that the window was left open...

I personally don't like the idea of monitoring the blower power alone...
It tells you the blower supply is there, but not that anything is happening (dead motor, blocked duct etc).

As for the original problem, why not a simple lightweight vane mounted on a micro switch that deflects when wind is blowing. Debounce it for a few seconds, then you have the simplest, one moving part sensor.

The MAF sensor is neat too, but requires a power supply and calibration.

allanhurst:
You could do it like a MAF meter in a car - a filament is heated to a costant temperature ( detected by it’s resistance) , and the current required to maintain it is proportional to the mass air flow…

Allan

It is possible to make on of those quite cheaply if you have the knowledge/skills.
Would be my choice.

[/quote]

jremington:
I would think a “smart thermostat” would know that the window was left open…

They are too smart by half.
Useful if you happen to be smart yourself and able to programme it.
These things are supposed to save energy but in use by most people i suspect not.
The override button gets used frequently.

I have seen sensors that consist of a lever operated micro switch with a piece of thin aluminum attached to the lever. When the air flows, it pushes the aluminum vane which trips the micro switch. You could put in front of the vent or remove the vent cover and place inside the duct.

Paul

Maybe use a good old fashioned proof of airflow switch?

lastchancename:
As for the original problem, why not a simple lightweight vane mounted on a micro switch that deflects when wind is blowing.

I like that idea.
Possibly a weathervane mounted off horizontal so that when no wind blows the mass makes it turn to the no blow position.

http://www.maplin.co.uk/p/maplin-replacement-wind-direction-sensor-for-n96fyn96gy-n81nf?cmpid=ppc&gclid=CjwKEAiA1ITCBRDO-oLA-q_n8xYSJADjBQfGxzZkrzpPnEgdt879oXpGCmDcLIsKeqNLQ_eVpj4Y3RoChnPw_wcB

Not too dear.

As an HVAC controls guy - there are a couple options. Monitoring power to the blower proves only power to the blower - we don't use that for proof of air flow. We generally use one of 3 things: 1 (least often) sail switch. A little outdate, but a it's a litterally microswitch with a sail that actuates the lever. 2. More often - pressure switches - these have little rubber tubes and a diapghram and a spring - the best place to put this would be across your air filter becasue that's where the greatest pressure differential is and also can tell you when your air filter is due to be changed. Aside from your locatio requirement I'd call this your best choice by far. One side "pressurizes" one side of the switch and the other tube relatively "suck" the other side. 3. The last choice is really for measuring but might be the only one for use at an individual register - it's a tee-shaped pickup in the airstream and a sensitive/amplified device picks up the pressure differential of pickups facing into and away from the flow.no flow = no differential). Great for measuring actual feet per minute of flow but way overkill for "is it running." I'd suggest sailswitch for simplicity, but I honestly don't think there is enough velocity at a standard duct to actuate it. Reconsider the location of at least the switch and use a pressure switch at the air filter for the best bang for your buck/time/effort.

Flex sensor mounted in the air steam.

Stick it to a "post it note" for extra sensitivity.

mikb55:
Flex sensor mounted in the air steam.
Flex Sensor 4.5" - SEN-08606 - SparkFun Electronics
Stick it to a "post it note" for extra sensitivity.

hmmm, that's might be a good idea.. I can may be use it as part of a "Sail"

ElEscalador:
As an HVAC controls guy - there are a couple options. Monitoring power to the blower proves only power to the blower - we don't use that for proof of air flow. We generally use one of 3 things: 1 (least often) sail switch. A little outdate, but a it's a litterally microswitch with a sail that actuates the lever. 2. More often - pressure switches - these have little rubber tubes and a diapghram and a spring - the best place to put this would be across your air filter becasue that's where the greatest pressure differential is and also can tell you when your air filter is due to be changed. Aside from your locatio requirement I'd call this your best choice by far. One side "pressurizes" one side of the switch and the other tube relatively "suck" the other side. 3. The last choice is really for measuring but might be the only one for use at an individual register - it's a tee-shaped pickup in the airstream and a sensitive/amplified device picks up the pressure differential of pickups facing into and away from the flow.no flow = no differential). Great for measuring actual feet per minute of flow but way overkill for "is it running." I'd suggest sailswitch for simplicity, but I honestly don't think there is enough velocity at a standard duct to actuate it. Reconsider the location of at least the switch and use a pressure switch at the air filter for the best bang for your buck/time/effort.

Ok, thx. I 'll do a little experimentation with micro switches and pressure switches.

I'm standing by my advice to use industry standard for the BEST choice, BUT upon further pondering:

If I REALLY wanted a microcontroller to determine airflow on/off at a single register, I might toy with putting a pinwheel or very small, light, well bearing'd/balanced propeller blade in the airflow and have an optical sensor count if the thing is turning or not. IR emitter/receiver pair...white reflective thing...or the propeller just breaks the beam when passing by. I'd probably get fired for trying something that wonky on the job but playing around the house I might.