Compare Indoor/Outdoor Sensors to Activate Attic Fan?

I would like to find an existing app that compares outside temperature/humidity to tha attic’s temperature/humidity and turn on a fan when outside conditions are favorable. Does anyone know how I might do this? Thanks all -b

WebCoRE is an option you can look into…

That’s very helpful, thanks. It looks powerful. But I’m hoping there’s something already available…something consumer-ready that doesn’t require coding…?

Thanks again - I appreciate the help!

Just talking out loud…

There’s some weather apps that let you check outside temp (weather tile). If you put a temp/humidity sensor in your attic…

You’d have to set what “favorable conditions” are, but I don’t see why you couldn’t say “if 100 degrees outside AND 80 degrees in attic THEN turn on attic fan”?

Is that what you’re trying to accomplish? It wouldn’t require any coding outside of setting up the automation within the ST app.

Attic Fan Smart Ventilation from @RBoy. It is part of very useful collection of paid apps, but they are all worthy.


Check out this app:

Maybe so. But to your point, i’d have to define “favorable” and that varies month-by-month, even day-by-day.

Many thanks for all ideas! I appreciate everyone’s coaching!!

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Can it compare indoor and outdoor temperatures to each other? For example - when the outdoor air is 10 degrees cooler than the indoor air, turn the fan on. That’s ideally what i need. To compare indoor/outdoor temperature and humidity. If we have cooler and/or drier air outside, i want to run the attic fan full blast.

Many thanks!

What about winter when is -10F, would you cool your attic that is above 60F?

I monitored my attic fan usage compared to the runtimes of a/c and what I discovered is that it cost more to use the attic fan because it pulls the cool air from the conditioned space into the attic.

WebCoRE was very useful for this.

I only turn it on now when I have to go up there just to cool the space because it’s HOT up there in the spring, summer and fall!

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We are way down south so if we are at -10, the world has ended LOL…!

But i get your point. In general, I’m trying to reduce moisture. Temperature also, but that’s not as big a problem.

If i could just set a test to identify when i have dry air outside - say 10% lower than in the attic - that would be perfect.

If we could add other criteria, that would be nice, but…just nice-to-have.

@rontalley…do you have any soffit or ridge vents in your attic that would allow the attic to be ventilated? Most of my experience has been in the south where most of the attics are ventilated by inducing outside air thru some type of vents and out thru the attic fan. This keeps the attic temperature within reason. It will never be lower than outside but it keeps it from getting really hot. You might want to check to see if any of your attic vents are covered up. This would reduce your AC costs.

We do. We have a lot of soffit vents all around. So if we could generate a draw up high at the ridge through a gable fan, it appears we should be able to get a lot better airflow from outside.

Yeah, it’s pretty well vented. Even has the foam “thingies” that keep that blow-in insulation off of the soffit vents. Also has the appropriate ridge vents in the hips and gables. I gave consideration in getting the rafters insulated with the spray insulation stuff but the ROI wasn’t worth it.

Never-the-less, the attic fan definitely made the attic much cooler but made the a/c run longer.

Screenshot of Upstairs System in 2019 showing runtimes. (Don’t know why the set point and outside temps are not showing…

Screenshot of Upstairs System in 2020 showing runtimes. (System no longer constantly runs since I stopped using the Attic Fan Automation…

Previous years, I was in the 50% range and last month I was in the top 1%!

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@rontalley - when you ran the attic fan, did you run it 24x7 in the hot months only?

No, I had an Iris contact sensor tied to a string hanging from the rafter and an Iris SmartPlug. I set the thermostat, that’s connected to the attic fan, to it’s lowest setting.

At first, it was simply turning on if the attic got too hot and off when it cooled down in the attic and outside temp dropped. However, this resulting in my upstairs a/c pretty much running all day.

I then used several rules based on outside temperature vs attic temperature vs time of day vs inside temp vs presence vs motion vs contacts, etc. to turn on/off the attic fan.

My attic fan is fairly loud so you can tell when it’s running so I wanted to maximize the time it was on to pull heat out of the attic and also to minimize the time on if an upstairs room was occupied.

The ladder method reduced the amount of time the a/c ran but by the fan turning on/off so much, the amount of kw/h the fan used actually increased…

Now, it just stays off unless I have to go up there for an extended amount of time.

I discovered this BEFORE I started researching attic fans and why they actually cost you more.

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You can use the weather app to get outside humidity and a humidity sensor, Zooz 4-in-1 works great or the Xiaomi temp/humidity sensors (hard to pair), to compare the two. Again, this is a very simple task using webCoRE.

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