Saving energy by making a dumb fan smarter

Description

I wanted to cool my house without A/C - better for both the environment and my wallet. Similar to a whole house fan, I open windows around my house and use this fan to push air out which creates negative pressure, sucking cool air in through the other windows.

Here's what I used to get the job done:
- Home Assistant with the Node Red add-on
- WiFi smart plug running Tasmota
- Climacell free API key for weather info
- Local temperature sensor
- Window monitor
- AirVisual free API key for AQI info
- Telegram bot for notifications
- Home Assistant thermostat

The basic logic flows as follows:
- If the window is open and Climacell reports a lower temperature outside than inside, turn on the fan.
- If the temperature inside the house is 0.5 degrees F < the thermostat set-point, turn off the fan; > 0.5 F above the set-point, turn on the fan.
- If the outside AQI >= 100, turn off the fan and notify me to close the window.
- If window is closed, turn off the fa

Details

Living in a house with no central air conditioning can be difficult during a sunny San Jose, California summer, and running portable AC units with no solar is not kind to one's wallet.

Luckily for me, the outside air temperature at night typically drops to the low 60s (Fahrenheit) – and it's free.

A couple rooms in my house happen to use 20"x20" windows which is the same size as a standard three-speed box fan. I placed it in an upstairs window, and used it to push air out of the house. When it was on I opened other windows which, due to the negative pressure created by the exodus of hot air, sucked the cool outside air in.

After doing this manually, I decided I'd smarten up and in turn smarten up my cooling system. This took place over time.

The first step was adding a smart plug and connecting the fan to Home Assistant. From there, I realized I often wanted the fan to start working when I wasn't home, so I needed a way to monitor temperature so the fan could turn on at the exact right time. I connected to Climacell via API to get the outside air temperature (OAT), and to a local temp sensor for the comparison.

Once the window was open, the trigger for the fan to turn on was OAT < interior temperature. For a while, I had a hardcoded cutoff at 66 degrees so my house wouldn't get too cold. Once the fan turned off, it wouldn't turn back on until dawn. This was the logic for two summers.

--- Improvements over time ---

- When California had horrible fires, I added an AQI integration from AirVisual which, when the AQI >=100 not only turns on my air purifier, but also turns off the fan and notifies me via Telegram to close the window.

- Last year I had a lightbulb moment and swapped my hardcoded 66 degree shut-off trigger for a Home Assistant generic thermostat integration. Now when the fan is running and the temp drops below the set-point by 0.5 degrees the fan turns off. It turns itself back on when the temp hits 0.5 degrees above the set-point. This resulted in a perfectly cooled house when I got up every morning rather than playing a guessing game.

--- What could still be done? ---

- I could make this system faster by adding a second box fan to the other 20"x20" window in the room, but I've been perfectly happy with its speed so I haven't felt the need to do this.

Discussions

Rey wrote 04/11/2024 at 07:12

I do this at home as well, but unfortunately it's manual. I have an ESP32 connected to two ds18b temperature sensors, that collects environmental data (displayed on a simple mobile app). When temperature drops lower than 1c outside compared to inside, I open the window and turn the fan on. Would love to connect it to something smarter and control the window with a servo or stepper.

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benblb wrote 04/11/2024 at 18:47

I also have to open my window manually and have considered the same. Have you considered enabling a notification when the temps are within your rules so you don't have to manually check the screen? Easily doable with Home Assistant!

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Rey wrote 04/12/2024 at 15:52

I'm not using Home Assistant, but yes that's a good idea. As with all things: when I have the time ;)

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Yuri wrote 04/10/2024 at 19:44

I had an idea like this as well, people don't realise how much it cools down the space inside the home. I call it a "controlled draft". I used 12v universal car radiator fans because they are shallow, replaced the glass in the window with the plastic cut-out for the fans and also put a weather louvre on the outside to prevent rain from getting in when not in use. It works magic and I can direct the draft by opening the window in different rooms as desired.

Are you sure? yes | no

benblb wrote 04/11/2024 at 18:46

That sounds perfect! It was pretty warm here yesterday and my AC isn't up yet, so I slept with the window open and fan on. The room started at 73F and I woke up with the room at 61F. Very effective (albeit not as fast as AC).

Are you sure? yes | no

Saving energy by making a dumb fan smarter

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Saving energy by making a dumb fan smarter

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