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How can I get automatic notifications that the outside temperature (in Sydney CBD) is below a certain threshold, and therefore the best option is to turn off the air conditioning and instead open the windows?

The automatic notification should ideally be via Slack, but email is also fine. Nothing that requires a mobile phone, such as SMS alerts or a smartphone app. The service should be either free or very low cost.

Buying air conditioners that are intelligent about the external temperature is presumably impractical, as the air conditioners have already been purchased, as would changing the architecture of the building. (And please don't suggest we use fans - we're currently using a mixture of fans and aircon)

On r/frugal, a similar question was asked. The only worthwhile solution was using ifttt.com, but without any mention of specific "if" or "do" recipes.

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  • With a strong level of technical ability, or some persistence, one could rig up a Raspberry Pi to do it... would cost a couple of hundred dollars for the parts I'd guess. – Highly Irregular Dec 14 '15 at 9:04
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    Why is a smarthphone app out of the question? – THelper Dec 14 '15 at 9:23
  • The question appears to be asking for something web based and to rule out a thermometer on the outside of your house. If this is not intended, maybe adjust the wording. – mart Dec 14 '15 at 10:16
  • How about running a script on your home computer (or something like a raspberry pi) that polls a weather forecasting service that offers an API (for example, wunderground), then you can extract current conditions for temperature humidity as well as the day's forecast to decide if you want to open the window or not - use Slack's API to send notifications. – Johnny Dec 31 '15 at 5:52
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This doesn't use a notifiction service, it just replaces a dumb system with a slightly smarter one.

Ebay has lots of 20 buck gadgets that have a remote sensor and have a relay to control 5 to 15 A of current. Using one of these, you can keep the air conditioner off until outside temps exceed a certain value.

The 'open windows' criteria is much harder. Automatic window openers don't come cheap. If I had to hack one together, I'd start by ripping apart a car door electric window for the actuators.

One approach may be to get/make louvered openings. You can get fans with them already for greenhouses and barns. The louvers are closed when the fan is not operating, and are forced open by the wind of the fan. A pair of louvers, one set for in and one set for out, with a fan could automate the venting.

With a differential thermostat (http://www.mydtcstore.com/index.html) you can do things like, "run the fan as long as it's cooler outside than it is inside"

These start at US $70.

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Be aware it is not temperature only. If you fill the house up with water saturated air then you use electricity to condense the water out of the air when you turn the AC back on. Nominally you want air cooler than the thermostat and with a dew point of less than 55 Fahrenheit. If the air is 70 and the humidity is 62 it is not worth opening the windows for a short term. In a humid climate you are more driven by dew point and can get from the forecast. The dew point does not change rapidly. I live in a humid climate and there are actually very few days of the year that it is worth opening the windows at night during AC season and those days come in sets. It would be an absolute edge case that you open up the windows for a few hours and close. It is not something that needs to be micro optimized.

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