Comparing Outside and Inside Humidity

Hi. I’m trying to automate my attic fan to turn on if the outdoor humidity is 5% lower than my attic humidity. I probably want to do the same with temperature. Once the humidity or temperatures indoor and outdoor are close (within 1%?) then I want to turn off the fan. It seems like the theory behind attic fans is to pull in the outside air into the attic so I only want to do this when the outdoor humidity or temperature are lower than in the attic.

In the stock Smartthings automation manager, I can set a rule against the outdoor humidity (or temperature) and another rule against the sensor in my attic, but I haven’t found a way to compare the two.

Thanks for any help!

With Routines, I think the best you could do would be “If attic_humidity > 50% and outdoor_humidity < 45% Then fan:on”, but that would only cover those specific values. As you’ve seen, there’s no option to compare two device values. Maybe that will be a future enhancement.

I think Rules could handle this, but to set those up you have to write them in a json and then send that to the API. If that’s in your skillset, then it should be future proof and run local (if the devices involved are also running local).

The other option is WebCoRE, which could definitely handle this, and you’d get plenty of support if you have any trouble setting up your piston. However, WebCoRE relies on the Groovy platform, which will be shut down at some point in the not-too-distant future. So going that route will work, but would be a temporary solution.

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Thanks for your reply. I followed your advice and was able to put together a Webcore piston that did what I was looking to do. It seems to work well. I’m now a little bummed that it will be shutdown at some point. Seems to be a really nice dev tool.