Controlling Dumb Humidifier Using Forecasted Outdoor Temp and Current Indoor Humidity

Why Hubitat Elevation Here's a cool thing I did!
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This is something I've always wanted to get control of. In the past, I've had to constantly "tweak" my humidifier's "humidistat" during the winter. Down when its colder, up when it's warmer. Not anymore! Here's how I did it, including the note I typed in the rule for my future reference. I'm sure there are other (probably simpler and cleaner) ways to build a rule like this, but this one seems to be working for me.

-----Begin Rule Notes Here-----
Controls condensation on windows by using forecast data to adjust indoor humidity. Uses mathematical relationship between outdoor temperature and indoor humidity as developed by @jmaxxz in his app "Ideal Indoor (Winter) Humidity Calculator." Only runs if outdoor temperatures are expected to be below 50 degrees. Requires a humidity sensor, weather forecast data (OpenWeatherMap), and a humidifier controlled by a smart plug.

Step 0: Create and assign variables as necessary.
Step 1: Pull forecast data from OpenWeatherMap and determine lowest temperature in the next 48-72 hours.
Step 2: Do the math. The form of the relationship is Ax^2 + Bx + C, where A, B, and C are constants as determined by @jmaxxz. Since variable math is limited to a single operation per line, variables X, Y, Z, and U are established as intermediate steps in calculating V, the target indoor humidity.
Step 3: Target humidity is compared to the established minimum and maximum allowable values and the global variable Humidity-SP is set accordingly.
Step 4: The humidity setpoint is compared to the actual humidity. If the value is outside of the setpoint +/- the set deadband, the humidifer device is turned on or off as needed. If within the deadband, no action is taken.

Notes: The hum-offset has a direct effect on the target humidity calculated (variable V). Example, setting a the value of hum-offset to a value of 5 will increase (V) by an equivalent amount. Negative values will decrease (V). This can be used to raise or lower the humidity based on comfort, observed condensation, etc. Note that Humidity-SP will still be limited to values between hum-min and hum-max.

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You might want to turn off the humidifier plug when you exit the rule to temp being above 50. Just in case the plug is on when the forcast changes. Probably low risk but...

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Thanks for the tip... just added that to the rule.

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Just an update to confirm that this has been working as expected. Temperature here was somewhere around -20 degrees F. And no condensation on my windows!

The calculated humidity setpoint is something like 11% which is less than the minimum of 15%, so the setpoint is 15%. However, my humidity sensor is reporting 22% (too much cooking, showering, etc.) so the humidifier has basically been off. I may adjust my offset up 5% and see how things react. Supposed to be fairly cold for the next several nights.

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I used your rule for as a guide but did not want to pull forecast temperature from an outside source. I live in a coastal area with moderate winter temperatures but very high humidity (this winter has been downright soggy). So I divided the expected low temperatures into 3 tiers and assigned the related RH for each as suggested on the Aprilaire website:
Outside Temp / Recommended Humidity 40°=45% | 30°=40% | 20°=35%

I set up "Forecast Low Temp" as a global variable and also a dashboard tile where I can enter a new temp when I see a temperature shift coming. The rule seems to be working as expected but the reality so far this winter is that the dehumidifier needed to run non-stop.

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hello is possible to calculate relative humidity, from my two aqara sensor g/m3 I would like to have app that run my relay for fan in my basement and if is possible a tile (g/m3 and DEW point ) for dashboard.
When humidity is outside lower then inside. Like in aplication on link.

i find this link: Dew point calculator but is in Fahrenheit :confused:

i hop you can help me. Thanks