Hi Forum,
Does anyone know of a smart app for smartthings that measures the difference between two temperature sensors (inside and outside) and will allow you to turn a switch on if the inside temperature is greater than a set point?
Basically, if its cold outside and warm inside and the temperature inside is above a set-point, turn on the fan.
Thanks,
Neal.
“Whole House Fan” is intended for exactly that use case.
One) open the SmartThings mobile app
Two) tap the “marketplace” icon at the bottom right of the screen
Three) tap “smart apps” at the top right of the screen
Four) select “climate control”
Five) scroll down until you see the whole house fan smart app. Tap on it to start its set up wizard.
If that doesn’t meet your specific needs, I’m sure core could do it. It’s just much more complicated to set up.
OK thanks JDRoberts, the whole house fan is not quite right as I don’t think it lets you enter a temperature differential (i.e. if inside - outside <-10F and inside >70 then turn on. I will have a look into this CoRE.
OK so I have the CoRE APP installed on my phone via Github and Smartthing IDE but when I try and create a variable I get the error “You are not authorized to perform the requested operation”. Also the pistons I am trying to create disappear one I leave the CoRE SmartApp. Any ideas?
CoRE is quite complex… It has its own peer assistance thread where the CoRE experts hang out. Ask your question there and they’ll be happy to help. 
Have you ever solved this problem? I don’t have any experience with ST or CoRE, but I need something similar to what you described: While (TEMP_SENSOR_ATTIC > Outside_Temperature_Retrieved_From_Internet + 5 degrees) keep switch ON, otherwise, turn it OFF.
This is a very old thread but it comes up at the top of Google results.
As you noted, this is a VERY old thread, and a lot has changed in that time.
I’m pretty sure there’s a way to do this with the new architecture, but I’ll leave it to the experts to discuss.
Tagging @Automated_House just in case he knows.
Otherwise hopefully another expert will chime in.
I just tried to use this tutorial webCoRE for SmartThings - A-Z Installation walkthrough - YouTube because I understand that to set up a rule like what I need I need something called webCoRE. I install webCoRE on SmartThings. Add a little smartness to your things. from GitHub. But once I move to the mobile app, I don’t see any options to add webCoRE under Automations. Also, I briefly saw mentions of webCoRE not being available for ST anymore. Also, under the video mentioned above it mentions “In fact, don’t waste your time with SmartThings, switch to Hubitat instead”. But I don’t want to replace my hub just because of this one tiny rule that I need. I would like to know if it is possible to set up a rule in ST today.
SmartThings is in the middle of a huge transition from one architecture to another. The Samsung hosted free groovy cloud will be going away, which means anything that uses that will no longer work. And that includes all the custom smartapps like Webcore. 
The replacements for device type handlers will be “edge drivers“ written in Lua that run locally on your own hub.
And the replacements for smartapps, including Webcore, will be routines in the smartthings app (which have a lot more features than they used to), And the “rules API” which is a whole forum category in itself.
But with the exception of the new features in the routines in the app, everything else is still in beta and still undergoing changes.
So, yes, very few people are creating new stuff in Webcore for smartthings now. But you should be able to do the same thing another way that works with the new architecture, I just don’t know the details of that.
So to sum up, as of today there is no way to write a routine that I need? In that case, I guess I will be switching to Hubitat.
I’m not saying that at all.
I’m saying I think there probably is, I just personally don’t know how. So I hope that others who know more in this area will be able to help you.
Not entirely sure i get what you need but does this help ?
Meh, seems to address the first post more
Maybe 2 automations like this suit better
Or is that
If Device in attic and external temp has a + 5 degrees difference then keep a switch on
If different switch off
Perhaps approach it differently, if the attic gets warm/cold just switch on/off a device at a given temp regardless of ext temp
Trying to get my head round what you are trying to achieve, describe whats actually needed
Perhaps the attic gets too cold so a heater needs leaving on but if it warms up you dont need the heater on, examples like that make it easier to visualise the automation required
Its easy to overthink things sometimes
Under Automations, click on Add Routine then tap the Discover tab near the top of the screen and scroll down to find the webcore smartapp to set it up.
As mentioned above, we don’t know how long it will be around to use. Check out Rules API. 
Another option to check out… SharpTools rules engine.
Not OP but discovered this post looking for the same thing. My use case is about keeping my house cool. I want to know if the temp outside is warmer or cooler than inside to know if I should open my windows to cool the house down.
It’s currently 30C outside and 24C inside so I have all the windows and blinds shut. I have zero idea what temp I want the outside to be at for opening the windows to be a good idea this afternoon/evening I just know it should be cooler so setting a rule based on set temperature is challenging.
Hi @CoopUK
If you want to turn on a switch with a differential of 2°c with respect to a maximum internal temperature, for example 25°c, then you would make an automation like this:
IF:
THEN
You can also make another one to lower the inside temperature as long as the outside temperature is below 23°c or 22°c, at your choice.
IF
THEN
You have to do another one to turn off the switch when the outside temperature is >= 25°c
IF
THEN
You can limit all the automations between dates if you install the virtual Calendar Mc driver, for example between July 1 and August 15.
Comparing the outside temperature, either the TWC one for your Location or a sensor reading, with an indoor temperature from a sensor reading is something the Rules API can do. However, as far as I am aware, what it can’t currently do is determine the temperature difference between the two, or offset one or both the values. So you can know if it is colder outside, just not how much colder. Obviously you could rig an internal sensor to read artificially low if a fixed offset sufficed.
I wrote high first time around, I think I mean low.
