I cannot see there being any issues using HE in terms of device count and the ability to run any complex logic required from the data recorded.
It will come down to finding devices that have the reliability and accuracy you are looking for and also being able to be used by HE. Devices using Zwave and Zigbee wireless protocols have a good chance of being supported, but even Ethernet based device could potentially be used. You can also go one step further and use devices connected to ESP8266 boards and then in turn connect the ESP board to HE. Have you got any devices in mind yet?
It’s always possible someone could take you up on this offer, but I think you’ll find many people are more than happy to help for free if you share some additional details .
How do you currently monitor and control the greenhouse?
A first issue you will have is finding temperature sensors of that accuracy. Most of the Z-wave and Zigbee sensors are far less accurate (e.g., the new Aeotech sensor: aërQ Sensor - Aeotec) is only 0.4 degrees accuracy. The 0.1 would also be a battery life problem (more accuracy means more reports).
If you can live with 1 degree accuracy, consider something like the Aeotech MultiSensor 7. While you probably don't need the motion sensing, it also does temperature, humidity, light, and UV light - 4 things that probably have some relevance to a greenhouse. Also, you can set it up to be USB powered so you wouldn't have to worry about changing batteries!
Where are you located? I am guessing from the time of your post, that you are not in North America. Are you in the UK? This matters for device recommendation and availability.
How do you plan to open/close the windows? Do you have actuators in place? What kind of windows are they?
Similarly, what kind of fans are they? And how are they currently controlled?
And note that these switches are generally for home "ceiling fan" type fans -- the kind that are sold at home depo and the like that are more decorative. Specifically, they are for fans with AC motors that are "capacitive start" fans. If you are talking about more of an exhaust type fan for venting the greenhouse to the outside, then a simple relay switch is likely what you need.
I get the impression they want to control fan speed (otherwise why mention they have variable speed fans). Without knowing what country they're in, it is impossible to recommend any product.
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