I bought some Venstar T7900 thermostats after moving into a new home. My main use-case was to be wifi connected without the cloud, by use of the local API and Home Assistant integration. They work exactly as they should, but shortly after the install I realized the location of the thermostats was terrible and I don’t really have an easy way to relocate. I ended up buying a bunch of Xiaomi LYWSD03MMC temperature and humidity sensors and replaced their firmware with the custom PVVX firmware. They check-in quite frequently and provide a real-time reading (every 15-30 seconds) that I average into a single sensor in each zone.
In comes my conundrum… Ideally, I would like to replace the temperature reading from the thermostat itself with that of the respective zone average. The Venstar does have the capability of utilizing external remote sensors that are hardwired to the thermostat, and the ability to only use the temperature from the remote sensor for driving cooling and heating. Via the thermostat API, my options are basic and I don’t have the ability to “calibrate” or set the sensor to a different temperature value. I can manually change the value within the touch UI, but this doesn’t give me a way to automate.
I would like to “simulate” a temperature readout using the zone averages. Perhaps with an 8266 or something small that I can wire in behind the thermostat. The official probe appears to be a 10K sensor (Venstar 10K Remote Sensor (ACC-TSEN)). If I can figure out how to simulate the reading, I will likely build some automation with NodeRED.
Curious if anyone has done something similar? It may be worth mentioning that I could also be overcomplicating this or not thinking through all of my options. I would like to avoid replacing the thermostats if at all possible.
Thanks again if you read this far! I appreciate any feedback you may have.
After a bit more research, it looks like I might need to simulate a 10K NTC thermistor curve. In the link provided, they show resistance values at 10F intervals from -20F to 120F. Any idea how to simulate this from a microcontroller? Will this even work as I expect?
I also just bought this thermostat from the rave reviews on this forum. After install I find the functionality a little lacking.
I’m looking to simulate sensors to the thermostat from the various sensors in HA. I think I can do this by making http calls to the thermostat pretending to be a sensor itself. Would this work for you?
Unfortunately, the API provides no ability to switch HVAC systems on/off. It works just like any other thermostat, cloud or local, which don’t provide this ability either. This core issue of simulating the sensor value is a bit more complicated as the thermostat is made to work with their existing thermistor from my original post. I’m still in the process of figuring things out, but essentially I’ll be using an ESP32 with a digital potentiometer to help simulate the necessary resistance values that are documented in the data sheet.
If you plan to go down this route, just make sure to get a digital potentiometer that supports the temperature range you need, with enough steps to support the resistance variations from degree to degree.
Have you checked Ohmigo.io ? They seem to have a device that generates any type of RTD temperature sensor, including Pt100,Pt1000, NTC etc and its now available with Home Assistant integration. Some have used it also directly with its integrated Web GUI. In my application I simulate temperature to reduce electricity consumption of my Heatpump during hours when Nordpool spotprices are high.