@Puneit-Thukral You're mixing up the sensor node and the Vaillant heat control system. The minimum input voltage is 5V for the heat control system (only). This has nothing to do with the used MCU, but the 7-8-9 Vaillant interface usually works with 24V. So usually the input voltage is 24V anyway.
The interface for the Vaillant heat system is usually as follows:
24V input => roughly 11V to 17V output on the CTL pin to regulate the boiler temperature
If you used 5V on the input you'd only be able to regulate up to max. 5V.
The Vaillant heat control system (it's the second picture, the one with the box) does not run from batteries. It gets power directly from the Vaillant (or compatible) heating system.
The single AA battery temperature/humidity node runs from a single 0.8V-1.6V battery. You can see the battery in the picture - in this case it's an Eneloop although I'd recommend Alkaline batteries for devices that run for years. To run from a single battery the node has a boost circuit integrated that boosts the voltage to 1.85V.
On all devices (sensor nodes, heating control, gateway) I'm using a variant of ARM Cortex M0+ or Cortex M0. (Eventually it will all be Cortex M0+). This 32 Bit architecture is superior to Atmega328P (the typical MCU used in Arduinos) in every respect (be it power efficiency, pure performance, sleep current with wakeup timer, price to performance/flash size ratio, peripherals quality and quantity, etc etc.). Just 1 example: the used ADC is 12 Bit with hardware oversampling support (vs 10 Bit without oversampling on Atmega328)
@monte I'll sum it up:
Android App - might be open source in the future
Gateway - not open source, but open
Server Code (cloud) - no
Sensor Node - no
Heat Control - no
In the future the Android App might become open source. Anybody can build a UI, because the data needed for that is available from the gateway.
I wish I'd use my own design for the gateway but to finance that, I'd have to sell thousands of these devices because it's a very time intensive task requiring specialists. Currently the gateway is running on an Orange Pi Zero board with a custom hat. This is why it's the most expensive part. The communication Orange Pi <-> Hat is via USART.