I have added some features to my board:
1. 24V power supply on MT3608 feeded by 5V from main board.
2. LEDs on a separate board that can be seen through existen holes on the enclosure
3. Outside Temperature Sensor connects over standart 3.5 mm audio jack,
that already exist on OPI's extension board.
4. Onboard PIC programming by using LEDs board connector.
5. Spring connector to connect boiler and thermostat wires instead 2XUSB.
Schematic of the boiler and thermostat interfaces and PIC firmware has no any changes.
I have bought almost all components in regular electonic components store, at the rest - on the aliexpress.
PCB manufacturing - http://www.allpcb.com
CH340G - USB to UART (https://ru.aliexpress.com/item/CH340G-S ... 0.0.rifIiP)
PJ327F 3.5 mm audio connector (https://ru.aliexpress.com/item/1x-New-P ... Title=true)
MT3608 (https://ru.aliexpress.com/item/in-stock ... 0.0.rifIiP)
PIC16F88 (https://ru.aliexpress.com/item/PIC16F88 ... 0.0.rifIiP)
LEDs (https://ru.aliexpress.com/item/100pcs-l ... 0.0.rifIiP)
12 MHz resonator (https://ru.aliexpress.com/item/2pcs-HC- ... 0.0.rifIiP)
SMD Tact Switch (https://ru.aliexpress.com/item/10Pcs-SM ... Title=true)
I am using it with Baxi Main Four 240F and Honeuwell Round Modulation.
I am glad to answer any question.

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OS images:
armbian root/orangepi dhcp otmonitor port: 8080
https://yadi.sk/d/OEDc_tvR3P9WzY
openwrt (pulpstone) root/root dhcp otmonitor port: 8080
https://yadi.sk/d/C_J2y1pv3P9XPh