Components

Here's what I used, but anything that works is obviously fine!

Component	Link
Navilink Cable	https://www.tanklessparts.com/product/navien-gxxx001659-navilink-t-extra-channel-cable
RS485/CAN Hat	https://www.amazon.com/dp/B07DNPFMRW
RJ45 Breakout	https://www.amazon.com/dp/B0CJHQNHF2

Wiring

You want to wire the Blue wire to A, and the Blue/White wire to B. If you're looking at your RJ45 with the latch facing down, these correspond to the 5th and 4th wires from the left respectively. You do not need to worry about using the other two wires for power.

Protocol

Some very smart folks have already reverse engineered the majority of the protocol. Here's some useful links from which I derived my own parsing:

1. https://community.home-assistant.io/t/navien-esp32-navilink-interface/720567/20
2. https://github.com/htumanyan/navien
3. https://github.com/dacarson/NavienManager
4. https://github.com/evanjarrett/ESPHome-Navien

Header

Byte Start	Length	Description	Values
0	2	packet start marker	always 0x05F7
2	1	direction	0x0F = control 0x50 = status
3	1	type	0x0F = gas 0x50 = water
4	1	unknown	always 0x90
5	1	length

Gas

NOTE: multi-byte numbers are in little-endian

Byte Start	Length	Description	Values
6	1	status type	always 0x45
8	1	unknown	always 0x0B
9	1	unknown	always 0x01
10	2	controller version	not convinced... byte 10 is the F/W version (0x0C = 1.2) but what is byte 11
12	2	panel version	not convinced... byte 12 is the F/W version (0x19 = 2.5) but what is byte 13
14	1	set temperature	C value in increments of 0.5
15	1	water outlet temperature	C value in increments of 0.5
16	1	water inlet temperature	C value in increments of 0.5
17	1	unknown	always 0x00
18	1	unknown	always 0x00
19	2	target burner power	kcal
21	1	unknown	always 0x01
22	2	current burner power	kcal
24	4	total gas usage	m3 in increments of 0.1
28	2	elapsed time since install	days
30	2	usage counter	increments of 10
32	4	total water usage	L in increments of 0.11
36	4	operation time	hours
40	1	unknown	always 0x00
41	1	unknown	always 0x00
42	1	unknown	always 0xAA
43	1	unknown	always 0x48
44	1	unknown	always 0x00
45	1	unknown	always 0x00
46	1	recirculation enabled	0x00 = no 0x01 = yes
47	1	unknown	always 0x00
48	1	CRC	2

Water

NOTE: multi-byte numbers are in little-endian

Byte Start	Length	Description	Values
6	1	status type	always 0x42
7	1	unknown	always 0x00
8	1	flow detected	0x08 = recirculating 0x20 = demand
9	1	system power	0x05 = on 0x25 = recirculation active
10	1	system stage	0x1- = idle   0x14 = stand-by 0x2- = start up   0x20 = demand   0x29 = pre-purge   0x2B = ignition   0x2C = flame on   0x2D = ramp-up 0x3- = active   0x33 = active combustion   0x34 = water adjustment valve operation   0x3C = flame off 0x4- = shut down   0x46 = post-purge 1/2 (15s)   0x47 = post-purge 2/2 (15s)   0x49 = dhw-wait (150s) 3
11	1	set temperature	C value in increments of 0.5
12	1	heat exchanger outlet temperature	C value in increments of 0.5
13	1	heat exchanger inlet temperature	C value in increments of 0.5
14	1	unknown	always 0x00
15	1	unknown	always 0x00
16	1	unknown	always 0x00
17	1	capacity	percentage
18	1	flow rate	lpm in increments of 0.1
19	1	unknown	always 0x00
20	1	unknown	always 0xA0
21	1	unknown	always 0xBE
22	1	unknown	always 0x00
23	1	unknown	always 0x20
24	1	system configuration	0x01 = internal recirculation 0x02 = external recirculation 0x08 = metric
25	1	unknown	always 0x00
26	1	unknown	always 0x00
27	1	system active	0x00 = no 0x01 = yes
28	2	operation time	hours
29	1	unknown	always 0x00
30	1	unknown	always 0x01
31	1	unknown	always 0x00
32	1	unknown	always 0x00
33	1	unknown	always 0x02 (even when recirculation is off)
34	1	unknown	always 0x00
35	1	unknown	always 0x00
36	1	unknown	always 0x00
37	1	unknown	always 0x00
38	1	unknown	always 0x00
39	1	unknown	always 0x00
40	1	CRC	2

Python Script

Setup the environment

1. python3 -m venv venv
2. source venv/bin/activate
3. pip install aiomqtt
4. pip install gpiozero
5. pip install lgpio
6. pip install pyserial

Running

1. source venv/bin/python3 npe240a2.py

FootNotes

Footnotes

1. This only updates when the operation time updates. ↩

2. The CRC was reverse engineered by @htumanyan - you can find an example implementation here. ↩ ↩²

3. To attempt to get names for the stages I used AI on a maintenance manual. They're most likely wrong but seem "good" enough. The post-purge and DHW-Wait are correct because they can be confirmed by timing what's shown on the unit's display. ↩