Some new reading later, What's all the fuss about? | MySensors - Create your own Connected Home Experience, I could not see exactly which board from the arduino range they have got working with Home Assistant, but it certainly isn’t the 328 nano in my boiler controller.
You have not explained why the same board can’t do both jobs (monitor temperature/cut the gas and send data to HA).
You keep comparing your device with Tuya, but I’m quite sure nobody here would ever recommend Tuya.
Why not compare your device to a ESP-Home device?
Why is your Arduino and the “extra non-existent” board better than an ESP?
The nano328 could not do both jobs because 2kB of variables memory, whilst plenty for boiler states and thermister to linear temperature conversion maths, is not suited to ethernet and later communications protocols. So it should not be used to send data directly to HA (and I’m too chicken to enable the gpio pins on the board hosting my HA). A newer board such as a newer type of arduino or an ESP might be able to do digital and analog and wifi, (meaning boiler and HA comms), but I’m not going to comment on what I’ve not seen and don’t know. I’ve wanted to go to a two microcontroller solution for ages so that they can do mutual heartbeat and reset if either hangs; that is the real gain of not locating the boiler control decision in the new chip or ESP; only adding those next to the existing arduino nano 328 to collate and de-clutter for HA the prolific data generation capacity of five temperature sensors and a boiler status byte. Thus home assistant will see one device with quite a few entities which can have their huge name or id, whilst the boiler control code gets a twelve line code change to write a few bytes to somewhere else instead of to its usb connector. I’m not inclined to put anything as safety-critical as overtemperature cutout into a loop which might get unpredictable waits or updates or a total hang for protocol or operating system reasons. Since nobody would tell us in advance whether the ESP will get those, I’d rather not risk it.
Right at the beginning of your your post I thought" why not have a second nano to do the I/O"
You could communicate to the second nano over an I2C bus . This second nano could also function as a my sensors seriall gateway. If the distance is too far or inconvenient. A third nano could be the serial gateway and the two nanos could communicate over the nrf24 radio.
I’ve been using my sensors for many years now and have found it to be very reliable.
I get confused when I see names like “serial gateway” due to misuse of some similar words elsewhere; not your fault. Indeed I sometimes do push a few bytes from one nano328 to another over the same 5 Volt i2c as does the 20x4 character LCD screen, and that all goes through wires. The same i2c bus ought to be capable of moving some bytes to a wifi-capable 3.3V board such as an ESP. The reason why I’m not overly keen on using a nano328 as any sort of gateway is that it is a bit too small for all the address and protocol nonsense which internet-of-things won’t work without, so something else in the downstairs wired group gets the gateway task. As a minimum, able to dance the right dance when the dhcp issues an address.
I’d not read about nrf24 radio before. I don’t like the look of the rather showy antenna on most of those and I might prefer to go through a compact thing. What’s good about them ?
I had the same thought that the nano doesn’t have enough processing power to be a gateway. In another place, I had this discussion. A gateway takes a signal or data stream, reconfigures and sends it on to the down stream device. It doesn’t store any data. So it requires no memory for the devices that pass data through it.
You might have concerns about over-running buffers. These are handled by the radio driver/protocol. ACK and NAK are used to ensure delivery.
If one puts a sensor or actuator on the gateway one may have problems. I use two gateways, one serial and one MQTT. Both have sensors on them. The former, runs on a nano has no radio and is used to push 5 buttons and one sync bit. Very simple and no problems. The latter, an RPi zero 2W, has a relatively sophisticated program to run a 25 zone irrigation system. It also has a radio to allow the other devices in my yard access to HA. I haven’t had any problems with MySensors. Corrosion being my worst enemy; the Pi is notorious for its WiFi failing. The guys that come to pick weeds and other curious critters cause minor problems.
I use the nRF24 with the built in antenna. The range is almost the same, but a bit further, as bluetooth. ~15m? The ESP devices have essentially the same built in antenna.
In theory, MySensors can handle 253 nodes. Because the mesh uses a layered addressing scheme, the response time to the outer layer might be a concern. When I built up a 20 node system, the time to connect after a power cycle was long, but ran smoothy after all the devices got connected. Giving them static address helped, but a real pain to maintain.
Consider that one node could have 8 sensors/actuators and a few I2C devices, those 20 nodes could have 200 gizmos!
One of my “sensors” is a Low Cost Energy Monitor It uses MySensors and runs on a nano.
I did a quick count of WiFi access points around my house … 12+ Sometimes my network goes nuts. MySensors/nRF24 can run just outside the WiFi band, There are a few other radios that work with MySensors that have ranges that have been reported in the kilometer distances.