Monitoring Vaillant ASHP without expensive heat meter

No, but there’s this:
https://github.com/openenergymonitor/emonTH/tree/master/firmware/user_contributed/emonTH_V1_5_DHT22_multiple_
However…
Looking at that sketch, there would appear to be a few problems:

  1. If you lose a sensor (it fails to respond on power-up), then the order that the sensors are detected, therefore the Inputs to which they are allocated, changes. I’d call that a big no-no. It will be far better to hard-code the sensor addresses, the order would then be preserved irrespective. But you’d need to edit and reload the sketch if you replaced a sensor. But that’s probably better than shuffling all the data back into the correct places.
  2. The message sent by radio could exceed the maximum allowed by the library we use. The sketch sends 8 bytes plus 2 for each external DS18B20, and you’re allowed 253 total (though I’ve never tested that many). You’re clearly OK with 8 sensors.
  3. “Star” wiring, with all sensors coming back to a central point - the breakout board, might be OK with short leads, but the recommended method is ‘daisy-chain’, with the bus wiring running from sensor to sensor, and only a short (a few inches) stub between the bus and the sensor itself.

I’d recommend starting again with a modern sketch for the emonTH V2 and expand it to take the required number of sensors.

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