Netatmo NTH01 reverts to off when told to heat, unless primed in Netatmo app

Configuration
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I can’t get my Netatmo thermostat to stick in ‘heat’ mode for more than a few seconds unless I first use the Netatmo app to trigger a manual override – and that lasts maximum 12 hours before it reverts anyway. What’s going on and how can I fix it?

Background: I have Eve TRVs (Matter) on all radiators and a Netatmo NTH01 thermostat with relay (HomeKit Device integration) controlling a gas combi-boiler. When a TRV opens, a “Boiler on” automation fires in HA and calls climate.set_temperature on the Netatmo with temperature: 30 and hvac_mode: heat in a single action. But a few seconds later, the thermostat always reverts to off.

I have the same problem if I adjust the temperature or heat mode of the thermostat in other ways within HA (Developer mode, editing entity states, etc).

What I’ve ruled out:

  • The automation YAML is correct — single action, temperature and hvac_mode set simultaneously
  • The Netatmo cloud integration is not in use — I’m using HomeKit Device for local control
  • The Netatmo app’s internal schedule cannot be deleted, but manual commands from the app also hold reliably

A possibly relevant detail: my “Boiler on” automation triggers multiple times simultaneously when several TRVs open at once. The first execution succeeds; subsequent ones are blocked with “only a single execution is allowed”.

Background: this setup worked reliably under Apple HomeKit. I’m not sure if the HomeKit Device integration is sending commands differently from how Apple Home did, or whether there’s something else going on.

Any ideas gratefully received!

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I should add that I’m not wedded to the Netatmo setup as the only solution here. Even if I can fix the problem, I realise I’m trying to use a Netatmo thermostat as basically a switch that switches the boiler on and off in response to TRV activation. I’m very happy to consider alternatives. My main constraint is that I have a two-wire connection for my thermostat, which is not conveniently close to a mains outlet, so I can’t install a switch that needs to draw power from the mains. The Netatmo thermostat has the advantage of using batteries to power its screen and wireless receiver, and a relay that’s plugged in elsewhere.