OpenTherm 2 zones
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Is it possible to have this set up, 2 thermostats hardwired to 2 zones valves (upstairs/downstairs) . Also the 2 thermostats connected wirelessly at same time to the extension box and then wired to boiler via opentherm. Same as diagram 2 on this page with the exception of final connection opentherm from extension box -- https://support.tado.com/en/articles/3481812-how-does-tado-control-a-multi-zone-radiator-heating-system
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