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I have bought 9 Telldus Thermo/hygro sensors (313160). When installed you set an ID ranging from 1-8. This makes it impossible to set unique ids for more than 8 units. Is it possible somehow to add more sensors?

Not with this type of sensor. You need to buy some other brand of sensor, Oregeon is a supported brand and they use a dynamic "channel" allocation on power up.

It’s sad that Telldus Thermo/Hygro sensors not support more than 8 channels.
(the old Clas Ohlsson ESIC WT450H has 60 channels (01-1 to 14-4), they use Mandolyn protocol.

It is possible to use 8 Telldus Thermo/Hygro sensors (also the dual channel F007TPH (Inside temperature/humidity + outside temperature) they use Fineoffset protocol Channel ID 135+136 where 135 are Modell: temperaturehumidity and 136 are only type. temperature. CH2=151+152, CH3=167+168,... ( increases by 16).

... and ...

Now if you modify the Telldus FT007TH (temperature/humidity), by open the box and unscrew the pcb
there are a J7 that could be soldered to closed,
this will change the sensortype from temperaturehumidity to temperature and there are no conflict
despite they both use the same channel(135, 151, 167,...),
but only the temperature are sent ( both are still on the sensor display),
... then you can add 8 more Telldus sensors


... or ...

The Telldus Fridge/Freze thermometer FT007TP are temperature only and this sensor also uses the Fineoffset protocol with Modell: temperature, means that they also share the same channel numbers without conflict as they use different modell in the communication, they are more expensive and does not show humidity on the display but works without modificatins.