[MikeM]

The type of temp / hum sensors supported by the RX and WD units are also specified on the respective parts list.
The only work around is adding support for the SHT31 on the RX and WD software. It's possible that at some point in the future I will do it, but I can't say when.
For use inside the house, the HTU21D is a really good and precise sensor.

Thank you Werk_AG,
I'll order some HTU21D's for indoor sensors.
When I decided on using the SHT31 for outside, I though it would be clever to use the same sensor for all and never looked any further into it.