Make sure the adapter is connected to CL+/CL- and not to the third (G+) pin: G+ will drive the LED, but it's not a data line.
+
Make sure you have powered on the microcontroller. You may think that the heating system powers the microcontroller because the LED on the BSB-LAN adapter is on, but it doesn't. You need to power it separately.
With the adapter for the Olimex microcontrollers: Make sure that the BSB-LAN adapter board sits exactly in the center of the UEXT connector. It will still fit in, if it's shifted one pin left or right, but it won't work.
Make sure the RX/TX pins are set/detected correctly.
Make sure the adapter is connected to CL+/CL- and not to the third (G+) pin: G+ will drive the LED, but it's not a data line.
+
Make sure you have powered on the microcontroller. You may think that the heating system powers the microcontroller because the LED on the BSB-LAN adapter is on, but it doesn't. You need to power it separately.
With the adapter for the Olimex microcontrollers: Make sure that the BSB-LAN adapter board sits exactly in the center of the UEXT connector. It will still fit in, if it's shifted one pin left or right, but it won't work.
Make sure the RX/TX pins are set/detected correctly.