REMEHA Timer thermostat User Manual page 78

Problem The display does not function.
Solution iSense OpenTherm: your boiler may not support OpenTherm Smart
Power. iSense RF: insert (full) batteries.
Problem The backlight for the display does not work.
Solution iSense OpenTherm: your boiler may not support OpenTherm Smart
Power. In that case fit batteries in the iSense (see paragraph 3.2).
iSense RF: insert (full) batteries.
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