I have replaced my danfoss ts7899 programmer with a neomitis RF wireless thermostat. Do I need to remove my dt90 room thermostat which was connected to my boiler and danfoss programmer in order for my new neomitis thermostat to work.
If you were to disconnect the old thermostat, then the heating wiring would be permanently 'open circuit' and the heating won't operate. So either the DT90 would need to be left wired up and set to max as per @CBW, in which case you would always have to maintain the batteries in it. Or....
....Join the two switching wires that are in DT90 terminals A and B together.
This can be done at the thermostat simply by putting both wires in the same terminal. The thermostat would still be in place but have no effect on the heating and you won't have to maintain the batteries. Alternatively you could trace the thermostat cable back to its source and link the connections there. Then the DT90 and the cable to it can then be removed.