Boiler heating above the programmed set back temperature ?

I have a boss therm wireless thermostat used all the advice regarding communication issues with this system however the boiler comes on during the night at random times and super heats the house. Setback temperature at 6deg thermostat showing 16deg. On checking the receiver z green light shows indicating connection and the yellow relay light is on showing a demand for heat. Yet why does it heat more than the set back specified temperature