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Sounds like there is not enough load leaving your zone to take the zone temperature below the heating thermostat setpoint (15 C in this case), which is the only time your heating equipment will come on and maintain that setpoint.

This is very common in interior zones since significant natural cooling really only happens in the perimeter zones of a building. If this is a perimeter zone, you should look at your window size and material properties, as well as infiltration and other zone loads (people, electric equipment and lights). The balance between those parameters are probably such that they are offsetting each other. For example, let's say your windows are providing -1000 W, but your internal loads are 1200 W, which will cause your zone temperature to go up despite the temperature outside.