THERMOS doesn't understand this in a meaningful way - it assumes that for whatever temperature heat you need, you have chosen flow/return temperatures that make this possible.
If you were modelling a low temperature system that only provided space heat you would want to do something to adjust the demands, as the builtin demand model assumes you are covering hot water. This would probably also interact badly with the diversity effect, as there would be lower load diversity in such a system. There isn't a way in the user interface to change the diversity parameter at the moment.