When you rent premises--even a single room--you are given possession and control over it. The landlord may not enter, except 1) with your permission; 2) for an inspection, routine mainteance, or to show the premises to realtor or prospective buyers or renters, on reasonable (e.g. 24 hour) notice; and 3) for emergencies, which are things like fires, electrical faults disabling power or causing hazards, water  or case leaks, etc. Simply wanting to use your heater is not an emergency. If the landlord continues to enter without permission, you might have grounds to either terminate your rental without penalty, or to sue him for compensation.