You will need to heat the greenhouse, preferably with an electric heater. Cold, damp conditions will create mildew etc.

Paraffin heaters can produce small amounts of acetylene gas, which acts as a plant hormone. Not sure what effect it would have on geraniums.

For every gallon of paraffin you will get half a gallon of condensation which can cause damp. Also the fumes will damage many plants as well as yourself.