Set the temperature to 20º C during the day and 17 º C at night. Better yet, use a programmable thermostat. Seal cracks and drafts around windows, doors and electrical sockets with caulking or weather stripping. Replace or clean your furnace filter every three months. Dirty filters restrict airflow, forcing furnaces to work harder and use more natural gas. Let the sunshine in keep your curtains and blinds open during the day so the sun can naturally warm up your home.