Many gas companies offer an annual maintenance package for your furnace. These plans include a yearly diagnostic check to make sure your heating system is working safely and efficiently. If your furnace is being serviced once a year, these plans generally cost less than paying for each visit individually. For example, a one-time furnace check up will cost you about $140. While, annual maintenance plans cost up to $120, but they are ongoing. Plans generally include a technician's visit once a year.