Signs that one needs to get their furnace serviced include: Unusual Noises: If the furnace starts making unusual noises, it could be an indication of a loose belt or other mechanical problems. Higher Utility Bills: If the utility bill suddenly increases without any changes to heating habits, it could be an indication that the furnace is working harder than necessary. No Heat: If the furnace stops producing heat, it could be an indication that there's a problem with the system.