Arrived to furnace maintenance. Upon arrival I spoke with the customer who showed me the location of the furnace, ac, and corresponding thermostat. I began my maintenance by cleaning safeties and checking the overall condition of the furnace. This included a safety inspection of the systems heat exchanger. Upon doing this I found large holes had formed within the heat exchanger itself making this system unsafe to operate. I showed the customer these holes live and red tagged/shut down the furnace. My supervisor will be here to go over replacement. Furnace off upon departure.