Completed furnace, maintenance and inspection. I cleaned furnace and flame sensor. I tested airflow and temp splits. I checked components and amp draws. Current filter is very dirty and is also a highly restrictive allergenic filter. This was causing our airflow pressure to be way over max rating. I showed customer the airflow pressure ratings and mentioned to use a Merv eight filter instead of the allergenic filters so the system works better and doesn’t overheat. There was a lot of flu ash coming from the exhaust due to the furnace having to run hotter than it should because of the lack of airflow coming through the filter. Customer will replace filter. Furnace is running at departure.