Customer called because of a sewage odor when they run furnace and air conditioner. Return duct was very filthy and needs to be cleaned. Flex duct for most run but trunk line is where build up is mostly(attached photos). Also recommend REME Halo air purifier to get rid of any odors that could be present. All checks with the system were good. Signed customer up for maintenance agreement. Filter size is EAC washable but replaced with a 16-25-5 filter while I was there.