First of all, if you don't want to clean the entire AC system, don't waste your money clearing the ducts. The air you breathe flows not only through the air ducts, but also through the blower, through several rows of cooling coils (evaporator coils) and through the condensate container.
Make sure you find a reputable as well as trustable company for dryer vent cleaning in Markham that has a license to clean your entire system.
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The evaporator coil completely removes moisture and heat from the air. Coils are especially problematic in humid climates.
Duct cleaning alone does not take into account windings, which are very susceptible to build-up of dirt and grime as they are mostly wet in system operation, especially in humid climates.
If you don't clean the entire system, apart from the air you breathe through the wet and dirty rolls, you also have a less energy efficient air conditioning system.
Dirt actually isolates the windings, reducing the amount of heat and moisture they can remove from the air.
Due to the thickness of the rolls several rows, dirt also reduces the airflow through the windings. Your air conditioner has to work harder and longer to cool the air. Sewer cleaning is beneficial if it is performed while cleaning your entire system.