The duct cleaning service provider must have experience working with your specific HVAC system. They must also have a special license if required in your country, standards.
Perhaps the most important thing is to check the company references and see what other customers are saying. You can consider the most reliable duct cleaning in Vaughan at https://prestigeductcleaning.ca/duct-cleaning-vaughan/ for your duct systems.
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The supplier must open the door and open access so that the entire system can be inspected and cleaned. Before starting work, the supplier must ensure that there is no asbestos-containing material in the heating/cooling system.
If a vacuum leak indoors, only the highly efficient Particle Air Vacuum Cleaner (HEPA) should be used. Otherwise, they use a vacuum device that vacuums the house.
Carpets and furniture must be protected. Channel operation must be protected. This includes capping and resealing holes made by the supplier. Dust and other particles must be removed by controlled brushing along with contact suction.
Suppliers may only use soft-bristled brushes that must be used on fiberglass board and fiberglass-lined grooves. Some companies may recommend using chemicals and biocides. Make sure the supplier is willing and able to provide a good explanation of the effects of the chemical.
It is always recommended that you do your own research and understand the pros and cons of the chemicals in your home.
You may only need to clean the drain if a serious problem occurs and people around you are injured. The techniques used to clean air vents can be very expensive and tedious and time-consuming.
If the company you hire insists on using chemicals or cleaning products, you need to make sure they are certified for non-toxic and hypoallergenic HVAC chemicals and cleaning products.