Before you decide if you should get your air ducts cleaned, you need to know a little about how they work and why they collect dirt.

Most homes are heated using a furnace and a duct system that distributes hot air into different rooms. This system is made up of both "return" ducting that bring cold air into your furnace and "supply" ducting that distributes warm air throughout your home.

You can easily tell the difference between return and supply ducting by going down to your basement and inspecting the sheet metal ducting usually found hanging from the floor joists on the ceiling.

The return duct is usually a large rectangle that is attached to the bottom of your furnace, while the supply duct comes out the top of the furnace. The supply duct is usually made up of a large "trunk" duct on the ceiling, along with smaller metal ducts that branch off into each heated room. You'll find them often covered by a metal grate on the floor or the wall.

Since your duct work is open to your household air, it can easily collect dust, hair, pet dander and other debris from your home. This is especially true of the return ducts that feed air into the furnace.

But does this mean you should have your ducts cleaned?

A good duct-cleaning will help to remove dust, dirt, and hair, along with any toys, pens or things like jewellery that may have been dropped into the ductwork. Some believe that cleaning your air ducts will improve your indoor air quality, since your ducts should be spotless after they've been thoroughly cleaned, while others say it has little effect on air quality.

Research from the Canadian Mortgage and Housing Corporation has found that replacing the filter on your furnace can help keep the air in your ducts clean, and keep the components of your heating, ventilation and air-conditioning (HVAC) system from accumulating new dirt. However, it won't prevent dust from settling in your home.

Another reason to consider having your ducts cleaned is if you suspect water has gotten into your air ducts, either through a leak or through condensation. This can cause mould to grow in the ducts, which can be harmful. You will need to fix the leak or condensation problem and then have your ducts thoroughly disinfected -- or even replaced depending on how serious your mould problem has become.

New homeowners or people who have recently undergone renovations may also want to have their ducts cleaned if they suspect the contractors who worked on their home may not have done a good job of cleaning up. Sometimes construction crews allow small particles of firbreglass, drywall and sawdust to accumulate in the air ducts.

You should also consider having your ducts cleaned if you think there is a blockage that is affecting airflow to your furnace. Before you spending a lot of cash to replace your furnace, first have your ducts inspected and cleaned along with your furnace and air conditioner.

You can do some duct-cleaning yourself, especially the return duct, which is pretty large and often accumulates more dust that your supply ducts. To clean your return duct, first remove the grill on the outside of the duct and then use a vacuum to get rid of any debris.

However, if you feel that dirt in your ducts is affecting your air quality, causing mould to grow or blocking the flow of air to your furnace, you should call a reputable duct-cleaning service. Be sure to check if the service is a member of any trade organizations or affiliated with a reputable HVAC (heating, ventilation and air-conditioning) company before you hire them.