Often, simply cleaning your heater or air conditioner improves its performance. For fireplaces and chimneys, a thorough clean will improve efficiency and help to prevent dangerous chimney fires.
For ducts, however, this appears not to be the case. Research conducted by Canada Mortgage and Housing Corporation (CMHC) and the US Environmental Protection Agency (EPA) has shown that cleaning your central heating ducts doesn’t significantly impact on the quality of air or heat circulating in your home.
Dust and dirt in ducts
If they haven't been used for a while though, ductworks can become resting grounds for dust and dirt, or pet hair and dander (if your pet happens to be a mammal), so people with allergies may like to clean them out from time to time. Likewise, if you have a water leak in your home or from your air conditioner, mould can also build up (often accompanied by a foul smell).
Ductwork is fragile and often runs through either the ceiling or floor cavity. If you don't know what you're doing, cleaning ducts yourself can be very dangerous, and can put your ductwork at serious risk of expensive damage. If you do decide that your ducts do need to be cleaned, you will need to rely on a professional to do it for you. But, again, unless there's an obvious problem there may not be any measurable benefit in doing so.
How to clean heating or cooling vents
If you're experiencing poor performance from your central heating or aircon, or you think allergens are being circulated around your home, the problem could actually be as simple as a build-up of dust and debris stuck behind your vents. To clean vents is simple, and in most cases you only need a bit of water, a cloth and a screwdriver. In most cases all you'll need to do is to unscrew the vents from your wall, and wash them thoroughly in water. While you have the vents off, make sure you give the entrance to your ductwork a good clean with a wet cloth too. After doing so, you can simply dry the vent covers and screw them back on.