There are many potential benefits to having your ducts cleaned, there are also possible problems that could be caused by this process.
If you expect there is vermin activity in your ducts, then yes, as soon as possible have your ducts cleaned. You would be very surprised how often this is the case with ducts being a nice warm or cool place in a cold or hot crawl space or attic. It is just too attractive to small critters to pass up.
If you are concerned about dust and allergies:
You should first do a visual examination to determine if the service is needed. Do your ducts have visible mold or large build up of dust inside the duct work? Second, is anyone in your family experiencing allergy symptoms or unexplained health issues?
If you have mold present in the ducts you should first identify the cause of the mold or the cleaning will just temporarily fix the symptom but will not address the problem. The type of mold does not matter, if it is growing there is a problem. A properly performing duct system will not have moisture present and therefore will not have the right ingredients for mold growth.
If there is a build up of large amounts of dust, you should identify where is it coming from. The average duct system is not air tight and sucks dust and dirt in from the crawlspace / basement or attic in which it is installed. Again, cleaning ducts that are not tight that are inviting in dust will just result in repeated visit to address the symptom and not fix the problem. You should seal the ducts from the outside if accessible or from the inside if not accessible along with the duct cleaning.
If a family member is suffering from unexplained symptoms or allergies it is most likely due to air leakage throughout the entire home. Cleaning the ducts might temporarily alleviate these symptoms, but will not eliminate them as air leakage comes from many places around a home’s envelope. An energy audit would be a good first step to identify the sources of contaminants entering the home.
So should you get your duct cleaned – if they are sheet metal ducts, then yes if you have mold or large amounts of dust and deal with the other issues first. If they are flex duct, then only by someone who understands how to do it without destroying those very fragile elements and if you are willing to risk having to install new ducts if they are ripped or destroyed. I am not a fan of flex duct because it is so fragile but it is used in may homes due to its low-cost. If your ducts are made of duct board, then no, it is not worth your money as the duct board will never be air tight enough that cleaning them will make a difference – in my opinion. If your return air is not ducted then also don’t bother cleaning as it will never stay clean for any length of time.
If you decide to clean your ducts you should use someone who belongs to the National Air Duct Cleaners Association and know that they comply with NADCA’s air duct cleaning standards. Do not allow the use of chemical biocides or treatments unless you fully understand the pros and cons. You should also check the companies references that you are considering. Verify the company holds the appropriate state licenses to do the work you are requesting. You should also know there are no proven health benefits for cleaning ducts.