If you think it's time to clean those ducts, here are a few questions to ask yourself.

Air-conditioning and heating contractors often hear about duct-cleaning companies offering their services, but they may not be certain when clients actually need to get their systems cleaned.

Here are a few pointers on what to look for and what to do.

Replace the air handler

Remember this one. When you install a new air handler onto old ductwork, you run the risk of dirt blowing from the ducts. This can get very messy. The problem is simple: The new air handler blows harder. The old fan didn't disturb the dirt lying in the ducts because it was too weak. You come along and bang on the ductwork while yanking an old unit and hooking up a new, more powerful one. The dirt is now loose and ready to explode from the registers.

Many contractors get calls on this with some regularity. Sometimes the customers are mad. Even veteran HVAC contractors sometimes get caught doing this. It doesn't happen with every change out, but it happens often enough to warrant concern. If in doubt, get the ducts cleaned.

Even a simple motor change can cause this. When one client had a bad motor replaced in a unit that had gone unused for a year, dirt blew everywhere. All over the beautiful carpet. All over the grand piano. On the paintings. The insurance company settled for $25,000.

Consider the ducts on your next air handler change out, especially if there is a visible buildup of dirt in the system.

Bad filters

If you see bad filter setups of any kind, chances are your clients have a fair amount of dirt buildup in their ducts. The problem could range from gaps or holes in the filters to a missing filter, to no filters at all. If it's a short-term matter, such as a filter that fell down for a week, there's no need to worry. But if filters have been missing for some time, one look will tell you the ducts are filthy. They need to be cleaned.

Similarly, air handlers that have been neglected with no service for a long time are commonly quite dirty.

Images courtesy of Action Duct Cleaning Co.

Mold growth

If you see mold growing in the ducts or air handler, get it cleaned. If you see conditions that promote mold growth, such as standing water or wet insulation, fix it. If you don't and the mold spreads spores into the building, it could be a health hazard and you could have trouble.

The air handlers at one school were so bad that the building smelled like fish. Pupils were complaining of illness. Parents picketed outside with gas masks on, in front of the evening news cameras. Then came the lawsuit.

Fortunately, mold doesn't normally live in the ductwork itself, except in humid climates or where moisture is getting into the ducts. But the air handler should at least be cleaned when mold growth is evident. If the ducts are dirty or contaminated with mold, they should be included.

Images courtesy of Action Duct Cleaning Co.

Dirty ducts

Here the question is, "How dirty is dirty?" All ducts have some dust residue in them as a natural byproduct of air passing through. But when the dirt begins to pool, that is, you can see "puddles" of it in the ducts, it is starting to get out of hand. If it coats the duct interior one-sixteenth inches or more, you should probably consider cleaning.

If you see dirt and lint hanging from the walls of the duct interior, a cleaning was due long ago.

Dirt blowing out

This is one of the most common reasons duct cleaners are called. There is normally no alternative to duct cleaning in this situation. Usually the system has reached its dirt saturation point and the particles blow and blow. Sometimes a loose duct joint, filter gaps or leaky air handler can cause this. But the normal reason is too much dirt in the ducts. Clean them.

Bad smells

This can come from the air handler, the ducts or both. Some places like nightclubs accumulate cigarette smoke, perfume, human sweat and a host of other stale odors that can saturate the dirt in a duct system until the smell becomes quite distasteful. Cleaning the system gives the building a new life.

Before blaming the ducts, however, be sure bad odors aren't coming from elsewhere and simply being transported by the ducts.

Images courtesy of Action Duct Cleaning Co.

Sick occupants

With indoor air quality in the news, some building occupants blame their physical ailments on the air-conditioning system. They may be right. They may not. For some people, a cleaning is worth the cost - whether they need it or not - because it alleviates the liability of being accused of neglect.

Duct cleaners are sometimes asked to test duct systems for microbial growth. Sometimes they find it. Cleaning is a positive step in such a case, but this is not always the sole reason people are getting sick. The complex subject of "sick-building syndrome" has to be looked at from more than just one angle.

Some clients have reported that allergies can be improved by duct cleaning. Allergens can sit in the ductwork and be blown into an area daily, causing discomfort to sensitive individuals.

Air-conditioning contractors are often like doctors. They may see their client's system in poor physical health or the customers tell them of various symptoms.

Like physicians, well-informed contractors know when the remedy needed is duct cleaning. Hopefully, these guidelines will help them diagnose properly so their clients stay in tip-top shape.

(Dan Stradford is the chief executive officer of Los Angeles-based Action Duct Cleaning Co. Inc., and has almost three decades in the duct-cleaning industry.)