It's good to have your furnace and the ducts in your home inspected regularly; usually this means every year, but you can check the owner's manual of your particular furnace to note when the manufacturer recommends it in your case. However, between those inspections you might want to check for signs that your furnace is leaking or that your home's ductwork needs repair or maintenance. Note that your furnace doesn't need to quit altogether or only blow out cold air to need some attention, so consider a few surprising signs that it may be time to call a contractor. 

1. The walls are warm

The walls in the direct path of the airflow outside the vents might be warm, but if you feel warmth in other areas, this can be a problem. Usually this means that ducts behind the walls are leaking warm air so that it's collected by the walls; in turn, your furnace is working harder to push more warm air through the vents, or it needs to work longer to heat your rooms properly. Having the vents sealed or insulated or having loose connectors replaced is often needed in these cases.

2. You can see critters inside

If you inspect the ductwork yourself, you might notice rodents or a bug infestation. This can be a problem as it may mean that the ducts have openings that allow in these critters. The furnace may also not be working enough to push warm air through the ducts, so they don't get very warm themselves; in turn, rodents and other pests feel comfortable nesting there. If you see signs of a pest infestation, you first want to call a pest control service and then have your ductwork inspected by a heating and cooling contractor.

3. Excessive dust or a moldy smell

Dust in the ductwork of your home is not uncommon, but if you notice a lot of dust blowing out the vents or notice a musty smell every time the furnace comes on, this can mean that the filter is not working properly. In turn, the furnace is being overworked in order to blow air through the ducts. A moldy smell may also mean that there is a leak in the ductwork that is allowing for moisture to collect in the space and it then becomes moldy or allows for mildew growth. The ducts may need a good cleaning and then an inspection for these potential leaks.