Since your home’s ductwork is hidden behind layers of flooring and Sheetrock, you’ll need to keep a sharp eye out for these four signs that your air ducts are getting a little grimy.

1. Mold has been found in your furnace or air conditioning system.

Your HVAC system needs to be cleaned and serviced periodically—in fact, most manufacturers and HVAC service professionals recommend scheduling two service visits per year. Because both air conditioners and furnaces can develop condensation, mold growth is a possibility, especially if you live in a humid climate.

If your service technician tells you that your HVAC system has mold growing in it, first ask to see the evidence. Your next step should be to contact a mold expert. Your HVAC technician may not have the training and credentials to properly identify mold (or pinpoint whether the mold is harmful), so look for an indoor air quality expert, ideally a member of the Indoor Air Quality Association

Mold found on your furnace, air conditioner, or air handler unit isn’t a guarantee that your ductwork has been affected, but it certainly increases the chances. A NADCA-certified air duct cleaning technician will thoroughly clean your home’s HVAC system and ductwork to remove the mold and prevent its return.

2. Your home was just built or has undergone a major renovation.

Construction is messy. Even when contractors are diligent about cleaning up and closing off work areas to keep the rest of the house tidy, dust and debris will always find their way in. And unless the HVAC system is not turned on and kept completely covered during the entire construction process (which isn’t realistic for most homeowners), that dirt and dust will make their way into your ductwork.

Construction dust can contain some pretty nasty particulate matter that you don’t want to breathe, so don’t wait until you experience respiratory problems to call in the pros.

3. Your home is very dusty, no matter how often you clean.

Dusting isn’t the most entertaining task, but it’s something we all have to do from time to time. If, however, you notice that every flat surface in your house is coated in a layer of dust almost immediately after you’ve put away your cleaning supplies, you probably need to have your ducts cleaned.

Take a look at both the supply and return vents around your house. Are they visibly dirty and dusty? Carefully remove a vent cover and look into the ductwork. Is the metal covered in dust?

4. You’ve noticed your heating and/or cooling costs climbing. 

Your heating and cooling costs should be relatively predictable from year to year. If you receive a bill that is noticeably high, however, or if you look at your records and note that your utility bills are much higher now compared to the same time a year ago, your home’s ductwork might be (partially, at least) to blame.

In extreme cases, ductwork can become so dirty that air isn’t able to flow freely and efficiently through your home’s HVAC system. When this happens, your furnace and air conditioner are forced to work harder (use more energy, in other words) to maintain the temperature you’ve set at the thermostat. This extra energy expenditure translates to higher utility bills for you.

Beyond removing airflow-obstructing dirt and debris, your air duct cleaning technician will also keep an eye out for leaking ducts. Ductwork is usually installed in a home in sections, and these sections are sealed to prevent air leakage. Over years of use, however, these seals can deteriorate, which reduces the efficiency of your HVAC system.

Imagine trying to fill a small cup using a fire hose: You’ll get some water in the cup, but ultimately, most of the water will end up on the floor. Leaky ducts have a similar effect on your home. You’ll still have air moving through your ductwork, but some of that heated or cooled air will end up in the attic, walls, or basement.

The bottom line is that when your ductwork isn’t sealed properly, you’re paying for heated and cooled air that you don’t get to enjoy.