Musty odors, fogged-up windows and other clues your home’s air may need help

Many homeowners only think about their HVAC system when something obvious goes wrong, such as the house failing to cool, a sudden jump in the energy bill or an unfamiliar sound from the unit.

Some warning signs, though, are much less dramatic.

A musty smell, windows that repeatedly fog up, lingering humidity or a filter that seems to clog unusually fast can all point to problems with the air inside a home. That matters because Americans spend about 90% of their time indoors.

The U.S. Environmental Protection Agency says some indoor pollutant levels can be several times higher than outdoor levels. Those contaminants can come from consumer products, building materials and outdoor air, while temperature, moisture, humidity and HVAC maintenance also play a role in indoor air quality.

The EPA says there are three main ways to improve indoor air quality: control pollutants at the source, bring in clean outdoor air through ventilation and use filtration or air cleaning.

So what should homeowners watch for?

Start with the filter

A dirty filter is the simplest place to begin. The EPA recommends using at least a MERV 13 filter, or the most efficient filter an HVAC system can handle, and following the manufacturer’s guidance for replacement and upkeep.

Watch for moisture

Repeatedly foggy windows, condensation, damp areas and musty odors can all suggest moisture is building up inside the home.

The EPA advises keeping indoor humidity between 30% and 50%. The Centers for Disease Control and Prevention says humidity should stay at 50% or lower to help reduce mold growth. A low-cost digital hygrometer, often priced around $10 to $20, can help homeowners measure humidity instead of guessing based on how a room feels.

Bathrooms deserve special attention. Exhaust fans that vent outdoors can help remove moisture from showers and baths. Homeowners should also check for standing water near HVAC equipment and make sure air-conditioning drip pans stay clean and drain lines remain clear.

Don’t assume dust means the ducts need cleaning

If dust seems to come back almost as soon as it is wiped away, duct cleaning may seem like the obvious fix. But the EPA says routine duct cleaning has not been shown to prevent health problems.

The agency says duct cleaning may be worth considering in certain cases, including visible mold growth inside hard-surface ducts, a vermin infestation or ducts clogged with a large amount of dust and debris. It is also important to correct the underlying cause, or the problem may return.

Remember that a filter cannot do everything

Even a high-efficiency filter is not a complete solution. Household products, building materials, smoke and outdoor pollution are all possible sources of indoor contaminants.

The EPA recommends reducing pollutants at the source and improving ventilation in addition to using filtration. Portable air cleaners that are properly sized can also help in individual rooms.

Do not wait for something to fail

HVAC maintenance is about more than avoiding the miserable day when the air conditioner stops working.

Regularly checking filters and keeping the system maintained can help it work as designed. Moisture also needs attention: the EPA recommends keeping air-conditioning drip pans clean, ensuring drain lines flow properly and addressing leaks or standing water that could worsen moisture problems.

None of these signs by itself means a home’s air is unhealthy. But recurring condensation, a lingering musty odor, extra moisture or a neglected filter can be reasons to take a closer look.

The thermostat may be the most visible sign of how an HVAC system is performing. The air around it can be just as important.