When we close up our homes for winter, we expect to keep cold air out and warm air in. What often surprises homeowners is that the tighter the home, the easier it is for airborne viruses to build up indoors. The same heating system that keeps your family comfortable can keep germs circulating if the indoor air is not managed carefully.

At Sub Zero Heating & Air Conditioning, we focus on what you breathe as much as how warm or cool you feel. As a local, family-owned, full-service HVAC company, we help homeowners use their heating and cooling systems as tools to support healthier indoor air, especially during cold and flu season.

Why Winter Air Inside Your Home Can Work Against You

In winter, most of us seal up every crack we can find to conserve heat. That tight envelope keeps conditioned air in, but it also traps recirculated air carrying dust, pet dander, and virus-carrying airborne respiratory droplets that move through the system over and over again.

The Environmental Protection Agency estimates that Americans spend about 90% of their time indoors. During cold and flu season, the air inside your home is where you spend most days, most nights, and most of your breathing. That makes indoor air the primary environment that affects how often your family may get sick, not the time spent walking from the car to the store.

Most forced-air heating systems in homes do not mechanically pull in fresh outdoor air. Instead, they recirculate indoor air through the furnace or air handler. The only fresh air in many homes comes from small leaks around windows and doors or from occasionally opening them. Without a strategy for filtration, humidity, and ventilation, your system can move the same pollutants and germs from room to room all winter.

Humidity: The Hidden Variable That Affects Virus Survival

Relative humidity, the amount of moisture in the air compared to how much it can hold at a given temperature, has a major impact on how long viruses stay active. Research shows that the flu virus can retain its infectious nature in dry air below 40% relative humidity up to five times longer than it does when humidity is above 40%.

The EPA generally recommends keeping indoor relative humidity between about 30% and 50%. In that mid-range, the air is humid enough that virus-laden droplets fall out of the air more quickly, but not so humid that you create a friendly environment for mold or dust mites.

Heating systems tend to dry indoor air dramatically in winter. It is common for relative humidity to drop below 30%, especially in tightly sealed homes. In that dry range, airborne viruses survive longer, travel farther, and can irritate already dry nasal passages, which are your body’s first line of defense against infection.

A whole-home humidifier, which integrates with your HVAC system, can help maintain a consistent humidity level throughout the house. Unlike portable units that only raise humidity in one room and need constant refilling, a whole-home system adds moisture to the air as it circulates through your ducts. That consistent balance is important for health, comfort, and the condition of wood furniture and floors.

Air Filtration: Choosing the Right Filter for Virus Season

Not all air filters are designed to capture the same size particles. The Minimum Efficiency Reporting Value, or MERV rating, tells you how effectively a filter captures different particle sizes. Common household allergens like dust mites and pollen are usually captured well by MERV 8 to 11 filters.

Virus-carrying respiratory droplets, especially the smaller droplet nuclei that can stay suspended in the air, tend to fall in the 1 to 3 micron size range. MERV 13 filters are designed to capture about 85% to 90% of particles in that range, which makes them a strong choice for cold and flu season compared to lower MERV filters.

However, higher MERV ratings are not automatically better for every system. Denser filters can restrict airflow if the furnace or air handler was not designed to handle them. Reduced airflow may cause comfort problems, higher energy use, or even equipment issues over time. Before upgrading to a MERV 13 or higher filter, it is important to have a trained technician confirm that your specific HVAC system can handle the added resistance.

Filter replacement frequency also matters. During peak illness season, changing your filter every 30 to 60 days instead of waiting a full 90 days helps maintain its capture efficiency. A clogged filter not only lets more particles pass through but can also reduce airflow and strain the system.

Whole-Home Air Purification: A Step Beyond Filtration

Standard filters are your first line of defense, but they mainly catch particles that pass through them and are large enough for the filter media. Whole-home air purification systems add another layer of protection by targeting smaller particles and some airborne contaminants that a basic filter may not address as effectively.

Electronic air cleaners use an electric charge to attract and trap particles as air passes through, which can improve capture of fine particulates compared to a standard filter alone. UV light air purification systems use ultraviolet light installed inside the HVAC system to disrupt the DNA or RNA of some microorganisms that move past the light source, which helps reduce certain bacteria and viruses on surfaces inside the equipment or in the air stream.

Unlike portable room air purifiers that treat only the air in a single space, whole-home indoor air quality systems work with your ductwork and blower to treat air as it circulates throughout the house. Every time the system runs, the air in each room passes through the filtration and purification components, providing a more uniform level of protection across the home.

We service all makes and models, and we can evaluate which indoor air quality options make sense with your current equipment. In some homes, that might mean upgrading to a higher-efficiency media air cleaner. In others, it could involve adding an electronic purifier or a UV component as part of a layered approach.

Ventilation: Refreshing Stale Winter Air Without Losing Heat

Even with strong filtration and humidity control, you still need some fresh air to dilute indoor air pollutants and reduce the concentration of airborne germs. The challenge in winter is how to bring in outdoor air without sacrificing all the heat you are paying for.

Energy recovery ventilators, or ERVs, and heat recovery ventilators, or HRVs, are designed to solve that problem. These systems exchange stale indoor air for fresh outdoor air through a heat exchanger, so most of the heat in the outgoing air is transferred to the incoming air. This helps you increase air changes per hour, the number of times indoor air is replaced with outdoor air, without a big penalty on your utility bill.

ERVs transfer both heat and some moisture between the air streams. They are often a good fit for more humid climates, since they help keep indoor humidity from climbing too high as you bring in fresh air. HRVs transfer heat only, which tends to work well in colder, drier climates where retaining as much heat as possible is the priority while you add ventilation.

On milder winter days, you can also use simple cross-ventilation. Briefly opening windows on opposite sides of the home to create a short, strong airflow can flush out stale air and lower indoor pollutant concentrations. The key is to choose times when outdoor air quality is good and temperatures are reasonable, so you do not lose too much comfort.

Preventive HVAC Maintenance as an IAQ Strategy

Indoor air quality is not only about what add-on equipment you have. The condition of your existing furnace, heat pump, and air handler matters just as much. A dirty blower wheel, coil, or heat exchanger can recirculate accumulated dust and debris every time the system runs, adding to the indoor pollutant load all season long.

Our Comfort Club maintenance plan is designed with air quality and performance in mind. During seasonal visits, we inspect and clean critical components, check airflow, verify filter fit and condition, and look for any issues that could cause the system to move dust or other contaminants through your home. Keeping the equipment clean and tuned helps it support healthier air instead of working against your other indoor air quality efforts.

Because our technicians are trained from the ground up and we work on all makes and models, the recommendations we give are based on how your system is actually operating. We do not rely on a generic checklist. We look at your equipment, your filters, and your home to suggest practical steps that fit your situation and your goals for comfort and health.

Bringing It All Together for a Healthier Winter Home

The most effective indoor air quality strategy for cold and flu season is layered. Balanced humidity makes viruses less resilient and your body’s defenses more effective. Right-rated filtration, properly matched to your HVAC system, helps capture virus-carrying particles. Whole-home purification adds another level of treatment as air moves through your ducts, while smart ventilation refreshes stale indoor air without wasting energy. A well-maintained heating system ties all of these pieces together so they work the way they should.

Our Comfort Club is one of the simplest ways to keep those layers in place year after year, with regular maintenance that supports both performance and the air your family breathes. If you would like to talk through indoor air quality options for your home this cold and flu season, you can always reach us at Sub Zero Heating & Air Conditioning at (501) 984-7253.