1. The HVAC-Radon Connection
Most homeowners do not realize that their heating and cooling system plays a significant role in how much radon accumulates inside their home. Your HVAC system does not produce radon. Radon comes from the natural decay of uranium in the soil beneath your foundation. But your HVAC system can dramatically influence how much of that soil gas gets pulled into your living spaces.
The connection comes down to air pressure. Your HVAC system moves large volumes of air around your home, and that movement creates pressure differences between the inside of your house and the soil beneath it. When the air pressure inside your lower levels drops below the pressure of the soil gas outside, radon gets pulled in through every crack and opening in the foundation.
About 70 percent of the radon in most homes enters through the foundation, driven by exactly these pressure differences. Understanding how your HVAC contributes to this process can help you make informed decisions about testing and mitigation.
2. How HVAC Creates Negative Pressure
Negative pressure is the key concept to understand. When air pressure inside your home (especially at the lowest level) is lower than the pressure in the soil outside, your home essentially acts like a vacuum, pulling soil gas in through the foundation.
Your HVAC system creates negative pressure in several ways:
Combustion Air Consumption
Natural gas furnaces, water heaters, and fireplaces consume indoor air for combustion. Every cubic foot of air burned up the flue must be replaced, and that replacement air gets drawn from wherever it can enter, including through the foundation.
Return Air Imbalances
If your return air ducts pull more air from lower levels than the supply ducts deliver back, those lower levels develop negative pressure. This is extremely common in homes where the HVAC system was not properly balanced after installation.
The Stack Effect Amplification
In winter, your furnace heats indoor air, which rises and escapes through the upper building envelope. This stack effect creates a continuous cycle of warm air leaving from the top and soil gas being drawn in from the bottom. Your heating system amplifies this natural process.
Exhaust Fan Effects
Bathroom exhaust fans, kitchen range hoods, and dryer vents all push air out of your home. When these run simultaneously with your HVAC, the combined effect can create significant negative pressure at the foundation level.
The pressure difference does not need to be large. A difference of just 5 to 10 pascals between your basement and the soil is enough to pull radon through cracks as small as a hairline fracture in the concrete.
3. Ductwork and Radon Distribution
Beyond pulling radon in, your HVAC ductwork can also spread radon throughout your home. This is a problem that many homeowners overlook.
Leaky Return Ducts in Crawl Spaces
If your return air ducts run through a crawl space and have leaks (gaps at joints, disconnected sections, or holes), they will pull crawl space air directly into the HVAC system and distribute it to every room in the house. Crawl space air often has elevated radon concentrations.
Basement Return Air Registers
Return air vents in basements pull air from the area with the highest radon concentration and cycle it through the entire HVAC system. While this provides some dilution, it also distributes basement-level radon to upper floors that might otherwise have lower levels.
Supply Duct Leaks in Crawl Spaces
Leaking supply ducts push conditioned air into your crawl space, which pressurizes the crawl space and actually helps push radon away from the foundation. However, this wastes energy and can cause moisture problems, so it is not a practical radon strategy.
Air Handler Location
When the air handler is in the basement or crawl space, any leaks around the blower compartment draw in surrounding air, including radon-laden air. The entire air volume of your home then gets a dose of radon every time the system cycles.
A Common Georgia Problem
Many Georgia homes have their HVAC systems and ductwork in the crawl space. Research suggests that homes with crawl space ductwork can have 20 to 30 percent higher indoor radon levels compared to homes with ductwork in conditioned spaces, primarily due to duct leakage drawing in crawl space air.
4. Heating vs Cooling: Which Is Worse?
Both heating and cooling affect radon levels, but they do so through different mechanisms.
Heating Season (Winter)
Heating creates the strongest radon-pulling effect. The thermal stack effect is at its peak, combustion appliances consume indoor air, and closed-house conditions prevent dilution. Radon levels are typically 2 to 5 times higher in winter than summer in most homes. Gas furnaces are particularly impactful because they consume large volumes of air for combustion, increasing negative pressure at the foundation.
Cooling Season (Summer)
Air conditioning has a more nuanced effect. On one hand, AC does not consume combustion air or create the thermal stack effect that heating does. On the other hand, AC keeps the house sealed up for months and can create its own pressure imbalances. In Atlanta, where AC runs from May through October, summer closed-house conditions can keep radon levels elevated even without the stack effect. Some studies have found that homes in hot climates have less seasonal variation in radon than homes in cold climates, partly due to heavy AC use.
Shoulder Seasons (Spring and Fall)
When the HVAC system runs less frequently and windows are more likely to be open, radon levels often drop to their lowest. Natural ventilation dilutes indoor radon, and reduced HVAC operation means less mechanical pressure influence. In Atlanta, these comfortable stretches are relatively short, maybe a few weeks in April and October.
The bottom line: heating season is generally worse for radon, but do not assume your home is safe in summer just because the furnace is off. If your AC runs constantly and the house stays sealed, you may still have a radon problem.
5. HVAC Modifications That Help
While HVAC changes alone will not solve a radon problem, there are several modifications that can reduce the impact your system has on radon levels. These are worth considering whether or not you install a dedicated mitigation system.
Seal duct leaks
This is the single most impactful HVAC modification for radon. Seal all joints, connections, and gaps in ductwork, especially return ducts in crawl spaces or basements. Use mastic sealant or metal-backed tape (not standard duct tape, which degrades quickly). This can be a DIY project with zero material cost if you already have sealant on hand.
Balance supply and return air
Have an HVAC technician check that each room receives roughly the same volume of supply air as the return system pulls out. Imbalanced rooms create localized pressure zones. Closing bedroom doors at night with no return air path is a common cause of pressure imbalance.
Seal the air handler cabinet
If your air handler is in the crawl space or basement, seal the access panel and any gaps in the cabinet. This prevents the blower from drawing in unconditioned crawl space air (and any radon it contains) and distributing it through the house.
Consider combustion air supply
If you have a natural gas furnace or water heater, ensure it has a dedicated outside air supply for combustion. This prevents the appliance from depressurizing your home as it burns fuel. Many newer furnaces are sealed-combustion units that already draw outside air directly.
6. Why HVAC Is Not a Radon Solution
It is important to be direct about this: no amount of HVAC modification will reliably solve a radon problem. Here is why.
HVAC changes address part of the pressure dynamic, but they do not address the source. Radon enters your home because the soil beneath your foundation produces it continuously. As long as there are pathways from the soil to your indoor air and any pressure differential at all, radon will find its way in.
The proven solution is sub-slab depressurization, which creates consistent negative pressure beneath the foundation slab itself. This reverses the pressure relationship: instead of soil gas being pulled into your home, it is pulled away from your foundation and vented safely above the roofline.
The Right Approach
Think of HVAC modifications as a complement to proper radon mitigation, not a replacement. Sealing duct leaks and balancing airflow are smart for energy efficiency, comfort, and indoor air quality in general. But if your home tests above 4 pCi/L, you need a dedicated radon mitigation system. No HVAC adjustment alone will reliably bring levels below the EPA action level.
That said, combining proper HVAC maintenance with a radon mitigation system gives you the best possible indoor air quality. A well-sealed duct system means your HVAC is not working against your mitigation system by creating competing pressure zones. The two systems work together rather than against each other.
