Have you ever wondered how fresh air gets into your home when you are running your air conditioner or heater (HVAC) and closing all the doors and windows? Because you seal off the entire room, it seems like air can’t come in from outside. But in reality, even in a closed room, some fresh air still manages to sneak in. This is because the house is not completely sealed, and there are always some openings for air to circulate.
In fact, some air continues to flow in due to natural ventilation and infiltration. The walls, windows, and floors of a house have small holes, joints, and cracks. Even if we close the windows, outside air continues to flow in through these cracks. This is called infiltration. For example, when the AC is running, the blower in the room pushes air outward, creating a slight pressure difference with the rest of the room or the outside, and cold or warm air from outside is drawn in through the cracks.
Sometimes, everyday things also bring in outside air. For example, when you run a bathroom exhaust fan, a kitchen range hood, or a clothes dryer, they expel dirty air. When you run these fans, air is expelled from the room, drawing fresh air back into the room. In other words, bathroom or kitchen fans expel dirty air and bring in fresh air from outside. Similarly, opening doors and taking things out also causes a small air change.
Fresh Air Intake in HVAC Systems
Some modern HVAC systems also have a system called a fresh air intake. This means that the system has a special hole or vent (duct) connected directly to the outside vent. When your AC or heater is on, the system’s blower (air blower) creates a small negative pressure (vacuum) outside. This pressure forces fresh outside air to be drawn through this special vent to the home’s air filter.
For example, many newer homes have a small pipe near the furnace or air handler that leads directly to the outside of the house. When the system is running, this pipe draws fresh air mixed with the return air into the room. In older homes, when leaking, this was unnecessary because small cracks would automatically let outside air in. But now, newer homes are built much tightly, blocking natural outside air outlets. Therefore, installing a simple vent is required by code.
This fresh air intake system thins the existing air in the room and also removes any impurities from the air. This means your AC not only recycles the room’s air, but also adds fresh air in a proportionate amount. This keeps the indoor air fresh.
Main Sources of Fresh Air
Considering the above methods, there are three main sources of fresh air entering a closed room:
Infiltration (Cracks and Voids)
Any home is bound to have small cracks or gaps around walls, windows, and doors. When the AC or heater is running, this creates a slight pressure change in the room, allowing outside air to enter through these cracks.
HVAC Fresh Air Inlets
Many HVAC systems have separate vents for fresh air. When the system is running, the blower draws fresh outside air through these vents and mixes it with your return air (return vent).
Ventilation Fans and Appliances
Bathroom exhaust and kitchen hood fans regularly exhaust stale air from the room. When they exhaust air, fresh outside air enters the room. Similarly, dryers and other appliances work in a similar way.
Because of these methods, the air in the room is not completely stagnant. Small inlets are constantly occurring. Therefore, even if you close the windows, air circulation isn’t completely stopped.
Overall,
In short, your HVAC system allows some fresh air to enter even a closed room. Whether it is through cracks in the walls, the HVAC system’s inlet, or pressure changes caused by a fan, all of these factors allow outside air to flow into the room. This is why, even in a completely sealed room, the air remains fluid and doesn’t become completely stagnant.
The matter does not end there. Also, keep in mind that the air supplied through these methods may be insufficient, so if fresh air is not available for a long time, it is wise to occasionally introduce additional fresh air by opening a window or using the ventilation system. But the answer to the main question is that when the AC or heater is running, the home’s infiltration, HVAC intake, and vent fan work together to ensure fresh air flows into even a closed room.
