| ASHRAE Minimum Ventilation Rate | ~0.35 ACH or 15 CFM per person (whichever is greater) (ASHRAE Standard 62.2) |
| Typical Bathroom Fan Size | 1 CFM per square foot of bathroom area (Home Ventilating Institute (HVI) general guidance) |
| ERV vs. HRV Climate Fit | ERV for dry/mixed climates; HRV for humid climates (General HVAC industry guidance) |
| Permit Requirement | Whole-home ventilation installations typically require a permit (Verify with local building authority) |
Why Ventilation Has Its Own Vocabulary
When a contractor walks through your home and starts mentioning ACH, ERV, or CFM, it can feel like a different language. That unfamiliarity often leads homeowners to nod along without fully understanding what they're agreeing to — or paying for.
Ventilation is not the same thing as heating or cooling. As explained in our article on how ventilation and HVAC work together, your HVAC system manages temperature while ventilation manages the exchange of air. Both matter for comfort and health, but they serve distinct functions. This glossary gives you the language to discuss both clearly.
Use this reference before your next contractor visit, and you'll be better prepared to ask the right questions — and understand the answers.
ACH (Air Changes per Hour)
A measurement of how many times the entire volume of air in a room or home is replaced within one hour. Building codes and ASHRAE standards often specify minimum ACH levels for different spaces to maintain acceptable indoor air quality.
CFM (Cubic Feet per Minute)
A unit measuring the volume of air moving through a system every minute. Contractors use CFM to determine whether a fan, duct, or ventilation unit is appropriately sized for a given space.
ERV (Energy Recovery Ventilator)
A ventilation device that brings fresh outdoor air inside while transferring both heat energy and moisture from the outgoing stale air. ERVs help maintain indoor humidity levels and reduce energy loss in tightly sealed homes.
HRV (Heat Recovery Ventilator)
Similar to an ERV, but transfers heat only — not moisture — from exhaust air to incoming fresh air. HRVs are generally preferred in climates where indoor humidity needs to be managed downward, such as humid regions.
Makeup Air
Replacement air intentionally supplied to a space to compensate for air that has been exhausted. Without makeup air, powerful exhaust systems can create negative pressure, which may pull combustion gases back into living spaces.
Negative Pressure
A condition in which air pressure inside the home is lower than outside, causing uncontrolled air infiltration through gaps, cracks, and openings. It can affect comfort, air quality, and the safe operation of gas appliances.
ASHRAE Standard 62.2
A widely referenced guideline published by the American Society of Heating, Refrigerating and Air-Conditioning Engineers that specifies minimum ventilation rates for residential buildings in the United States.
Spot Ventilation
A targeted ventilation strategy that removes pollutants or moisture from a specific area — such as a kitchen or bathroom — using a localized exhaust fan rather than a whole-home system.
Whole-Home Ventilation
A coordinated ventilation approach that controls fresh air supply and stale air exhaust across the entire house, often integrated with or alongside the central HVAC system.
Static Pressure
The resistance to airflow within a duct system, measured in inches of water column (in. w.c.). High static pressure can strain equipment and reduce efficiency; contractors check it to verify the duct system is properly sized.
Exhaust-Only Ventilation
A simple ventilation strategy that uses fans to pull stale air out of a home, relying on natural infiltration to supply replacement air. It is inexpensive but can contribute to negative pressure if not carefully designed.
Balanced Ventilation
A ventilation approach that supplies and exhausts roughly equal amounts of air, maintaining neutral pressure inside the home. ERVs and HRVs are common examples of balanced ventilation equipment.
Key Terms You'll Encounter During Any Ventilation Discussion
Beyond the glossary definitions, it helps to understand how these terms appear in practice. Here's what to keep in mind when you hear them used on-site or in a written estimate.
ACH and CFM: Measuring Air Movement
ACH (Air Changes per Hour) tells you how many times the full volume of air in a space is replaced within one hour. A higher ACH can improve air quality but may also increase energy use. CFM (Cubic Feet per Minute) is a flow-rate measurement — contractors use it to size fans, ducts, and vents to the square footage of your home.
ERV vs. HRV: Choosing the Right Recovery Unit
Both an ERV (Energy Recovery Ventilator) and an HRV (Heat Recovery Ventilator) bring fresh air in while capturing energy from outgoing stale air. The difference: an ERV also transfers moisture, which is useful in dry climates or tightly sealed homes. An HRV exhausts moisture with the stale air, making it a better fit for humid climates. Ask your contractor which is appropriate for your region and home type.
Makeup Air and Negative Pressure
When powerful exhaust fans (like a range hood) push air out without a planned replacement source, your home can develop negative pressure — meaning air gets pulled in through gaps, cracks, and even combustion appliances like gas furnaces. Makeup air systems are designed to supply controlled replacement air and prevent this imbalance. It's a safety concern worth discussing explicitly. For more on how filters interact with these systems, see our air filter types and ratings reference.
Spot vs. Whole-Home Ventilation
Spot ventilation addresses a single area — a bathroom exhaust fan, for example. Whole-home ventilation is a coordinated system that manages air quality throughout the entire house. If a contractor recommends one over the other, ask them to explain why based on your home's layout and existing ductwork.
When you're ready to sit down with a contractor, our guide on what to ask before agreeing to a ventilation upgrade walks you through the right questions to protect yourself from poor outcomes.
This article is for informational and educational purposes only. Ventilation work often requires permits and must be performed by licensed professionals. Always verify local code requirements with your contractor or municipal building department before beginning any project.