Bathroom Ventilation

Determining the required Air Changes per Hour (ACH) for a bathroom exhaust fan starts by first calculating the necessary air movement in Cubic Feet per Minute (CFM), as this is the standard unit for fan ratings.

The common recommendation for residential bathrooms is 8 Air Changes Per Hour (ACH) for intermittent spot ventilation. This is typically achieved using one of two methods, depending on the bathroom size:

1. The ACH-Based Calculation Method

This method directly calculates the CFM needed to achieve a target of 8 ACH.

8 ACH requires a turnover of air once every 7.5 minutes.

Take the cubic feet of the space (L x W x H) and divide by 7.5

Average bathroom is 8L x 5W x 8H = 320 Cubic Feet

320 CF / 7.5 = 42.67 CFM

2. The Area- and Fixture-Based Methods

• A. For Bathrooms ≤100 Square Feet

Use the Area Method, which is roughly equivalent to 8 ACH for an 8-foot ceiling:

Minimum Requirement: Fans must provide at least 50 CFM for intermittent operation, even in small bathrooms.

• B. For Bathrooms >100 Square Feet

Use the Fixture Method to account for the larger amount of moisture generated:

For instance;

Each Toilet = 50 CFM

Each Shower = 50 CFM

Each Standard Tub = 50 CFM

Each Jetted Tub = 100 CFM

As an example for a standard 5' x 8'  bathroom that contains both a toilet as well as a standard tub/shower combination:

This would call for 100 CFM (roughly 2x what the ACH - based method called for)

Note: For bathrooms with ceilings higher than 8 feet or long, complex duct runs, the calculated CFM may need to be adjusted upward to account for the larger air volume and fan performance degradation due to static pressure. Always select a fan based on its rating at 0.25 in. w.g. static pressure, as this is more representative of real-world duct installation performance. 

Bathroom Exhaust Fan Placement

The primary goal of a bathroom exhaust system is to remove moisture and any associated contaminants (odors) as efficiently as possible. Achieving this requires a strategic fan location that maximizes the capture of humid air.

1. Near the Source of Moisture

The fan should be located as close as possible to the primary source of wet, humid air, which is typically over or directly adjacent to the shower or bathtub.

• Rationale: By placing the fan directly above the shower/tub, it captures the warm, moisture-laden air before it has a chance to fully disperse and condense on cooler surfaces (walls, mirrors, ceilings) throughout the rest of the room. This direct capture is the most effective method for immediate humidity control.

2. Maximizing Airflow Sweep (Away from Makeup Air)

Your suggestion to place the fan as far as possible from the source of makeup air (the air entering the room) is fundamentally correct from an air movement perspective.

• The Concept: Effective ventilation relies on establishing a predictable air current that sweeps across the room.

  • Makeup Air Source: In most bathrooms, the makeup air enters through the gap under the door (or a dedicated vent).

  • The Path: Positioning the fan near the shower and at the opposite end of the room from the door ensures the incoming (dryer) air must travel across the entire room, passing the wet zone before being exhausted. This setup creates a more complete air exchange, effectively removing contaminants and refreshing the air in the whole space.

Why Not the Center?

Placing the exhaust fan in the center of the room is often an aesthetic choice but is less efficient from a scientific standpoint.

• Dilution vs. Capture: A center-mounted fan primarily dilutes the moist air with drier air, rather than directly capturing the highest concentration of humidity at its source.

• Reduced Sweep: If placed centrally, the fan is close to both the source of moisture and the source of makeup air, leading to a phenomenon called short-circuiting. Air enters the room and is immediately pulled to the center fan without adequately sweeping the moisture-generating area, thus leaving pockets of stagnant, humid air behind.

In nearly all standard residential applications, a bathroom exhaust fan should not be set up near the floor. The ideal placement is high up, on the ceiling or high on a wall, and as close as possible to the primary source of moisture (the shower/tub).

Here is a breakdown of the reasons why:

1. The Physics of Moisture and Heat

• Hot, Moist Air Rises: The excess moisture in a bathroom—steam from a shower—is initially composed of hot, saturated water vapor. As a hot gas, it is less dense than the cooler, drier air in the room, causing it to naturally rise toward the ceiling.

• Targeting the Source: The fan's primary job is to capture and exhaust this humid air before it has a chance to cool, condense on surfaces, and cause damage. Placing the fan high up allows it to immediately draw in the rising steam, making the process highly efficient.

2. Airflow Dynamics

• The Best Airflow Path: A properly placed fan draws makeup air from the entire room. In a bathroom, this makeup air usually enters through the gap under the door.

  • Placing the fan high up and far from the door (ideally near the shower) ensures the airflow path travels across the entire room, pulling fresh air in low and sweeping the moist air out high, maximizing air exchange and minimizing "dead zones."

• Near the Floor is Inefficient: A fan near the floor would mainly be drawing in the cooler, drier air that is naturally at the bottom of the room, leaving a stagnant, humid layer trapped at the ceiling.

3. Practical and Safety Concerns

• Dust and Debris: A fan near the floor would be much more susceptible to drawing in hair, dust, lint, and other debris, which would clog the fan grille and the ductwork much faster, reducing performance and requiring more frequent, difficult cleaning.

• Electrical Code: Electrical codes place specific restrictions on the placement of electrical components, especially those that are switch-operated, near the floor. Placing an appliance at floor level often poses safety and code-compliance challenges.

• Odor Removal: While moisture removal is the primary goal, a high fan location is also effective for pulling lighter, warmer air with odors up and out of the room.

The Exception (For Heavier Contaminants)

In industrial or laboratory settings, ventilation is sometimes positioned low to remove heavy gases or vapors (contaminants denser than air). Since the primary contaminant in a residential bathroom (steam/water vapor) is lighter than air, the rule remains: place the exhaust fan up high.

Installing a humidistat-triggered switch (often available as a wall control) is a highly recommended practice for residential bathroom ventilation as it ensures proper ventilation when it is needed and prevents excessive, wasteful operation when it is not.

Here is a breakdown of the key reasons why this is a wise strategy:

1. Automated Source Control (Addressing the "Forgotten On")

The primary failure mode of a standard exhaust fan is human error—the user forgets to flip the switch before or immediately after a shower.

• The Humidistat Solution: The humidistat is a moisture-sensing device that continuously monitors the relative humidity (RH) in the bathroom. When the RH rises rapidly (like at the start of a hot shower) or crosses a pre-set threshold (e.g., 60% RH), the switch automatically triggers the fan to turn on.

• Building Science Benefit: This provides immediate, consistent source control. It ensures the fan begins removing moisture at the very beginning of the steam generation process, preventing water vapor from having a chance to migrate and condense on cooler surfaces (like gypsum walls, wood trim, and behind mirrors) where it causes mold, mildew, and material decay.

2. Energy Efficiency and Optimized Runtime (Addressing the "Excessive Run")

A common recommendation is to run the fan for 15 to 20 minutes after a shower to clear residual moisture. However, a person using a manual or simple timer switch often runs the fan for far too long—or not long enough.

• The Humidistat Solution: The fan runs only until the humidity level drops back down to the target level (the "set-point") and then shuts itself off automatically.

• Building Science Benefit: This optimizes runtime.

  • In a large bathroom or a high-humidity climate where it takes 30+ minutes to dry out, the humidistat ensures the fan runs for the full required duration.

  • In a small bathroom or dry climate where the air clears quickly, the fan shuts off early, preventing unnecessary motor wear, heat loss, and wasted electricity. This balances the need for moisture removal with energy conservation.

3. Protection Against External Humidity

The humidistat provides a protective function beyond just showering:

• Continuous Baseline Control (Optional Mode): Many modern humidistat switches can be set to operate in a "low-speed continuous" mode, or simply respond to general room moisture. This is especially useful in humid basements or bathrooms in high-humidity seasons, where the RH can climb even without a shower. The fan will cycle on intermittently to prevent long-term moisture buildup in the building materials.

The necessity of makeup air for a bathroom exhaust fan to function is governed by basic physics: a fan cannot draw air out of a space unless an equal volume of air can be drawn in to replace it.

When an exhaust fan turns on, it creates negative pressure inside the bathroom. For the fan to efficiently flush out hot, humid air with fresh, drier air, it must overcome this pressure. If the bathroom is sealed tightly (no pathways for air entry), the fan will struggle or fail entirely to move the air, leading to the following:

• Ineffective Ventilation: The fan will run, but its actual airflow (CFM) will be drastically reduced, meaning the moist air remains trapped and cannot be replaced.

• Structural and Health Risk: This lack of exchange allows steam to condense, promoting mold growth, peeling paint, and eventual structural damage.

The pathways that allow for proper makeup air include:

1. Door Undercut (or Gap): A necessary 1/2-inch to 3/4-inch gap under the bathroom door provides the primary and most common path for replacement air to be drawn from the adjacent house areas.

2. HVAC Supply Register: The supply air register, even when not providing conditioned air, allows a path for air to be drawn through from the rest of the structure.

Without an adequate opening, the fan essentially chokes, leaving the hot, humid air trapped inside the room where it condenses & leaves surfaces wet.

Testing To Confirm Performance:

As mentioned above, sometimes complicated installations such as long runs, damaged/crushed vent lines, stoppages from bird nests, or any other reason can restrict a properly sized exhaust fan. On the other hand, we can't assume that a homeowner or their contractor (Even HVAC professionals may use the wrong formula for choosing the appropriate sized exhaust.

The most accurate way to test the actual Cubic Feet per Minute (CFM) of an installed bathroom exhaust fan is with specialized HVAC testing equipment, such as an exhaust fan flow meter (flow hood).

Here is an explanation of the professional method, along with a simple way to check if the fan is moving air at the correct volume.

Professional CFM Test Method

The professional method for testing an installed fan uses a flow hood or flow pan connected to a pressure and flow gauge (like a digital manometer). This equipment is typically used by HVAC technicians or home energy auditors.

1. Close the Bathroom Door: The system must be checked as it will operate. People rarely shower with the bathroom door open, so testing must be conducted with the door shut to get an accurate test. 

2. Set Up the Gauge: The pressure and flow gauge is calibrated and set to the correct function for fan flow measurement.

3. Cover the Fan: The flow hood is a large, square or rectangular piece of fabric or rigid material on a frame that is placed directly over the exhaust fan's grille. This essentially captures all the air being drawn into the fan.

4. Read the Measurement: Once the fan is running and the hood is in place, the gauge will provide a direct and accurate reading of the air flow in CFM.

This method accounts for all system resistances, such as duct length, bends, and external vent caps, which is why it provides the true installed CFM, which is often lower than the fan's advertised rating.

CFM relative to a predetermined volume: CMHC Garbage Bag Airflow Test

The trash bag test, often called the CMHC Garbage Bag Airflow Test, is a simple, low-cost way to estimate the cubic feet per minute (CFM) of an exhaust fan. The deflation method for exhaust fans is generally considered less accurate than the inflation method used for supply registers, but it can still provide a good approximation.

Here is how you would test the CFM by deflating an inflated 35-gallon trash bag:

1. Convert the Bag Volume to Cubic Feet

First, use the information you already have to get the volume of the 35-gallon bag in cubic feet (ft3):

Volume of bag ~ 4.68 cubic feet

2. Prepare and Time the Deflation Test

Follow these steps to conduct the test:

1. Close the Bathroom Door: The system must be checked as it will operate. People rarely shower with the bathroom door open, so testing must be conducted with the door shut to get an accurate test. 

2. Prep the Bag (Optional but Recommended): Use a wire coat hanger or a piece of cardboard to create a rigid opening for the trash bag and secure the bag to it with tape. This helps keep a good seal against the fan and maintains the bag's shape.

3. Inflate the Bag: Fully inflate the bag with air. Do not tie it shut.

4. Position the Bag: Turn on your bathroom exhaust fan. Hold the rigid, open end of the fully inflated 35-gallon bag tightly and squarely against the fan's exhaust grille. It is crucial to create the best possible seal around the grille to ensure all the air is pulled out through the fan.

5. Start Timing: Start a stopwatch or timer the moment you place the bag tightly against the grille.

6. Stop Timing: Stop the timer the moment the bag is completely deflated and suctioned flat against the fan grille.

7. Repeat: For a more reliable estimate, repeat the process at least three times and calculate the average deflation time (t).

3. Calculate the Fan's CFM

CFM is defined as cubic feet per minute. Your measured time (t) is in seconds. The formula to calculate the fan's CFM is:

CFM  = Bag Volume (Cubic Ft)/ time (Seconds) x 60 seconds/ 1 minute

A 35 gallon trash bag has an approximate volume of 4.68 cubic feet.

4.68 cubic feet x 60 seconds = 280.8 (how many CFM required to evacuate the bag in 60 seconds)

So if the average deflation time was 4 seconds

280.8 / 4 = 70.2 CFM

If the average time was 8 seconds

280.8 / 8 = 35.1 CFM

So for small bathrooms you want to see about 5.5 seconds which means the fan is removing just a bit over 50 CFM

For larger bathrooms, you'll want to see about 2.75 seconds which means the fan is removing just a bit over 100 CFM

Simple Function Check

While this doesn't provide an exact CFM number, this is a quick way to check if the fan is working well enough to move air and clear moisture.

The Toilet Paper Test 

This is a simple qualitative test to see if the fan is generating enough suction.

1. Close The Bathroom Door: The system must be checked as it will operate. People rarely shower with the bathroom door open, so testing must be conducted with the door shut to get an accurate test. 

2. Turn on the Fan: Ensure the exhaust fan is running.

3. Hold Up the Paper: Take a single sheet of toilet paper or a lightweight paper towel.

4. Check for Suction: Hold the paper up to the fan grille.

5. Result: A properly functioning fan should hold the paper to the grille on its own suction when you let go. If it immediately falls, the fan's performance is likely compromised due to a weak motor, a clog, or a disconnected duct.

Why Actual CFM is Important

The rated CFM on a fan's box is often measured in a laboratory with zero static pressure (no ductwork). An installed fan's actual performance will be lower because the ductwork, bends, and vent cap create static pressure (air resistance). Testing the installed CFM gives you a realistic measure of your ventilation.