How Much Does a Carbon Filter Reduce CFM?

When cleaning up the air in a space, you must consider many factors before choosing an air purification system. The fan size and CFM (Cubic Feet Per Minute) are necessary values you should know if you’re using a carbon filter. So, how much does a carbon filter reduce CFM?

A carbon filter reduces CFM by about 25% due to the airflow resistance the filter presents. This percentage reduction is vital to note when selecting a fan that’ll meet the airflow requirements of your space. The thousands of pores on the filter may be responsible for this resistance to airflow.

This article delves into whether your air purifier carbon filter reduces CFM. I also discuss how much a carbon filter reduces CFM and ways to calculate CFM for an air filter.

Does My Air Purifier Carbon Filter Reduce CFM?

The US EPA’s (Environmental Protection Agency) IAQ (Indoor Air Quality) report states that the average American spends approximately 90% of their time indoors. However, the report also says that some pollutants are often two to five times more concentrated indoors than outdoors. For this reason, every household needs a method to improve their indoor air quality.

Air purifiers decrease or eliminate air pollutants like smoke, dust, pollen, VOCs (volatile organic compounds), pathogenic microorganisms, and other allergens. Their basic design and inner workings make them able to achieve this. Nonetheless, air purifiers need to do two things to clean the air effectively:

Create airflow through the air purifier: This airflow is measured in CFM, and the unit must suck in the unclean air before removing the air pollutants. Air purifiers use powerful fans to suck in air from all corners of the room.

Use effective filters: The filters are the “stars” of the air purifier’s cleaning system. They trap the pollutants in the air the device pulls in, and the purifier expels clean air by continuous airflow.

The fan speed and airflow (measured in CFM) and filters’ airflow capacity determine how fast and efficiently a purifier works. Also, filters capture pollutants differently, and no filter can capture all pollutants. Hence, many air purifiers use one or more filter types to tackle a wide variety of contaminants at once.

Your air purifier with a carbon filter is likely a hybrid filter that combines the carbon filter with a mechanical filter like HEPA (high-efficiency particulate air) filter. If an air purifier has a greater filtration efficiency, it can reduce the device’s airflow (CFM).

Simply put, air passing through a filter needs adequate time to interact with the filter, enabling it to remove enough pollutants per cycle. The more the filter layers (like a hybrid air purifier) or the finer the filter media, the greater the reduction in airflow.

Summarily, your air purifier carbon filter can reduce CFM, especially with thin carbon filters. However, this doesn’t pose a significant issue with the regular use of your air purifier.

How Much Does a Carbon Filter Reduce CFM?

Carbon filters comprise beds of activated carbon (most commonly), with which they trap odors, gases, and particles. You’ll find carbon filters in furnaces and air conditioning systems, where they trap contaminants and allow clean air to recirculate in the room.

They play a crucial role in ventilating indoor cultivation environments, usually as part of an air exhaust system. 

CFM (cubic feet per meter) is the most common way to measure airflow, and it’s important when estimating the ventilation requirements of a grow room. It determines how much cubit fit of air a fan can exchange or move in a minute and how quickly air flows through a purifier. Higher CFM ratings correspond to a faster airflow, meaning more air changes per hour.

Without a target CFM, you’d be unable to find a suitable filter, and you can’t match it with the correct fan rating.

It might seem light a lot to wrap your head around, but it’s pretty straightforward once you understand each parameter separately. A good grasp of what CFM and carbon filters represent makes it easier to understand their mutual connection.

How Carbon Filters Work

Like other filter types, carbon filters capture unwanted odors and dust particles. They use various materials to trap these pollutants, but activated carbon (charcoal) is the most common choice.

Activated carbon is slightly modified regular charcoal with hundreds of pores for trapping molecules. The modification process primarily involves injecting hot steam or air into the charcoal (oxygen), creating millions of tiny pores between the carbon atoms.

It’s like making popcorn from corn kernels. Charcoal is the dry corn kernel, and activated carbon is the fluffy, light popcorn with a larger surface area. The massive surface area of pores traps molecules in the air by adsorption.

Adsorption allows dirt, odor, and dust molecules in the air to stick to the carbon atom and form a bond. Because of the bond, the molecules cannot travel back into the air freely. The exhaust fan in a grow room forces all the air through the filter, trapping the contaminants on the carbon filter.

Whether it’s a greenhouse, a grow room or a growing tent, bad odors and toxic chemicals like VOCs would make the space unusable without carbon filters.

A grow room or tent is an indoor location designed to house horticulture in a controlled environment. Components of the grow room system include ventilation and odor control, light fixtures, temperature and moisture control, and a hydroponic system. Grow rooms find extensive application in the commercial and personal cultivation of cannabis.

Recall that ventilation and odor removal is of the utmost importance in a controlled horticulture environment. Getting accurate measurements ensures you provide adequate ventilation for the room size. It’s ideal to consider all potential factors that can reduce the required measurements. 

One of such factors is the carbon filter used in the ventilation system. Without calculating the minimum CFM required for exchanging air in a grow room, your fan size might be smaller or larger. If the fan cannot extract the correct amount of air per minute, it’ll compromise the grow room’s ventilation.

The CFM value reduces by 10% to 25% because of the extractor fan’s airflow resistance when pulling air through the carbon filter. The CFM, in this case, represents the fan’s efficiency. You’d need to add  25% more CFM rating to your carbon filter so that the fan can extract the required amount of air per minute.

For example, if your initial CFM rating were 266 CFM, the actual rating you need would be: 

266 CFM x 25% (1.25) = 332.5 CFM

Knowing this reduction allows you to calculate better the minimum CFM required for proper ventilation. Other factors that reduce the CFM or fan efficiency in a grow room ventilation system include:

  • The ducting material and length: For example, a straight hard cast 25 feet ducting may reduce the fan’s CFM efficiency by 3%.
  • The ducting angle: The number and sharpness of bends in a ducting setup affect the airflow resistance since airflow reduces the further it must travel. So, a straighter duct route minimizes airflow resistance, and a sharper bend increases the efficiency reduction. A 30° angle reduces airflow by 20%, and a 90° bend can reduce it by 60%

In essence, the size, diameter, and type of connector or duct can affect a carbon filter’s resistance to the fan’s performance at extracting air.

How Do You Calculate CFM for Air Filter?

I cannot overemphasize the importance of correctly ventilating your greenhouse or grow room. Proper ventilation caters to some needs, including humidity, temperature, fresh/stale air exchange, and disease and odor control.

Designing a ventilation system can be challenging because of the numerous known variables and complex calculations. With a focus on carbon filters for this kind of ventilation, here’s how to calculate CFM for air filters:

  • Room volume: You need to calculate the room volume by multiplying the room’s length x width x height. For example, a space that’s 9 inches x 9 inches x 9 inches will have a volume of 729 cubic feet.
  • Required CFM: This value is essential for determining the fan size based on how often the air exchange should occur in the room. Suppose you expect one air exchange every three minutes. Therefore, 729 cubic feet/3 minutes = 243 CFM.

The CFM value here will be the absolute minimum required for exchanging air in the room. 

Unfortunately, that minimum value would be insufficient for ventilating a room of that size if you don’t factor in some variables. The additional variables are:

  1. CO2—add 5% if the room has CO2 enrichment.
  2. Carbon filter—add 25% if the exhaust system uses a carbon filter.
  3. Ambient temperature—add up to 40% for humid climates (like Florida) and 25% for hot climates (like Arizona).
  4. The number of HID lights—add 10-15% for each non-air cooled light and 5% for each cooled light.

If the 729-cubit feet room has 1 x 1000 Watt air-cooled light, a carbon filter, CO2 enrichment, and uses air from an air-conditioned room, here’s the actual CFM:

(243 CFM) + (243 x 5% (cooled light)) + (243 x 5% (CO2)) + (243 x 25% (carbon filter)) + (243 x 0 (no ambient temperature)) = 328 CFM

328 CFM is the accurate minimum CFM required to ventilate a room of 729 cubic feet. Once you have this value, you can select a fan closest to this CFM rating. Again, the fan should also match the carbon filter to facilitate a neat fit.