CFM calculation in HVAC

 

        In this post we will see about cfm calculation in hvac.

The CFM (Cubic Feet per Minute) number is used to determine how much air can pass through the ducts of your heating system. This measurement determines how efficient your system is at moving heated air throughout your home. To calculate the CFM, multiply your total square footage of the space you are measuring times 0.8. For example, if you have a room that is 10 feet x 12 feet then multiply this number times.8 (80%) and add that amount to the volume of the building.

For instance, if you live in an 800-square foot apartment then the formula would look like this..

800 sq ft x 80% 6400 cubic feet

6400 cubic feet plus the volume of the building equals the CFM.

CFM: Cubic Feet Per Minute

The Cfm calculation is based upon two factors; the first factor is how much energy your home uses, and the second factor is the amount of time that your home spends heating or cooling. This calculation works out the cost per hour spent using energy.

The Cfm calculation takes into account CO2 emissions associated with the equipment being used and compares them to the current average emissions rates from those units. If there are increases in emissions over the past year, then the Cfm rating may increase. A higher number indicates a greater contribution towards global warming, and therefore a need for increased efficiency in order to reduce global warming.

For homes with multiple stories, sound insulation levels are measured as well as overall noise levels. The Cfm rating takes this noise level into consideration and compares it to the current national standard.

CFM is an industry standard unit used to measure cooling capacity. A typical system can run at 100-150 CFM. A higher CFM rating means that your air handler has greater capacity to move air through the house. Air handlers are rated by their cubic feet per minute (CFM) output and the average CFMs vary from around 200 CFM to 350+ CFM.

The calculation factor (CFM), is used to determine how many cubic feet per minute of air are actually moving through the duct. In order to calculate the CFM, you need to know the square footage of the area that is being measured. Next, divide the total number of cubic feet into the number of square feet. Then multiply that amount by.07. This gives you the CFM rate.

How should I calculate CFM?

In order to calculate CFM (Coefficient Of Fractional Mixture), you need to first determine how much air you are using at your current CFM rating. To do this, you can divide your AC's total BTU usage by the number of hours that your AC runs. So if you ran your AC for 40 minutes today, its BTU consumption would equal 600 watts. If you have a 20 amp circuit, then 600 * 0.4 240 Watts. This means that your AC consumes 240 Watts per hour. Using our previous example, 240 Watt-hours equals 120 CFMs. Multiply 120 CFMs by 1.25 to find out how many CFMs you are using from your AC. In this case, 12CFMs are coming from your AC. Now subtract any CFMs that come from other sources like lighting and fans. You want to know what amount of CFMs comes from your AC, not from anything else. Thus, we end up with 12CFMs being used by the AC.

What is the value of 1.25?

The constant 1.25 is called the Coefficient Of Fractional Mixtures (CFM). CFM represents the percentage of air flow relative to the overall intake airflow. By multiplying the CFM by 100, you get the CFM %. In this case, since the AC only provides 12CFMs, multiply that figure by 1.25 to determine how much of the total air supply is coming from the AC.

How much air am I using?

To find out how much of the air you are using, you take the AC's total BTUs divided by the time period that your AC is running. For example, let's say you run your AC for five days and use 1200 watt hours. Your AC uses 1200 Watts / 5 240 Watt Hours per day. Since 240Watt Hours equates to 120 CFMs, divide 120 CFMs by 5 days to obtain the daily CFM averages. We ended up with 24CFMs/day.

Cfm (or cubic feet per minute) refers to air flow rate that can be measured using CFM meters. This is a measure of how much air is being pushed through a duct system at any given time. As air moves throughout your home, it encounters different obstacles (like walls, furniture, etc.) where it changes direction and velocity. By measuring this change in speed, you can determine how many cubic feet per minute of air are moving through a specific space. This number can then be used to calculate how large of a room needs to be to maintain a certain volume of air.

1. Measure the square footage of your total space (room) being cooled/heated.

2. Add together the width, length, and height of the room.

3. Multiply that number by 0.6. This is how much air volume we need per hour.

4. Divide this number by 10. (This represents how many cubic feet of air you need per minute.)

5. Use a conversion chart to convert CFM to LPM.

**CFM Cubic Feet Per Minute. 

**LPM Liters Per Minute.

Density (D) Volume (V) / Area (A)

Volume (V): Take your volume measurements from your building plan and find out the total volume of your space.

Area (A): Measure the area of each vent that needs to be calculated. This can easily be done by using either a tape measure or measuring the width of the opening at its widest point.

Example: If your room measures 10 feet long x 8 feet wide x 6 feet tall, then your volume would equal 10 feet^3 1 cubic foot. Your air handling unit has a maximum airflow rate of 400 CFM.

CFM is short for Cubic Feet Per Minute. To figure this out, divide 400 by 10 40 CFM's per minute. To convert 40 into decimal form, multiply by 0.1, resulting in 4 CFM. So if you have a fan that is capable of delivering 400 CFM's, it means that the fan is running at 100% power. However, fans are not 100% efficient, so if you do the math, you'll realize that 400 CFM actually equals 80%. In other words, if you want to run your fan at 100%, you'll need to divide 400 by 80, resulting in 50 CFM.

A little background information first. In a typical residential application, we have a central air conditioning unit that cools down the whole house. This unit has a fan blade inside that pulls air from the room through duct work and out into the outside environment where it gets cooled before returning back to the room. This cycle continues throughout the day, and at night, this same blade turns off, allowing the indoor air to exhaust directly out into the environment without recirculation. This is how the system works.

Now that you know a bit about how the unit functions, we can move forward and discuss some simple calculations required to get the airflow rates of your home. Using this method, you can easily determine what size blower motor to buy if you are not sure of your exact airflow requirements.

First step is to find the square footage of your home; then multiply this number by 9.5. This will give you the total cubic feet per minute (CFM) of air flow. Then divide this number by 16, which gives you the CFM per hour. If you live in a smaller space, use 15 instead of 16. You would calculate like this:

10 x 9.5 = 95 total CFM per hour. 

Divided by 5 = 19 CFM per hour.

This calculation is accurate but does not take into account any additional factors that may increase the volume of air being moved, including window openings and ceiling height. A good rule of thumb is 1/4 of the ceiling height times 20 percent of the area of windows allows you to approximate the CFM needed based on the size of your home. So a 4 foot ceiling height equals 25% of the area of the windows, therefore 40 CFM per hour is adequate.

If you are unsure of what percentage of ceiling height is allowed in your home, call a local licensed contractor and ask them what they recommend.

1. Airflow rates are calculated by multiplying cubic feet per minute (CFM) by the square footage of the space being ventilated.

2. This calculation should be done over 24 hours.

3. This calculation can then be divided by 1,000 to become the number of CFM's that are needed each hour.

4. To find the average CFM needed per hour, divide this total by 4 and multiply by 100 to get your desired result.

5. If you want to know how many CFM's are needed for only one person, divide the total by 10.

6. Calculate these numbers using CFM's instead of cubic feet if they are more accurate for your situation.

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