How Much Fresh Air is Enough?

Everyone knows it’s important to introduce fresh air into buildings. However, there are two questions to be answered: How much air does a building need, and if you don’t have the proper measurement tools, how can you be sure you’re delivering it?

First, things first: if you don’t have the air balancing tools, how do you measure the volume of fresh air?

Some field test conditions don't allow for accurate or convenient measurement of outside air cfm. However, there is a simple and fairly accurate method of determining the amount of fresh air by using only a thermometer as your test instrument.

The percentage of outside air can be calculated using these three simple temperature measurements.

OAT- Outside Air Temperature. This is the temperature of the air entering the system or equipment from the outdoors.

RAT - Return Air Temperature. This is the return air entering the equipment. This temperature may be different from the temperature entering the return grilles due to duct loss or gain.

MAT - Mixed Air Temperature. This is the air temperature past the outside air inlet where the temperatures of the return air and the outside air have mixed together. This may be in the return plenum, or in the blower compartment.

The formula is:

Fresh Air % = ((MAT - RAT) / (OAT - RAT)) X 100

Here’s an example of how the formula works. let's say the Outside Air Temperature is 100F, the Return Air Temperature is 75F, and the Mixed Air Temperature is 80F.

Apply the formula:

Fresh Air % = ((80 - 75) / (100 - 75)) X 100

Fresh Air % = (5 / 25) X 100

Fresh Air % = 0.2 X 100

Fresh Air % = 20%

In this example, 20% of the total airflow of the system is being pulled into the system from outside.

Next, to find the fresh cfm, multiply the percentage of fresh air by the fan airflow. Let’s say we have a 3 ton system moving 1,200 cfm. Multiply the 20% fresh air by the 1,200 cfm to find a fresh air cfm of 240.

The final question is how much fresh air is needed? There are many rules of thumb and a host of tests that can be taken and calculations that can be made, but most fresh air calculations stem from a certain cfm per person rate. The rate changes depending on the type of building and the activity or processes going on in them.

The following table was derived from a large volume of mostly commercial air balancing jobs that we completed over a 15 year period. I’ve found it to be a great starting point when determining the fresh air needed in any particular building. Simply select the type of building you are considering adding fresh air to, and estimate the typical number of people occupying the building. Multiply the number of people by the required cfm per person to determine the required fresh air flow.

Building use and required fresh air (in cfm) per person:

  • Homes 5-15
  • Offices 15-20
  • Light commercial buildings 15-25
  • Retail stores 15-20
  • Classrooms 15
  • Churches 15
  • Restrooms 35
  • Conference rooms 20
  • Restaurants 20
  • Restaurants, smoking 25-30
  • Bars 30
  • Exercise rooms 30-40
  • Manufacturing 25-40
  • Dry cleaners 30
  • Hotel rooms 20-30
  • Dance clubs 25-35
  • Makeup air = 80% to 100% of exhaust air

One of the best rules of thumb for residential fresh air requirements is 100 cfm for every 600 to 900 sq.ft. of living space. The number varies depending on the tightness of the home and the outdoor weather conditions. And remember, this article is a brief response to inquiries I've received, and is not an engineering course!

The HRV/ERV Option

Heat recovery ventilators (HRVs) and energy recovery ventilators (ERVs) are equipment designed to facilitate the delivery of fresh air into buildings. The idea is to recapture the heat loss of exhausting conditioned indoor air. The idea is a reasonable principle, and will provide some relief from the energy cost of fresh air.

However, as with all equipment, measured performance is the end result of good engineering and is the final word in performance and efficiency. Look for a balancing procedure article to verify HRV and ERV performance in the next CB Hotmail newsletter.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician and have questions or comments about this article, contact Doc at [email protected] or call him at 800/633-7058. Go to NCI’s website at www.nationalcomfortinstitute.com for free information, technical articles and downloads.

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