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The History and Application of Ventilation

The oil embargo of 1974 caused major changes in the way buildings are constructed.  Prior to 1974, many buildings had enough air leakage through windows, doors, and even walls, to provide adequate ventilation.  The cost of fuel to condition this “leaked” air was so low that the additional expense to prevent this leakage provided little or no payback.  The oil crisis of 1974 changed all that.  Electricity, gas, and oil prices soared.  Building engineers and designers sealed up the building shell to reduce infiltration and exfiltration of air.

Unfortunately, there was a negative effect.  Indoor air quantity had been addressed, but indoor air quality (IAQ) was overlooked.  It wasn’t until 14 years later, in 1998, that the “sick” building syndrome was recognized.  Buildings constructed between 1974 and 1988 were not adequately ventilated.  Workers in many of these buildings experienced headaches, dizziness, drowsiness, fatigue, eye irritation, and respiratory disease.  A building that was “sick” in air quality almost always had 20% or more of the occupants displaying one or more of the mentioned symptoms.  A large number of people with these complaints reported relief upon leaving the building. 

Ultimately, these building-related illnesses were attributed to airborne pollutants within the building.

The American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) consists of people who design and establish standards for mechanical equipment used to heat, cool, and ventilate buildings.  ASHRAE formed a committee to study sick buildings, and in 1989, published standards for all building designers to use.  These standards are guidelines, not legal requirements.  However, most state and local ventilation codes use the ASHRAE standards as the basis for their codes.  The standards are also used in litigation involving IAQ lawsuits.  The standards set minimum ventilation rates in CFM required per person per type of environment.  (Always check local codes for ventilation rates for a specific building.)

Poor IAQ is not always design related.  Blocked outdoor air dampers, defective actuators, or incorrectly set control systems may cause inadequate ventilation.

Most maintenance personnel will respond to occupant complaints concerning IAQ with only temperature in mind.  If a space is within some acceptable range of temperature, it is felt no adjustments are needed.  Complaining occupants do not usually have the knowledge to say “open the outside air dampers” or “increase the supply CFM to this room.”  The service technician needs to be able to recognize inadequate ventilation and know how to correct it!

Two simple formulas can be used to determine how much outside air is needed and how much is being brought into a building through its air handler or handlers.

Most air handlers have some kind of economizer control system.  They should have a minimum position control that can be adjusted to ensure proper ventilation.  An example will best illustrate proper ventilation and how to get it.

ASHRAE recommends 20 CFM per person as a ventilation rate for occupants of an office building.  In order to find the minimum position of the damper or dampers to supply this amount of outside air, the needed percentage of outside air must be determined.  Multiplying the maximum number of people in the area served by the air handler by the volume of outside air per person, and then dividing by the total fan volume of the air handler will find this.

If 250 people are in the office building and the air handler’s total fan volume is known to be 20,000 CFM:


             250 People  x  20 CFM Per Person  =  5000 CFM



Now to adjust the minimum position control to get the required 5000 CFM, or 25% of the air handler’s capacity, two temperatures need to be taken, the return air temperature and the outside air temperature.  It is best to do this on a cold day when the two temperatures are far apart.  If the DT is only a few degrees, a small error in measurement can result in a large error in results.  If the DT is 30°F, 40°F, or more, a small error in measurement will not significantly affect the results.

The previous calculation found that 25% O.A. was needed.  This leaves 75% of the air handler’s fan volume for return air.  We now have all the information needed to set the minimum position control to supply the required minimum ventilation rate.  The formula for this is:


If, in the example, the return air temperature was 74°F and the outside air temperature was 30°F:

                                  [74°F x 75%] + [30°F x 25%] = 55.5°F + 7.5°F = 63°F

Adjusting the minimum position control until the mixed air temperature is 63°F results in the required 5000 CFM ventilation rate.

Taking only three temperatures and using two simple formulas can ensure proper ventilation.

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