Under Pressure

January 03, 2012
January/February 2012
A version of this article appears in the January/February 2012 issue of Home Energy Magazine.
Click here to read more articles about HERS

I find it puzzling that many energy raters and auditors are not curious about what causes the pressure test results they see. Frankly, I find it very discouraging as well, because if they understood the principles behind these tests, they could accurately assess the problem and come up with proper solutions. We find ourselves working backward from a test result to a hypothesis. Instead, we should be trained to spend our time on visual inspections, formulate a hypothesis, and then confirm or reject that hypothesis based on the test results. Unfortunately, most of us simply want someone to feed us an answer based on a test. For heaven’s sake, don’t make us think!

Brett Dillon
is the managing director of IBS Advisors, LLC. Brett has years of experience working as a HERS Rater, quality assurance designee, and rater trainer. He currently chairs the RESNET Technical Committee.

Our industry is under pressure to improve the skills and knowledge (or as my friend Maureen puts it, the “mental wattage”) of our workforce. These improvements will increase the quality and rigor of the service we provide the various programs and our clients.

HERS Raters must have 18 hours of professional development every three years to maintain their credential. The RESNET Training and Education Committee created core areas that we felt needed to be addressed in this professional development: HVAC, business management, communication skills, advanced diagnostics, ethics, insulation grading, advanced energy modeling, and federal programs such as Energy Star. The specific areas for training in advanced diagnostics include static pressure tests, infrared testing and interpretation, flow tests, and combustion safety tests.

At the same time, the RESNET Technical Committee has written a new standard for performing duct leakage and infiltration testing.


I think we should incorporate basic training in physical science into any certification training for raters and auditors, particularly certification training related to pressure testing. Our role as raters and auditors is not simply to record data, but also to solve problems—and for that, we need to understand the science behind what we do. Would you go to a medical doctor who simply ran tests, gave you a prescription, but never tried to find out why you were experiencing the symptoms you had?

Archimedes of Syracuse (287–212 BC) was tasked with discovering whether a goldsmith had committed fraud by creating a crown made of a mixture of gold and silver instead of the pure gold he had been given. While noodling on this problem in his bath, he observed the displacement of the water as he was submerged—leading him to discover the principle of buoyancy. “Eureka!” he shouted, running naked through the streets—something, thankfully, most building scientists do not do today. If an object is more dense than the fluid around it, it sinks; if an object is less dense than the fluid around it, it rises. Evangelista Torricelli (1608–47) discovered the pressure effect of the atmosphere in 1643, and with it the cause of wind. He noted that wind is caused by changes in the temperature, and therefore the density, of air between two adjacent geographic locations. Between Archimedes and Torricelli, we can now think about the stack effect and convective heat transfer, two issues that affect every home we build.

Blaise Pascal (1623–62) followed up on Torricelli’s experiments on atmospheric pressure and discovered that pressure diminishes as altitude increases—a fact that is now reflected in the new RESNET standard for infiltration testing. We will be using altitude correction factors for testing that takes place over 5,000 feet above sea level. Pascal also formulated Pascal’s law, which states that when a given quantity of fluid is contained, it exerts an equal pressure on the interior surface of the container at a 90° angle to the surface. We consider gases to be fluid, and this fundamental principle can be used to help properly design duct systems to move air.

In 1662, Robert Boyle formulated Boyle’s law, building upon Torricelli’s discovery of the relationship between the temperature and the density of air. Boyle made the connection between the pressure of a gas and its volume. As a gas is compressed, it heats up; as it expands, it cools down. This effect is used to produce the phase change in the vapor compression refrigeration cycle used in air-conditioning and heat pump systems in our homes.

Guillaume Amontons (1663–1705) proposed that there was a temperature at which all gases would become liquid, and the gas pressure would be zero. William Thomson Lord Kelvin (1824–1907) later developed a temperature scale based upon that idea, giving us an absolute temperature metric. The relationships among temperature, pressure, and volume were further analyzed by Jacques Charles in 1787, when he discovered that if a gas is heated, it expands (and increases the pressure on the container, if it is contained). In 1802, Joseph Louis Gay-Lussac discovered that under constant pressure, the volume of an ideal gas is proportional to its absolute temperature, as long as the pressure is constant. The Combined Gas law uses the discoveries of Boyle, Amontons, Charles, and Gay-Lussac to describe the pressure-volume-temperature relationship. When pressure is multiplied by volume and the result is divided by the absolute temperature, the quotient is constant. Because of the relationship between pressure and temperature, RESNET is now also including an adjustment based on differences between the outdoor and indoor temperatures when performing infiltration testing.

The new RESNET standard for infiltration testing uses correction factors to account for density differences in the air caused by temperature differences and altitudes greater than 5,000 feet. The pretest baseline reading of pressure differences between the house and ambient conditions is recorded and is then subtracted from the measured building pressure to produce the induced building pressure. Relax—if your manometer has a baseline adjustment feature, the displayed building pressure is the induced building pressure. The test result produced is calculated by this equation:

Nominal CFM50 = (50 / Induced building pressure)0.65 x Recorded fan flow

Again, if your manometer has a baseline adjustment feature and the test is performed with the manometer in @50 Pa mode, the nominal CFM50 is displayed directly on the manometer. However, if the altitude of the home is greater than 5,000 feet above sea level, an altitude correction factor is applied, using this equation:

Altitude-corrected CFM50 = Nominal CFM50 x (1+ [0.000006 x Altitude in feet])

If we have a home in Idyllwild, California, that has a nominal infiltration rate of 1,235 CFM50 tested in the summer at an altitude of 5,253 feet, the

Altitude-corrected CMF50 is Altitude-corrected CFM50 = 1,235 x (1 + [0.000006 x 5,253]) = 1,273.92.

Now we have compensated for the decrease in atmospheric pressure discovered by Blaise Pascal.

To adjust for temperature differences greater than 30°F, we use temperature correction factors for pressurization and depressurization (yes, they are different) that are calculated based upon ASTM Standard E779-10. These factors are listed in a table incorporated into the RESNET standard; the equation used is

Temperature-corrected CFM50 = Nominal CFM50 x Temperature correction factor

If we move that home to San Antonio, Texas, and test it with an indoor temperature of 70°F and an outdoor temperature of 105°F, the temperature-corrected CFM50 is

Temperature-corrected CFM50 = 1,235 x 1.059 = 1,307.86.

This accounts for Boyle’s law, which predicts that warm gases will have a higher volume than cold gases, and compensates for the standardized air density assumed by the manometer when it performs the traditional calculation to arrive at the nominal CFM50. Let’s place that same home in Aspen, Colorado, and test it in the winter. Now we have to correct for altitude (8,500 feet) and temperature (5°F outside, 70°F inside), using this equation:

Corrected CFM50 = Nominal CFM50 x Altitude factor x Temperature factor Corrected CFM50 = 1,235 x (1 + [0.000006 x 8,500]) x 0.892 = 1,157.80

Changes in temperature and altitude have a real effect on natural infiltration, and these calculations (which are to be incorporated into the RESNET-accredited software tools) are an attempt to gain repeatability while accounting for air density changes.

In 1738, Daniel Bernoulli (1700–82) described the effect of volume on pressure (the Bernoulli effect). When the volume of moving air goes up, the pressure drops. Pressure is caused by gas particles colliding with the surface of their container. If the container has a larger volume for a given quantity of gas, the particles spend more time traveling and less time colliding with the container surface. If the container has a smaller volume for the same quantity of gas, the particles spend more time colliding with the container surface and less time traveling, causing the increase in pressure (and temperature) described by Boyle. This led Bernoulli to describe static pressure as the pressure applied on the container as these gas particles bounce around.

We can measure static pressure with the pitot tube, thanks to Henri Pitot, the engineer charged with calculating the flow rate of the Seine. Pitot invented this instrument in

1732, when he was working out the relationship between pressure and velocity. Giovanni Venturi further explored that relationship in 1797, when he included the cross-sectional area of the container in calculating flow. Venturi discovered that if the cross-sectional area of a container with gas flowing through it is decreased, the velocity goes up, but the static pressure drops. Venturi and Bernoulli described the effects of volume, velocity, and cross-sectional area on pressure. Because of the work these gentlemen did in the 1700s, we can design duct systems that deliver air effectively, and we can test the flow rates through those systems.

Unintended Effect of a Damper
Unintended Effect of a DamperFigure 1. This illustrates looking down at a horizontal slice of a duct with the damper position vertically. The position of the damper relative to the direction of the forced air movement is most important. Installing the damper as above actually causes air to be sucked out of the branch back into the trunk.

A classic mistake made by many HVAC installers is placing manual dampers vertically at the trunk-branch connection. They think that if they install the damper vertically, it will scoop air from the trunk into the duct. However, due to the Bernoulli effect, installing the damper in this position actually causes air to be sucked out of the branch back into the trunk (see Figure 1). The velocity of the air moving past the damper creates a negative pressure zone behind the damper, drawing air back out and decreasing the flow rate delivered to the destination. Placing the damper horizontally alleviates that situation and, according to my HVAC friends, increases the airflow up to 30%. A properly designed and installed duct system will work by taking advantage of three factors:

  • The Bernoulli effect (horizontal dampers).
  • Pascal’s law (no branches in the last 1–2 feet of trunk). The last 1–2 feet of trunk needs to be takeoff free, so that pressure can build up in the trunk to send air down the takeoffs upstream. Envision what happens with a garden soaker hose. As the water hits the end cap, pressure builds up, pushing water out of the pores in the hose at a steady pressure (Pascal’s law). If the end cap has a hole in it, the water comes out in the direction of the force and less water oozes out through the pores.
  • The Venturi effect (reducing trunks to maintain pressure after volume has been decreased with branches). Bernoulli deals with airflow across an opening, creating a negative pressure relative to the direction of the airflow (crack your car window while going 60 mph and watch the pesky fly get sucked out). Venturi deals with increasing velocity due to decreased cross-sectional area (the effect of putting your thumb over the end of a garden hose). When a trunk has the same cross-sectional area for its entire length, the volume of air per time decreases after every branch, decreasing pressure and velocity. By reducing the trunk after a certain CFM of air has been released down branches, the velocity and pressure can be maintained.

By understanding the scientific laws and principles that govern the causes of the symptoms we see, we can better diagnose and treat the cause instead of the symptom. The pressure testing we do becomes something far more than finding a number to plug into the software model. It becomes a way for us to confirm our hypothesis based upon the evidence we see when dealing with stack effect, wind, and mechanical systems. It also becomes a means of predicting what is going to happen, allowing us to develop scenarios for our clients that lead to solutions, instead of throwing money down every rat hole.

learn more

Contact the author at BDillon@IBSAdvisorsllc.com.

I would like our industry to move to a more scientific, method-based approach to diagnosing building performance problems and predicting performance under actual conditions. RESNET is creating processes for performing infiltration, duct leakage, and combustion appliance zone testing that account for the laws that govern air pressure. And RESNET is accrediting education programs that support a deeper understanding of the science behind what we do.

Understanding the laws that govern pressure; increasing that mental wattage in our workforce so we can use test results to verify our hypotheses; increasing our ability to perform a differential diagnosis and arrive at the correct and least expensive solution—all these abilities are priceless.


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