In Denver, People Work Harder To Breathe but HVAC Equipment Doesn’t
Joel Erway posted on October 24, 2013 |
Calculating the power needed to run HVAC systems

In 2007 Pittsburgh Steelers safety Ryan Clark lost the remainder of his football season when he became gravely ill after playing the Denver Broncos. This was partially due to a rare genetic abnormality called sickle cell.

Meanwhile back in Pittsburgh, building owners' utility meters spin a little bit faster than in Denver for equipment performing identical tasks.


With changes in altitudes comes a change in air pressure and air density. This affects more than you might think.

From the Ground Up

We live with a force (air pressure) that is literally squeezing our bodies. You don't notice it because our bodies are used to it. But this force varies upon elevation. In science, our reference point for pressure begins at sea level. As we rise in elevation the air molecules become further apart and are less dense.

When we breathe, oxygen enters our lungs and passes through tiny permeable membranes into the blood. Air pressure helps push the oxygen. The less dense the air, the less assistance the oxygen has to enter our bloodstreams.

Over time our bodies adjust. But symptoms often occur from the lack of oxygen at higher altitudes, such as sickness, nausea, and fatigue. In the rare case of Ryan Clark, the combination of high altitude and extreme physical stress was almost fatal. Sickle cell complications cost him his spleen and gallbladder.

Moving Heavy Air Costs More Money

Fans are stupid machines. They will spin at a given speed and produce a constant volume of airflow regardless of altitude. What changes is the power required to drive it. This is due to the fact that air in higher altitudes weighs less than at sea level. We must account for these differences with correction factors as shown in Table 1.

We compare a calculation at standard sea level and correct it depending on the altitude. For example, say we calculate that a fan requires a Brake Horsepower of 5.28 at sea level and we want to know how it would perform at 5,500 feet. According to the table, we would simply divide by 1.22 to get 4.33 BHP.

Table 1 - Air Density Correction Factor, Courtesy of Twin City Fans

For comparison purposes, if we assume both areas pay $0.10/KWh and operate 24/7, the annual difference would be $620 – not bad if you're the owner in Denver. Commercial buildings commonly have larger horsepower fans, so that difference can be much greater.

Who knew we'd find a common thread between sickle cell and utility bills?

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