Air Pressure Measurements Explained

When discussing heating and cooling coils, as well as air handling units (AHUs), air pressure is often a common topic. In fact, there are many types of air pressure that are important to the design of coils, AHUs and complete HVAC systems in general.

These pressures can range from very low pressures to very high pressures, and are measured with three different scales, which we will explain in great detail below. The first two, which are somewhat exclusive to HVAC and other industrial fields, is inches of water (inH₂O) and feet of water (ftH₂O). The third, which is in much more widespread use, is pounds per square inch (psi), such as the air pressure is a car’s tires. Smaller and more precise pressures are typically measured in inH₂O, while larger pressures are listed in (ftH₂O) or psi.

Before we get into the details of these three main units of measurement, let’s look at a few examples of air pressure in the world of HVAC coils and air handling units.

Air Pressure Drop (APD)

Air pressure drop, or APD, in a commercial heating or cooling coil refers to the reduction in air pressure as it passes through the coil. This drop occurs due to resistance created by the coil’s tubes, fins, and overall design. A typical air pressure drop for a Marlo coil would be a very small number, so it is measured in inches of water. For example, 1.0” maximum of air pressure drop is a good place to start, with most of our coils ranging from .5 to .75 inH₂O of APD. 

APD in a commercial or industrial air handling unit (AHU) refers to the loss of air pressure as it flows through all the components inside the unit. This pressure drop occurs due to resistance from filters, coils, dampers, duct connections and other internal components. Even when totaling all of these factors, the total pressure drop in a typical MarloAIR air handling unit would be a small number, so it is measured in inches of water (an average of under 6 inH₂O). 

Depending on the size of the air handling unit, this number can be as high as 10” in certain process conditions for smaller units. In larger AHUs, higher pressure becomes a significant factor in the structure required, and consequently, the cost of the unit.

Internal Pressure Ratings

A completely different pressure measurement, used to define strength and durability, is the pressure the internal tubes of a commercial or industrial coil can withstand. These are much larger pressures than those discussed above, so they are most often referred to in terms of psi or feet of water.

For example, typical Marlo hot water coils and chilled water coils are rated for internal pressures up to 195 psi (equivalent to 450 feet of water), while direct expansion (DX) coils (refrigerant coils) require higher pressures, depending on the refrigerant and operating conditions. Evaporator coils typically have pressures up to 150 psi (346 ftH₂O), while condenser coils must withstand pressures up to 450 psi (1040 ftH₂O).

Another example of pressure ratings used as a measure of strength are the pressures our airtight coil housings are designed to hold. Mainly used in industrial heating and cooling processes, our airtight housings can be built for a wide range of pressures – from 10 inches of water to 1 psi is standard, but can be made to withstand more than 10 psi – which is why they use both of the pressure measurement scales.

Now that we’ve discussed a few specific examples, let’s get into the two units of measurement, how they compare, and how to measure them.

Inches of Water (inH₂O) Measurement

Definition: Inches of water column (inH₂O) refers to the pressure exerted by a column of water of a specific height. It is typically used for low-pressure measurements in HVAC systems, as discussed above.

How to Measure: Inches of water is measured using a manometer, a U-shaped tube partially filled with water. When pressure is applied, the water level shifts, and the difference in height (in inches) represents the pressure. Digital manometers are also available for more precise measurements.

Common HVAC Applications: Measuring pressure drop in coils, pressure drop in AHUs across filters and other components, and airflow in ducts.

Feet of Water (ftH₂O) Measurement

Definition: Feet of water column (ftH₂O) refers to the pressure exerted by a column of water of a specific height. It is typically used for high-pressure measurements in HVAC systems, as discussed above.

How to Measure: Feet of water can be measured using a manometer, a U-shaped tube partially filled with water. When pressure is applied, the water level shifts, and the difference in height (in feet) represents the pressure. Digital manometers are also available for more precise measurements.

Common HVAC Applications: Measuring pressure drop in coils, pressure drop in AHUs across filters and other components, and airflow in ducts.

Pounds per Square Inch (psi) Measurement

Definition: Pounds per square inch (psi) is the pressure applied on a surface if one pound of force is applied to a one square inch area. It is commonly used in high-pressure applications such as tires and compressed air systems, as well as in certain HVAC pressure measurements.

How to Measure: Pressure gauges (analog or digital) are used to measure psi. When connected to a pressurized system, the gauge will display the pressure directly in psi.

Common HVAC Applications: Internal tube pressure measurements in coils and sometimes pressure requirements for airtight coil housings.

Conversion Between inH₂O and psi

The two units of measurement can be converted as follows:

1 psi = 27.7 inH₂O

So, if you need to convert inches of water to psi:

psi = inH₂O / 27.7

And if you need to convert psi to inches of water:

inH₂O = psi × 27.7

Here are two examples showing how to convert between inches of water (inH₂O) and pounds per square inch (psi).

Using the conversion formula: psi = inH₂O / 27.7

Substitute the given value: psi = 55 / 27.7 = 1.99 psi

So, 55 inH₂O is approximately 1.99 psi.

Example 2: Convert 3.5 psi to inH₂O

Using the conversion formula: inH₂O = psi × 27.7

Substitute the given value: inH₂O = 3.5 × 27.7 = 96.95

So, 3.5 psi is approximately 96.95 inH₂O.

Conversion Between ftH₂O and psi

Similar to the examples above, these two units of measurement can be converted using the following formula: 1 psi = 2.31 ftH₂O.

To save you from doing the math, here are some quick reference charts showing conversions: