3 Ways to Measure Airflow for Rooftops and Other HVAC Equipment

One of the most common reasons for rooftop service calls is incorrect airflow. 

Low airflow will often lead to frozen evaporator coils, low pressure trips, sweating ductwork and low capacity. 

High airflow can lead to condensate blowoff, high pressure trips, high humidity issues and more.

As you can see, setting airflow properly at startup (and verifying on service calls) is extremely important. 

In this article we will share with you three ways to measure airflow. 

Airflow Charts 

The best way to measure airflow is to use the airflow chart in the installation manual. Most manufacturers will provide a chart or graph that plots cfm (airflow) vs static pressure drop across the DRY evaporator coil.

All you have to do is give the unit a fan (G) call (make sure the unit is running at 100% speed if it has a VFD), and then measure the pressure drop across the evaporator coil. Check the installation manual to see if the chart is based on a dry or wet coil (most will be DRY).

Here is an example from a York installation manual, in this case a ZF120 Predator unit. 

This rooftop is a nominal 10 tons, so unless you have specific instructions from a submittal or design engineer, it is best to set the airflow to 400 cfm/ton = 4,000 cfm. 

As you can see in the chart, 4,000 cfm is achieved at approximately 0.21″ static pressure drop. 

If the measured static pressure drop is not close to 0.21″, adjust the blower speed as necessary to get within 5% of the target cfm. 

Enthalpy Calculation

Another great way to measure airflow is to calculate how much total cooling the unit is doing (based on the current conditions) and compare that to the technical guide (or with selection software). 

First you will need to measure dry bulb and wet bulb temperatures for both the leaving air temperature (LAT) and the entering air temperature (EAT). Make sure that the EAT is measured immediately before the evaporator so that you take into account any outside air that might be leaking through (I also recommend fully closing the economizer or manual damper to get the most accurate readings). 

You will also need to get an outdoor dry bulb temperature reading as well. 

Next, locate the capacity tables in the technical guide to determine the cfm based on the field measured ambient air temperature and EAT/LAT. Since most technical guides force you to interpolate the data, it’s much easier to calculate this with selection software. 

Example:

Ambient dry bulb temperature = 90 degF 

EAT = 78/65 degF

LAT = 57/55 degF

Next, run performance with different cfm values until the selection matches the field data. 

Based on this final selection, this unit is supplying around 4,300 cfm.

You can double check this with the following formula:

Total Capacity = cfm x 4.5 x (Hi-Hf)

Where:

Hi = Enthalpy initial (entering air enthalpy)

Hf = Enthalpy final (leaving air enthalpy)

Total Capacity = cfm x 4.5 x (Hi-Hf)

134,000 = cfm x 4.5 x (30.0-23.3)

cfm = 4,379

Based on our findings, we can safely assume this unit is operating between 4,300 and 4,400 cfm. 

A good rule of thumb is to make sure that the enthalpy difference is between 6.0- 7.5 for a standard comfort cooling rooftop or split system. In this example our delta-H is 6.7. 

Temperature Rise Calculation 

If the unit has a gas heat exchanger or electric heating coil, you can calculate cfm by measuring the air temperature rise. 

Make sure the unit is calling for ALL stages of heating, then measure the leaving air and entering air temperatures. Just like the enthalpy calculation, make sure that the entering air temperature is measured immediately before the evaporator.

Once you have these values, solve for cfm in this formula:

Total Capacity* = cfm x 1.085 x (LAT-EAT)

*Make sure to take into account the heating efficiency in the Total Capacity variable. For example, if the unit has a 240,000 btuh rated input and an 80% steady state efficiency, then the total capacity would be 240,000 x 0.80 = 192,000 btuh. 

In this example, if the LAT = 105, and the EAT is 65, then

240,000 x 0.80 = cfm x 1.085 x (105-65)

cfm = 4,423

NOTE: The enthalpy calculation and the heating calculation both make assumptions that the cooling and heating systems are operating properly. If the unit is low on charge, or the combustion process isn’t working correctly, these figures might be off. Just keep that in mind!

There are other tools as well that can help measure airflow, like an anemometer or an air balancing hood. 

How do you measure airflow?

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Brad Telker

Vice President of Sales, Applied Systems Group at cfm Distributors, Inc.

Brad was born and raised in St Louis Missouri, where he spent the first 18 years of his life. At the ripe young age of 18, he embarked onto his next big adventure, traveling to the University of Missouri Columbia for a bachelor’s degree in Mechanical Engineering. Brad joined the cfm team in 2006, and now as the Vice President of Commercial Sales, he focuses on business development, as well as helping contractors and engineers find creative and unique solutions to any size project. When he’s not at work, Brad enjoys photography, running and spending time with his family.

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