Low airflow is the primary cause of the most unsolved complaint in the HVAC industry – hot or cold rooms. To diagnose this problem, you can measure the room’s airflow with an air balance hood and compare the results to the required room airflow. The difference between the two numbers reveals the problem and points you towards a solution. Let’s take a look at the missing link you need to diagnose this problem.

What Causes Hot and Cold Rooms?

High-Performance HVAC Industry Influencer Rob Falke
Rob Falke

A typical hot or cold room may require 150 cfm (cubic feet per minute) of supply airflow to keep it comfortable. NCI contractors report the typical hot or cold room’s airflow is about 33% of what’s required.

This means the typical room that needs 150 cfm only receives 50 cfm (150 cfm x .33 = 50 cfm). It’s no surprise the room is uncomfortable!

Room Airflow Diagnostics

Room airflow diagnostics combine quick and easy testing with a test report to produce an unforgettable customer experience.

Here’s how it works. Let’s say that you and your customer estimate the uncomfortable room needs airflow of 180 cfm for them to be comfortable. You show your customer how to measure airflow with a balancing hood. They expect the measurement to show that 180, but suddenly discover their supply register only delivers 40 cfm!

To help describe airflow, describe it as beach balls per minute. A 7” beach ball contains one cubic foot of air.

If needed, use a little math to clarify the problem in up to three separate ways. First, the room is 140 cfm low on airflow (180 cfm – 40 cfm = 140 cfm).
Second, the room only has 22% of the airflow it needs to be comfortable (40 cfm/180 cfm = 22%). Third, the room is 78% low on airflow (100% -22% = 78%).

Don’t forget, airflow is difficult for your customers to understand. We use cfm as an acronym to describe a volume and time measurement (Let that sink in).

To make airflow easy to understand, you may explain how a 7” beach ball holds one cubic foot of air. Then tell the customer how the room needs 180 beach balls of air per minute but is only getting 40 beach balls per minute.

This diagnostic method and explanation are straightforward and easy to understand. Everyone instantly recognizes the problem and wants to find a solution. Help them understand when airflow is increased to the required amount, the room will become comfortable.

Before you can use this diagnostic method, you’ll need to estimate required room airflow.

Estimate Room Airflow

Purists will assess the wall insulation R-value, window U-value, and roofing material to complete a load calculation. Granted, there is a time and a place for this, but this isn’t the time. This is simple air diagnostics using the NCI method of room airflow estimation with your customer participating.

Figure1: This is a sample report your technicians can use in the field to help them with their estimates and measures room airflow measurements.

Traditional load calculations estimate airflow using calculations based on a wide variety of engineered variables. This room airflow estimating method compares only against similar features built into this home.

Your customers usually live there and have the most knowledge of how the home operates. Be sure to invite their input into the decision.

There are six steps to estimate room airflow in the field. As I describe each step, please refer to Figure 1, the completed Estimate and Measure Room Airflow Report, to see how it works.

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