Blog

Zoning Evaluation: Measuring Ductwork Capacity

When you’re considering zoning a building, one of the most important steps of zoning evaluation is measuring ductwork capacity, or how many Cubic Feet Per Minute (CFM) the duct can deliver. This post will discuss the importance of measuring the CFM capacity of a building’s ductwork and provide an overview of the process for calculating CFM to properly evaluate a structure for zoning.

What is Zoning?

Zoning is when a building is divided into different temperature zones to allow custom comfort profiles for the occupants. This will enable them to take control of their airflow and improve the effectiveness of their heating and air conditioning system.

Importance of Ductwork

The purpose of zoning is to create controlled restriction; if too much restriction is made, there is a possibility that the HVAC equipment could be damaged; by measuring the ductwork, the size of a zone can be evaluated to prevent overstressing the equipment.

Ideally, the size of a zone should be 35% ductwork (CFM) to equipment capacity for a single-stage blower and 25% ductwork (CFM) to equipment capacity for a two-stage blower when utilizing the Arzel HeatPumPro’s zone weighting feature. This makes three zones the most common number of zones.

When following the minimum zone size the ESP of a system with the smallest zone should be around 0.2″ w.c. when all zones are opened, the ESP should be about 0.08″ to 0.1″ w.c. if we get too high, say greater than 0.25″ the customer may have a complaint about airflow noise, often when airflow noise is a problem we’ll find that the TESP of the blower unit exceeds the maximum according to its blower performance data chart usually around 0.8″. This is where equipment damage can occur and why static pressure is essential.

How to Measure Ductwork Capacity

Airflow capacity can be calculated by multiplying the velocity (FPM) by the sq. in. (Cross-section) and dividing by a square foot or 144 inches. The nominal design velocity should be 600FPM on the return air, 700FPM on the supply air, and 450FPM on branch ducts. If we want to see how a system operates in real time, we can use an anemometer to see the actual velocity rather than design velocities. This can help us diagnose air delivery problems more effectively.

For example, if we have an 8×14″ supply duct, we would calculate that as 8×14 = 112 sq. in. With a design velocity of 700FPM, the formula would look like this (112×700)/144 = 544; we would know that 8×14 as a supply duct can handle 550CFM. If we have a single-stage 5-ton blower (2000CFM), the zone would be too small to meet the 35% requirement. Additionally, we would know the zone needs to be bigger. However, if the blower were upgraded to a two-stage blower, then being over 25% would be a good result.

Proper Equipment Operation in Zoning

When we evaluate a home or building for zoning, we must remember the input of the occupants; we’re ultimately looking to optimize their comfort. First, however, we must be mindful of proper equipment operation to offer long life and reliable operation of their comfort system.

Recent Posts
View All

Arzel Zoning is committed to offering innovative zoning solutions, and we have lots of options. So which zoning system should you choose? It can be hard to wade through all of the choices when it comes to zoning, and we wanted to help. Here is a guide to installing Arzel zoning systems. We will talk […]

Our unique retrofit dampers make it possible to install dampers in existing ductwork even when it seems impossible. Here’s a brief rundown of our zoning damper options, and some tips on how to install dampers in existing ductwork. Air Makes It Possible Arzel dampers are pneumatically driven, meaning they are operated by air. There’s no […]

We take care to offer a variety of excellent HVAC contractor training opportunities at Arzel Zoning, because we know that the contractors who install and service Arzel Zoning systems are an important part of our success. We want to be a part of what makes you successful too. The more you know about zoning, the […]

Are you working with commercial clients yet? If you’re ready to add commercial clients to your roster, but unsure of the options out there, this blog is for you. We’ll discuss the different ways to ensure optimal comfort and efficiency in commercial buildings by installing the right HVAC system. Commercial spaces don’t always have the […]