Whether you’re moving into a new home that you recently built or you’re simply worried about your existing home’s ducting, one of the most common questions that homeowners have about ducting is whether or not its size really matters. The short answer is yes, it certainly does. However, there’s a lot that goes into determining the right size of ducting and many reasons why getting the right size is imperative.

What is Ducting?

Ducting or ductwork is simply a series of ducts that run throughout a home. These open passages are used to deliver heated or cooled air to different locations throughout your home. Each main room of your home should have two different registers. One is meant to supply temperature-controlled air to the room, and the other is meant to remove stale air.

Ducting can be constructed of galvanized steel, aluminum, phenolic insulation panels, and polyurethane. Each section of ducting that gets installed in your home needs to be appropriately sized for your HVAC unit to perform efficiently. If your ducting is oversized or undersized, it could lead to premature wear out of your heating and cooling units and an uncomfortable living environment.

The Problem with Oversized Ducting

The bigger the ducting size, the more airflow can go through it. While this may seem like a big advantage on the surface, it’s really not when you dive deeper into utilizing it. If your current HVAC system isn’t capable of supplying a large amount of air to fill the oversized ducting, you’re going to have a problem.

Your HVAC system is specifically designed to work with a particular air pressure in your ducting. This helps to ensure that the temperature-controlled air is effectively distributed throughout your entire home. When your ducting is oversized, it will struggle to deliver that temperature-controlled air to various areas of your home.

In fact, you’ll find that your HVAC system has to work much harder to produce enough hot or cold air to get your entire home up to your desired indoor temperature. Unfortunately, this can lead to frequent equipment breakdowns, increased energy usage, and premature wearing out of your heating or cooling unit. You never want to have ducting that is considered oversized for your existing HVAC equipment.

The Problem with Undersized Ducting

Just as oversized ducting can wreak havoc on your heating and cooling systems, so can undersized ductwork. When the ducting is too small for the specific HVAC system that you have, air flowing through it will start to back up. The air pressure within the ductwork will become higher.

As the air pressure increases, this produces resistance in the blower fan. This causes poor system performance, decreased efficiency, and an immense amount of stress on your system’s components. Within a short amount of time, you’ll likely start experiencing frequent breakdowns with your heating and cooling units. Even worse, you’ll be shortening their lifespan due to the excess wear and tear on their internal components.

How is Ducting Sized?

When you call in an HVAC professional to install new ducting in your home, they will follow a specific formula to ensure that you’re getting the right size ductwork for the HVAC system that you’ll be utilizing. The HVAC industry standard formula is known as Manual D.

Developed by engineers at The Air Conditioning Contractors of America, Manual D is responsible for appropriately sizing supply and return ducts as well as registers for residential installations. Via this formula, your chosen HVAC contractor will analyze the various rooms throughout your home to determine optimal airflow and ensure there’s no excessive noise so that your ducting can have the right pressure for your HVAC system to work efficiently.

Calculating Ductwork Size

When your HVAC contractor is working to determine the needed size for your new ducting, there are various factors that they’re going to take into consideration. Some of the most obvious are the square footage of your home, the square footage of each main room, the size of your HVAC units, cubic feet per minute, friction loss, and the static pressure of your HVAC units.

Square Footage

Probably the easiest measurement to take is the square footage of each room. You simply need to take the length and multiply it by the width. In rooms that are abnormally shaped, it can be much easier to divide the room into smaller rectangular pieces to calculate the room’s total square footage.


Cubic feet per minute, known simply as CFM for short, is a measurement of air velocity. A single cubic foot of air is considered to be one foot by one foot by one foot deep. Determining the total CFM for each room of your home starts with multiplying the air velocity by the total cubic volume of your room. This CFM calculation will basically determine how many cubic feet of treated air passes over a particular stationary location over one minute.

Friction Loss and Static Pressure

A system’s friction loss rate is based on numerous factors. Some of these include the length of the ducting, the number of turns, and so forth. An HVAC professional will have to take into account each section of ducting needed. They’ll typically utilize duct sizing calculators to get the friction loss rate because it can be very complicated to calculate by hand.

Common Signs of Missized Ducting

If you already have ductwork installed in your home, you may be worried that it isn’t the right size. Fortunately, there are a few obvious signs that will let you know that you have missized ducting. If you suspect that you do, it’s always best to call in the HVAC professionals to assess your ducting and fix it accordingly.

Varying Temperatures

One of the most obvious signs that your ducting is not appropriately sized is that you experience different temperatures in different rooms of your home. In some cases, this temperature difference can be over 10 degrees Fahrenheit. This noticeable difference is the result of insufficient amounts of treated air not reaching all areas of your home.

Air Imbalances

When your ducting is undersized, it can allow different air pressures to be present in different rooms of your home. As you walk from one room to the next, you may experience physical imbalance in your ears. Your ears may feel like they need to pop or are overly stuffy.

Excessive Repairs

Another common indicator that your ducting is not appropriately sized for your home is frequent breakdowns. When your HVAC system is working harder to deliver air through undersized or oversized ducting, it can lead to excessive wear and tear on its internal components. You should have your HVAC technician examine your existing ductwork to see if it’s inadequately sized.

Let the Pros Help

TR Miller Heating, Cooling & Plumbing offers top-quality HVAC ducting installation services for the entire Plainfield, IL region. We can also help with all of your plumbing, drain, heating, cooling, indoor air quality, ductless mini-split, sump pump, and water heater needs. Simply call our office today, and we’ll put you on our schedule.

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