Why HVAC Return Air Door Undercuts Are a Bad Idea

In our last post, we covered how restricted return airflow can wreak havoc on your HVAC system, leading to inefficiency and uneven heating or cooling. If you’ve noticed whistling vents, rooms that never reach the right temperature, or a system that seems to run nonstop, you might be searching for an easy fix. Maybe someone even suggested HVAC return air door undercuts as a quick solution.

Let’s pump the brakes on that idea.

Cutting down your doors might seem like a simple way to improve airflow, but in reality, it’s an ineffective, inefficient, and sometimes downright dangerous approach. Let’s break down why return air door undercuts are not the answer—and what you should be doing instead.

The Myth of HVAC Return Air Door Undercuts: Why It Sounds Good (But Isn’t)

The logic behind this practice is simple: If a room doesn’t have a dedicated return vent, air still needs a way to get back to the system. Cutting the bottom of the door seems like a reasonable way to create that airflow path.

But here’s the problem:

1. HVAC Return Air Door Undercuts Don’t Provide Enough Airflow

Your HVAC system needs 50-100 square inches of return air to keep everything balanced.

Let’s put that in terms of door gaps:

• ¾-inch undercut on a 30-inch door → ~19-22 square inches of airflow (Not even close)
• 2-inch undercut → ~50 square inches (Starting to get drafty)
• 4-inch undercut → ~100 square inches (Welcome to your new saloon-style bedroom)

Unless you want to crawl under your bedroom door like a horror movie escape scene, a simple undercut won’t provide enough return air.

2. Door Undercuts Destroy Privacy

You know that feeling when you close your door for some peace and quiet? Well, with a larger undercut, you can kiss that goodbye.

Sound travels freely under the door, meaning:

• You’ll hear everything happening in the hallway (and they’ll hear you).
• Cooking smells, pet dander, and allergens from other rooms will sneak right in.
• Forget about light control—if someone turns a hallway light on, you’ll see it through the gap.

3. HVAC Return Air Door Undercuts Reduce Energy Efficiency

Instead of circulating properly, conditioned air just leaks into the hallway, forcing your system to work harder than it should. This means:

• Higher energy bills because your HVAC has to run longer.
• Uneven temperatures between rooms.
• Shortened lifespan of your system due to increased wear and tear.

4. Fire Safety Risks of Undercuts

Fire-rated doors are designed to slow the spread of fire and smoke—but a large undercut completely undermines that protection.

What Are the Best Alternatives to Undercuts?

1. Wall-Mounted Transfer Grilles

These allow return air to flow while maintaining privacy and come in fire-rated options. Learn more about our home inspections.

2. Jump Ducts

A short duct connecting a room to a return path. Includes fire-rated dampers for safety.

3. Dedicated Return Vents

The best way to eliminate airflow restrictions. Contact us to discuss HVAC solutions.

Bottom Line: Don’t Rely on HVAC Return Air Door Undercuts

HVAC return air door undercuts might sound like an easy fix, but they are an ineffective and unsafe solution for improving return airflow.

Instead of resorting to questionable modifications, let’s focus on solutions that work. In our next post, we’ll cover the best modern return air solutions.

👉 Check out Part 3: Modern Return Air Solutions That Actually Work

Or, if you’re ready to fix your HVAC airflow issues, schedule a home inspection with us today.

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