Duct Sizing for 3.5 Ton R-32 AC Systems: Why Your Airflow Sucks (And H

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Hey There, It’s Tony Again.

Let’s talk about one of the most overlooked and misunderstood parts of installing a 3–5 ton R-32 AC and gas furnace system: ductwork.

You can drop five grand on a beautiful new high-efficiency system like the ones over at The Furnace Outlet, but if your ducts are choking airflow like a crimped garden hose—you’re just burning cash and time.

I see it all the time:
System looks great on the outside. Then I measure static pressure and find the airflow's garbage. Coils freeze. Furnaces overheat. Customers get mad. And the ductwork is the silent killer.

Let’s break it down.

Why Duct Sizing Matters (A Lot More Than You Think)

When you’re pushing 36,000 to 60,000 BTUs of cooling power, that air needs to move. Your system’s designed to handle 400 CFM per ton—so a 5-ton unit should be moving about 2,000 CFM.

If your ducts can’t handle that?

Your compressor runs too hot
Your blower motor works overtime
Your system short cycles
Your utility bill skyrockets
And worst of all—your house still feels like a sauna 😡

It’s like putting a fire hose through a drinking straw. You can do it… but it’s gonna be a mess.

Common Mistake #1: Undersized Returns 🫠

I don’t care how fancy your system is—if you’ve got a 5-ton R-32 Goodman and your return plenum is 14", you’re starving that blower.

For reference:

A 3-ton system needs about 1,200 CFM
A 4-ton system needs 1,600 CFM
A 5-ton system needs 2,000 CFM

And to move that much air quietly and efficiently, your return ducts should be 20" or larger—sometimes even dual returns depending on layout.

Need a cheat sheet? This residential duct sizing chart from Hamilton Home is your new best friend.

Common Mistake #2: Long Runs with Too Many Elbows

Every foot of duct and every elbow adds resistance. Ever tried breathing through a curly straw? Same idea.

If your supply run travels 40 feet through flex with four 90° bends, it’s no wonder your airflow sucks.

Quick tip from yours truly:

Use hard pipe where you can
Minimize 90° elbows (use two 45s instead)
Keep your ducts as straight and short as possible

The longer the run and the more turns you have, the more external static pressure you’re adding—which leads me to my next point.

Static Pressure: The Silent System Killer ⚠️

Static pressure is the resistance your blower has to fight against to push air. Think of it like blood pressure—but for air.

On most residential systems, you want total external static pressure (TESP) to be under 0.5" WC. But with poor duct design, I’ve seen systems hitting 0.9" or even 1.2" WC—and you bet those blower motors are crying.

Want to learn how to measure static pressure? Check out this awesome guide from AC Service Tech. It walks you through it step-by-step.

The Role of Filters (Yep, They Matter Too)

Filters affect airflow. Every filter has a pressure drop. If you go slapping in a high-MERV pleated filter without accounting for duct sizing and airflow—you just added another choke point.

If your system is already borderline on airflow, switching to a high-efficiency filter without upsizing your return duct is like throwing a pillow over your vents.

My advice?

Stick to MERV 8–11 unless your system was designed for higher
Use deep media filters (4" or 5") for better airflow
Keep those filters CLEAN—change ‘em every 1–3 months

For more on filter selection and pressure drop, HVAC School has a great explainer.

What About Zoning? 😬

Zoning is great when done right—but in a 3–5 ton setup with R-32 refrigerant, it complicates duct design a lot. You need:

Bypass dampers or variable speed blowers
Pressure relief strategies
Ducts that support simultaneous open zones

Mess this up, and one closed damper can spike static pressure so high that your blower just gives up.

Thinking of zoning your new system? Do yourself a favor and read Energy Vanguard’s zoning guide. It’s eye-opening.

Tony’s Field Fixes (Real Talk) 🔧

Here are some real fixes I’ve done for duct disasters:

Case 1: 5-Ton System, Single 16” Return

Static pressure = 0.88" WC
Fix: Added 2nd 18" return, swapped filter to 4" MERV 8
Result: Dropped static to 0.42" WC. Night and day.

Case 2: Flex Duct Octopus in the Attic

System was short cycling every 4 minutes
Fix: Replaced flex with straight rigid runs, used long-radius elbows
Result: Blower ran quieter, more even temps, homeowner thrilled

Case 3: Oversized Supply Plenum, Undersized Branches

Air was dumping near the furnace, starved elsewhere
Fix: Balanced branch sizing, added manual dampers
Result: Even airflow and improved coil temps

How to Check If Your Ducts Are the Problem

Here’s your checklist:
✅ Hot and cold rooms
✅ System runs but can’t keep up
✅ Blower is loud or vibrating
✅ High utility bills
✅ Furnace overheating
✅ Frozen indoor coil

If you’re ticking off two or more of those boxes, you’ve probably got airflow issues tied to poor duct design.

Wrapping It Up from Tony 👋

Listen, I’ve said it a million times—a system is only as good as the ductwork behind it. You wouldn’t buy a race car and slap on bicycle tires, right?

So if you’re investing in a 3–5 Ton R-32 AC and gas furnace combo, like the killer systems over at The Furnace Outlet, don’t cheap out or cut corners on the ductwork.

Proper sizing, smart layout, and airflow balancing will make your system: