Crushed Airflow and Rattling Vents: Guide to Duct Sizing for Your 5-Ton R-32 Combo System

🧢 Hello from Jake

Hey there, it’s Jake again—and today, we’re talking about something folks don’t see but always feel when it’s done wrong: ductwork. That’s right, your 5-ton R-32 combo system can be top-of-the-line, but if your ducts are the wrong size or shape, you might as well be trying to drive a semi through a bike lane.

I’ve been on a lot of calls where people are blaming their brand-new system for hot rooms, low airflow, or noisy vents—but it ain’t the equipment. It’s the ductwork. If your return’s too small or your supply’s all twisted up like a pretzel, your system’s gonna struggle like it’s wearing a belt two sizes too tight.

So whether you’re designing a brand-new install or fixing someone else’s mess, I’m gonna show you how to size and run ductwork the right way for a 5-ton R-32 combo. We’ll talk CFM, friction rate, static pressure, and how to avoid turning your living room into a wind tunnel or a sauna.

Let’s get into it. The air’s not gonna move itself.

 

🌬️ Section 1: Why Proper Duct Sizing Matters for a 5-Ton System

Your 5-ton R-32 system moves around 2,000 CFM of air. That’s a whole lotta airflow—and if your ductwork can’t handle it, you’re asking for trouble.

Too-small ducts cause:

• High static pressure

• Weak airflow to distant rooms

• Noisy vents and whistling grilles

• Overworked blower motors

• Short cycling or overheating

Too-large ducts reduce air velocity, mess with airflow balance, and waste energy trying to push air through oversized space.

Ductwork is the delivery system for your comfort. You wouldn’t put race fuel in a lawnmower, so don’t choke out a high-efficiency furnace with tiny flex ducts and undersized trunks.

ASHRAE recommends designing ducts based on Manual D principles, using friction rates, equivalent lengths, and balancing methods. You don’t need to memorize formulas—but you do need to understand what works.

 

📐 Section 2: How Much Airflow Does a 5-Ton System Need?

Let’s keep it simple.

1 ton of cooling = roughly 400 CFM.
5 tons = 2,000 CFM total airflow.

That’s what your duct system needs to handle—supply and return.

So your total ductwork should be sized to move 2,000 CFM at reasonable velocities:

• Main supply trunk: at least 20 x 20 inches or two 14-inch round ducts

• Main return trunk: 24 x 20 inches or dual 16-inch returns

Remember, it’s not just about trunk size—it’s about the whole path. Every elbow, flex duct, filter grille, and transition adds resistance. That’s why pros use friction rate charts and Manual D calculators to fine-tune the layout.

 

🧮 Section 3: Key Duct Sizing Rules of Thumb

Here’s what I’ve used in the field when you don’t have a laptop and spreadsheet handy.

• 6-inch round duct = ~100 CFM

• 8-inch round = ~160 CFM

• 10-inch round = ~300 CFM

• 12-inch round = ~450 CFM

So if you’ve got a room that needs 300 CFM, you’re looking at a dedicated 10-inch duct to that room. No more, no less.

And don’t forget the return side. That return plenum has to move the same volume back to the furnace or the whole system goes out of balance. Undersized returns are one of the biggest airflow killers I see—especially when folks upgrade to bigger systems without adjusting the ductwork.

You can also use Ductulator apps on your phone now—great for checking sizes on the fly.

 

🔧 Section 4: Avoid These Common Ductwork Mistakes

I’ve seen some real horror shows out there. Here are the big ones to avoid:

• Too many 90-degree elbows – adds static pressure

• Long runs of flex duct – slows airflow, sags over time

• Crushed ducts in attic or crawl – instant bottleneck

• Using the same size ducts for every room – airflow won’t be balanced

• No return in key rooms – creates pressure imbalance, sucks in outside air

• Return grill blocked by furniture – sounds simple but it happens all the time

Also—check your filter size. A 5-ton system should have at least 1,000 square inches of return filter surface. That means a 20x25 filter isn’t gonna cut it unless you’re running two of them.

Want a real-world visual? Energy Vanguard has a killer breakdown on airflow restrictions caused by poor duct and filter setups.

 

📊 Section 5: Static Pressure and Air Velocity—Why They Matter

Airflow isn’t just about size—it’s about pressure. Your blower motor creates a pressure difference that moves air through the system. If the pressure is too high, airflow drops and parts wear out. Too low, and the air never reaches where it should.

Target external static pressure (ESP) for a residential system is usually 0.5 inches of water column (in. w.c.). Go higher and you risk burning out motors.

High ESP usually means:

• Ducts are too small

• Filters are dirty or undersized

• Grilles are blocked

• Coils are clogged

Air velocity should be:

• 600–900 FPM in the main trunk

• 400–600 FPM at room registers

Higher than that and you’ll hear it. Lower and you won’t feel the air.

You can learn more about this from Contractor Business’s guide on static pressure.

 

🧰 Section 6: What to Do if You Inherited Bad Ductwork

If you just installed a shiny new 5-ton R-32 combo system and the ductwork came with the house... well, odds are it wasn’t designed for what you’ve got now.

Here’s what I tell homeowners:

• Measure airflow at the registers using an anemometer

• Check static pressure at the blower (your HVAC tech can do this)

• Look for hot and cold spots—they usually mean airflow imbalance

• Consider adding returns to large rooms

• Seal all joints with mastic, not duct tape (which isn’t rated for pressure)

Sometimes, it’s worth biting the bullet and upgrading that 30-year-old octopus in your attic. Good ductwork doesn’t just help comfort—it boosts efficiency and lowers your utility bill.

Want help designing the new layout? ACCA’s Manual D software is what the pros use, and many contractors will offer a duct audit if you ask.

 

🧢 Wrap Up from Jake

Well there you have it—ductwork sizing in plain English. Ain’t glamorous, ain’t shiny, but it’s the backbone of every good HVAC system. Without it, your 5-ton R-32 combo is just a muscle car with flat tires.

If you’re planning a new install or fixing airflow problems in your current setup, take the time to get the ductwork right. Size it properly, keep it sealed and supported, and balance that return air. You’ll get quieter operation, longer equipment life, and comfort that actually reaches every corner of your house.

And if you’re just starting your upgrade journey, don’t forget to check out The Furnace Outlet’s 5-ton R-32 AC and gas furnace systems. These systems are efficient, modern, and built to move air—just make sure your ductwork can keep up.

Need tips on how to fix refrigerant leaks for your 5-ton R-32 systems? Visit my guide: Sniffing Out Trouble.

Stay cool, stay balanced, and I’ll catch you on the next job.

– Jake 📏