I’ve been in attics hotter than a Texas skillet and crawlspaces tighter than a manhole. If there’s one mistake I see over and over, it’s folks installing the wrong-sized HVAC system for their home. Let me be real clear: bigger isn’t always better, and smaller definitely isn’t cheaper when it comes to comfort and efficiency.
In this guide, I’ll walk you through how to size your AC and furnace combo the right way — using the same principles pros like me follow in the field. If you’re looking at a 3–5 ton R-32 AC and gas furnace system, you’re already in the right weight class — now let’s make sure it fits like a glove.
📏 What Does “Ton” Mean in HVAC?
First off, let’s clear up the confusion. HVAC “tonnage” has nothing to do with weight. A ton refers to the amount of heat a unit can remove from your home in one hour. One ton = 12,000 BTUs per hour.
So the numbers look like this:
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3 ton = 36,000 BTU/hr
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3.5 ton = 42,000 BTU/hr
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4 ton = 48,000 BTU/hr
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5 ton = 60,000 BTU/hr
That BTU rating isn’t just about cooling. Your furnace also needs to produce enough heat to offset your winter load. That’s why it’s essential to balance both the heating load and cooling load when selecting a system.
📐 The Square Footage Rule (and Why It’s Not Enough)
Sure, you’ve probably seen the rule of thumb:
1 ton per 500–700 square feet.
But that’s just a starting point, not gospel. For example, two homes with the same square footage could have wildly different HVAC needs depending on:
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Insulation quality
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Ceiling height
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Window type and shading
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Attic ventilation
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Ductwork layout
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Home orientation (north vs. south facing)
So don’t rely on napkin math. You want data — and that’s where Manual J comes in.
🧮 Manual J Load Calculation: The Gold Standard
Manual J is the industry-approved way to calculate how much heating and cooling your home really needs. It was developed by the folks over at ACCA (Air Conditioning Contractors of America), and it's the basis of every correct sizing job.
Here’s what Manual J considers:
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Square footage
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Local climate zone
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Roof material and slope
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Wall and attic insulation levels
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Duct condition and location
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Number and size of windows
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Indoor air quality and ventilation systems
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People and appliance heat load
A proper Manual J calculation usually requires software or a trained HVAC pro. But if you want to ballpark your needs at home, check out the HVAC Load Calculator from HVAC.com. It won’t replace a pro assessment, but it’s a great way to start narrowing down the right tonnage.
💨 Furnace Sizing: Don’t Forget the Heat
Most people get so hung up on the cooling side that they forget the heating.
Just like air conditioners are sized by tonnage (cooling capacity), furnaces are rated in BTUs of heating output. And while you may be comfortable with a 3-ton AC in the summer, you could need a 100,000 BTU furnace to stay warm in January.
Let’s say you live in the Midwest, in a 2,500 sq ft home with good insulation. Your Manual J calc might tell you:
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Cooling Load: 48,000 BTU (4 ton)
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Heating Load: 80,000 BTU (minimum furnace size)
But here’s the kicker — furnace ratings come with an AFUE efficiency number, and that matters big time.
For example:
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An 80% AFUE 100,000 BTU furnace delivers 80,000 BTU to the home.
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A 96% AFUE 100,000 BTU furnace delivers 96,000 BTU.
So always adjust your heating size based on the AFUE efficiency. Learn more about this from Energy.gov’s furnace overview.
📊 Why Oversizing Is a Bad Idea
Let’s say you say, “Heck with it, Tony — just give me a 5-ton system. Bigger is better, right?”
Wrong.
Oversized systems may cool or heat your home faster, but that leads to short cycling. That’s when your system turns on and off in rapid bursts. It sounds like this: Whirr… quiet… Whirr… quiet…
Short cycling causes:
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Poor humidity control
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Uneven temperatures
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Higher energy bills
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Extra wear and tear
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Shortened lifespan
Same goes for furnaces. An oversized furnace might heat the air fast, but it won’t distribute heat evenly. Some rooms bake while others stay cold.
🛠 Sizing Based on Climate Zones
Your region plays a massive role in sizing decisions. The U.S. is split into climate zones, and each has different HVAC load demands.
According to EnergyStar, here’s how climate affects tonnage:
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Zone 1–2 (Hot/Humid – FL, TX, LA): 1 ton per 500 sq ft
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Zone 3–4 (Mild – NC, TN, CA): 1 ton per 600–650 sq ft
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Zone 5–7 (Cold – NY, IL, CO): 1 ton per 700+ sq ft, with more focus on furnace BTUs
For colder climates, make sure you don’t undersize your heating system, especially if you have vaulted ceilings or old windows.
🔄 Sizing for a Dual-Fuel System
If you’re going with a dual-fuel system (gas furnace plus heat pump), you’ve got extra flexibility.
Here’s the trick: your heat pump handles the cooling and mild heating, and the gas furnace kicks in when temps drop too low (usually under 35°F).
That means your heat pump should be sized for the cooling load, while the furnace should cover your worst-case heating load.
This is where zoning and smart thermostats come in handy. A hybrid system gives you better efficiency but adds complexity — which is why you should consult a contractor trained in dual-fuel setups.
🔧 Don’t Forget the Ductwork
Your HVAC system is only as good as your ductwork.
If you’re going from a 3-ton to a 5-ton system but keep the same undersized ducts, you’ll choke the airflow. It’s like putting a firehose through a straw.
Common duct issues include:
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Undersized returns
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Poor sealing (causing leaks)
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Long runs without adequate boost
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No insulation in unconditioned spaces
Make sure your contractor does a duct sizing and leakage test during the install. If they don’t bring it up — you bring it up.
🧾 Real-World Sizing Scenarios
Let’s look at a few examples:
Case 1: Suburban Ranch Home – 2,200 sq ft (Dallas, TX)
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1 story, average insulation, 8’ ceilings
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Manual J cooling load: 42,000 BTU
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Manual J heating load: 60,000 BTU
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Recommended: 3.5-ton AC, 80,000 BTU gas furnace (80% AFUE)
Case 2: Two-Story Colonial – 2,800 sq ft (Cleveland, OH)
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2 stories, older windows, uninsulated attic
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Manual J cooling load: 48,000 BTU
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Manual J heating load: 90,000 BTU
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Recommended: 4-ton AC, 100,000 BTU gas furnace (96% AFUE)
Case 3: New Build – 2,600 sq ft (Atlanta, GA)
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High-efficiency windows, spray foam attic
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Manual J cooling load: 36,000 BTU
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Manual J heating load: 50,000 BTU
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Recommended: 3-ton AC, 60,000 BTU furnace (96% AFUE)
Every home is different — these are just examples. But they show why you can’t guess your tonnage just by square footage.
📦 Choosing the Right 3–5 Ton R-32 System
Once you’ve nailed down your sizing, the next step is picking a system that fits your budget, fuel type, and SEER2 goals. If you’re leaning toward something that’s efficient, future-proof, and available right now, check out the selection of R-32 AC and gas furnace systems in the 3–5 ton range.
R-32 gives you:
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Lower global warming potential (GWP)
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Faster cooling and better efficiency
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Smaller refrigerant charge
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Compliance with upcoming EPA and DOE regulations
And when matched with a properly sized furnace and coil, you get long-term performance that’s easy to service and easy on your wallet.
🧰 Final Sizing Tips from the Field
Here’s what I always tell my customers:
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Don’t oversize “just in case” — you’re asking for humidity problems
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Use Manual J as your guide, not guesswork or internet calculators alone
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Make sure your furnace size matches your AFUE rating
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Get your ducts inspected if you’re changing system size
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Always confirm that your contractor is familiar with R-32 refrigerant installs
If you take the time to size your HVAC system right, you’ll get better comfort, lower bills, and fewer breakdowns — plain and simple.
About the Author
Tony Marino is a licensed HVAC technician who’s been sizing, installing, and repairing heating and cooling systems for over two decades. He believes homeowners deserve the same straight talk he gives his own family. When Tony isn’t in the field, he’s writing guides to help folks make smarter HVAC decisions.
