Square footage is the laziest way to size a PTAC.
Tony hates it.
You should too.

Why? Because no two 300-sq-ft rooms behave alike. One might feel like a freezer in January. Another might turn into a sweat lodge at lunchtime. Same square footage—totally different load shape.

This is where Tony’s system-design brain flips the script:

“You don’t size the room to the unit. The ROOM tells you what unit it wants. You just gotta listen.” — Tony

In this guide, Tony breaks down exactly how he sizes PTACs using load shape, not generic square-foot charts that lead to short cycling, skyrocketing electric bills, and rooms that feel right only on paper.

Let’s get to work.

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🧭 What Most Homeowners Get Wrong About PTAC Sizing

Most people start with:

“My room is 350 square feet, so I need a 12,000 BTU unit.”

Wrong.
That’s like saying:

“I’m six feet tall, so I wear size 14 shoes.”

Square footage is a starting guess, not a design rule.

Tony has installed PTACs in:

• glass-heavy sunrooms

• high-ceiling lofts

• tiny bedrooms with massive west-facing windows

• hotel suites with two exterior walls

• insulated basements

• drafty old farmhouses

Square footage helped in none of these.

Why? Because PTACs aren’t just cooling air. They’re fighting:

• solar load

• thermal mass

• air leakage

• window quality

• humidity

• occupant behavior

• exterior exposure

BTUs ≠ square footage.
BTUs = load shape.

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🌞 Load Shape 101 — The Only Thing Tony Cares About

Load shape = how the room behaves throughout the day and year.

Think of it like the room’s personality:

• When does it heat up?

• When does it cool down?

• What materials is it built from?

• Where does the sun hit?

• What’s outside those walls?

• How much moisture is in the air?

Tony treats every room as a thermal story. You read it first, then you pick the equipment.

This is why Tony loves PTACs like the Amana J-Series 15,000 BTU PTAC with 3.5 kW Heat (https://thefurnaceoutlet.com/products/amana-j-series-model-15-000-btu-ptac-unit-with-3-5-kw-electric-heat-ptc153j35axx) — they’re flexible, forgiving, and built for rooms with tricky thermal personalities.

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🔍 Why Square Footage Lies (And How It Costs You Money)

Here’s a real Tony example.

Two rooms.
Both 350 sq ft.

Room A

• One small double-pane window

• Brick exterior wall

• North-facing

• Good insulation

Room B

• Two massive single-pane sliding doors

• West-facing

• Interior walls drywall + steel studs

• Air leakage under both doors

• Higher-than-average humidity

Same size.
Completely different BTU needs.

Room A might run great on a 9,000 BTU PTAC.

Room B might barely survive with a 15,000 BTU PTAC plus a heat kit.

If you sized both rooms using square footage only, Room B would cook you alive at 3pm every day.

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🌡️ Tony’s 7-Point Load Shape Formula

(This Is How He Sizes PTACs Without Guessing)

Tony doesn’t size by charts.
He sizes by conditions.

Here’s the exact checklist he uses before choosing BTUs.

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🔅 1. Solar Gain (The Sun Is Stronger Than Your AC)

Rooms with west or south exposure receive up to 40% more heat load than north-facing rooms.

Single-pane windows? Double the load.
Large glass areas? Triple it.

Tony’s rule:

“Show me your windows, and I’ll tell you your BTU.”

Bonus: You can use the U.S. Department of Energy’s guide on window performance here:
https://www.energy.gov/energysaver/design/windows-doors-and-skylights

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📏 2. Ceiling Height (High Ceilings Eat BTUs for Breakfast)

A 12,000 BTU unit in a 14-ft-ceiling loft is a joke.

Rule of thumb:
For every 1 ft above 8 ft, add 10–15% BTU load.

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💨 3. Air Leakage (Drafts Will Bankrupt Your System)

Old homes, basements, rental units, and converted garages often leak air like crazy.

Check:

• Under exterior doors

• Around electrical outlets

• Behind trim

• At attic hatches

• At old A/C sleeve cutouts

Every leak adds latent load (moisture), which is harder to remove than heat.

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🧱 4. Wall Construction & Insulation (Thermal Mass Matters)

Tony checks wall type before touching BTU calculators.

Brick or CMU walls
= absorb and release heat slowly
= more stable loads

2x4 or 2x6 wood framing
= less thermal mass
= faster heat swings

Metal studs
= act as thermal bridges
= move heat straight into the room

You can read more about thermal bridging here

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💧 5. Humidity Load (Moisture = Hidden BTUs)

Humidity eats BTUs. A 12,000 BTU unit in Florida doesn’t behave like a 12,000 BTU unit in Arizona.

Tony’s rule:

“If I walk in and the room smells musty, I’m adding BTUs.”

ASHRAE has great technical detail on humidity here

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🛏️ 6. Occupancy & Usage (People = 250 BTU Each)

People generate heat.

A guest room with two people sleeps fine on a 9k unit.

A studio where two people work remotely all day?
You’ll need more capacity.

Cooking, showers, pets, electronics, and laundry all change the load.

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🔌 7. Equipment Heat Load (Electronics Add Hidden BTUs)

TVs, gaming consoles, office equipment, hair dryers — all produce heat.

Tony always asks:

“What do you do in this room?”

Because a gaming PC adds more BTUs than a window.

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🔥 How Tony Uses Load Shape to Pick Between 9k, 12k, and 15k BTUs

Most PTAC sizes fall into:

• 9,000 BTU

• 12,000 BTU

• 15,000 BTU

Here’s how Tony decides, using real-world conditions.

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⚪ 9,000 BTU — Small Rooms With Low Load

Tony picks 9k when:

• North-facing

• Good insulation

• Small windows

• Low humidity

• Bedroom use

• No cooking or heavy electronics

Perfect for: standard hotel rooms and small studios.

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🟡 12,000 BTU — Medium Load Rooms

Tony uses 12k when:

• Mixed sun exposure

• Larger windows

• Normal humidity

• Two occupants

• Moderate electronics

Perfect for: guest rooms, home offices, short-term rentals.

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🔴 15,000 BTU — High Load Rooms

This is where big boys like the Amana J-Series 15k PTAC shine.

Tony uses 15k when:

• West or south-facing

• High ceilings

• Older windows

• High humidity areas

• Two exterior walls

• Sliding glass doors

• Loft-style layouts

Basically, when the room fights back.

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🛠️ Why Tony Hates Oversizing (Even If It Sounds Safer)

Oversizing feels safe, but it backfires.

Problems Tony sees weekly:

• Short cycling

• Poor dehumidification

• Higher bills

• Higher noise levels

• Coil icing

• Temperature swings

• Rooms that feel cold but clammy

When the unit is too big, it cools the air fast but doesn’t run long enough to dry the air.

Tony says:

“Comfort is a moisture game, not a cold-air game.”

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🧊 Why Undersizing Is Worse Than Oversizing

Undersizing leads to:

• Constant runtime

• Burned-out compressors

• Overheated electric heat kits

• Tripped breakers

• Comfort complaints

• Mold growth

A 9k unit trying to cool a 500-sq-ft west-facing sunroom is like giving a lawn mower a job plowing snow.

Tony never undersizes.
Ever.

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📐 Tony’s BTU Chart (But Based on Load Shape, Not Sq Ft)

This is NOT a square-foot chart — it’s a load profile chart.

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🧱 How Wall Construction Changes the Entire System Design

Let’s compare wall types.

Wood-framed walls (2x4 or 2x6)

• Fast heat transfer

• Need more BTUs

• Sensitive to outdoor extremes

Metal studs

• Even worse thermal bridging

• Tony adds 10–15% more BTU

Concrete block

• Slower heat gain

• More stable

• Often need smaller units

Brick

• Excellent thermal mass

• Helps buffer swings

• But increases humidity in some climates

This matters more than square footage ever will.

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🌬️ Why Return Air Path Determines Comfort (Not BTUs)

Tony emphasizes that PTACs breathe through a very restricted return path.

Block that return path, and:

• Temperature split drops

• Coil freezes

• Compressor overheats

• Room gets uneven

• Humidity spikes

Tony checks:

• Furniture placement

• Curtains

• Bedframes

• Desk alignment

• Behind-the-sleeve insulation

A PTAC breathes through the sleeve—you choke the sleeve, you choke the system.

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🧪 Tony’s 3-Minute Test to Confirm Correct Sizing

Tony uses this test after installation:

**Step 1: Set cooling to 65°F.

Step 2: Run the unit for 10 minutes.
Step 3: Measure the supply and return temps.**

Target temp split: 16–22°F.

• Less than 16°F = undersized or airflow problem.

• More than 22°F = oversized or low airflow.

Simple. Field-proven.

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⚙️ Why the Amana J-Series 15,000 BTU PTAC Works for Tough Load Shapes

Tony loves this unit because:

• High output cooling

• Strong compressor for high humidity

• Reliable 3.5 kW heat kit

• Good moisture removal

• Long duty cycle capability

• Works well with insulated sleeves

• Quiet operation even at high CFM

It’s the “do-everything” PTAC for complex rooms.

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📦 A Word About Wall Sleeves (Most People Get This Wrong)

Tony’s rule:

“The sleeve is the ductwork. Don’t pick bad ductwork.”

Cheap sleeves:

• Restrict airflow

• Cause back pressure

• Increase compressor temperatures

• Allow outdoor air leakage

• Kill coils early

A $90 bad sleeve can destroy a $900 PTAC.

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🛠️ Tony’s Field Checklist (Use This Before You Pick BTUs)

1. Window size & direction

2. Insulation type

3. Wall construction

4. Ceiling height

5. Occupancy pattern

6. Humidity conditions

7. Electronics

8. Return air clearance

9. Sleeve type & airflow path

10. Local climate zone

Only after he collects this does Tony pick BTUs.

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🏁 Final Word — The Room Chooses the Unit, Not the Chart

Square footage is quick, easy, and wrong most of the time.

Load shape tells the real story.

If you size your PTAC using Tony’s system-design method, you’ll get:

• Better comfort

• Lower energy bills

• Longer equipment life

• Fewer callbacks or complaints

• Correct humidity control

• Stable temperatures

• Balanced heating & cooling

If you size by square footage…

You’ll get whatever the chart thinks your room is.
Not what it actually is.

Buy this on Amazon at: https://amzn.to/47cH9ut

In the next topic we will know mmore about: Electric Heat Isn’t Backup — It’s Part of the Design. How Tony Chooses Between 3.5 kW, 5 kW, and Dual-Stage Heat Kits