Most homeowners size a PTAC based on what their room looks like right now — its current square footage, current insulation, current windows, current layout, and current use.
And that’s exactly how people end up with a perfectly good PTAC today…
that becomes undersized, oversized, inefficient, or uncomfortable after a simple remodel.
Amana J-Series PTAC Model 17,000 BTU PTAC Unit with 5 kW Electric Heat
Here’s the truth I’ve learned over 30+ years in this business:
🔧 Your room will change faster than your PTAC will.
Most PTACs last 10–15 years.
Most homes change every 3–7 years.
That means if you're not sizing with the future in mind, you’re basically building comfort on quicksand.
Rooms get expanded.
Walls get moved.
Windows get added.
Attics and garages get finished.
Offices turn into nurseries.
Guest rooms turn into home gyms.
Shaded rooms become sunrooms.
And each change can swing your heating or cooling requirement by 20%… 30%… sometimes even 50%.
This is the guide I give homeowners who want a unit that works today and tomorrow — the same forward-thinking approach hotels, commercial builders, and multi-unit property owners use to avoid being blindsided by remodel-related load changes.
Let’s get into it.
🔮 1️⃣ Why “Sizing for Today Only” Is One of the Biggest HVAC Mistakes Homeowners Make
The mistake almost everyone makes is this:
“Let’s size the PTAC for how this room is right now.”
The problem?
Rooms evolve.
Houses evolve.
Families evolve.
Lifestyles evolve.
A system that fits today’s room can become inadequate as soon as you:
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add a window,
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finish an attic,
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knock down a wall,
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add insulation,
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turn a bedroom into a home office,
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or change furniture layout in a way that affects airflow.
Hotels don’t size this way.
Commercial contractors don’t size this way.
Property developers don’t size this way.
They size for future load — not just current load.
Homes should be no different.
Here’s why future-proof sizing matters:
✔️ Remodels change BTU demand
Even a simple remodel can alter:
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airflow patterns
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sun exposure
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humidity load
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heat loss
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required heating wattage
✔️ You avoid replacing your PTAC too early
A future-proofed unit lasts longer and performs better across remodels.
✔️ You can maintain comfort no matter how the room evolves
No more “too hot after the remodel” or “too cold in winter after adding a window.”
✔️ It saves money long-term
Buying one well-sized PTAC now beats buying two over the next decade.
✔️ You get flexibility
A slightly larger unit with variable modes gives you more control as the room grows.
You’re not buying a PTAC for this year.
You’re buying it for the next decade.
Sizing for today is short-term thinking.
Sizing for tomorrow is how you keep comfort consistent.
🛏️🔧 2️⃣ Common Remodels That Dramatically Change Your Heating & Cooling Load
Here’s a dirty little secret:
Most remodels affect HVAC load more than homeowners realize.
Let’s break down the most common ones.
🖥️ ✔️ Converting a Bedroom Into a Home Office
Heat load increases because:
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computers
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monitors
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routers
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printers
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UPS backup systems
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task lighting
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electronics
A typical office setup adds 500–1,000 BTUs.
🛏️ ✔️ Turning a Space Into a Nursery or Kids’ Room
Kids = higher heat load from:
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movement
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humidifiers
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sound machines
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blackout curtains (trap heat)
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closed-door sleeping
Expect +400–800 BTUs, plus more overnight heating demand.
🪟 ✔️ Adding Windows or Sliding Doors
This is a BIG one.
A single window or glass slider can:
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change solar load
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increase heat gain
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increase heat loss
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increase humidity infiltration
For sun-heavy windows, expect 10–30% more BTU requirement.
🧱 ✔️ Removing Walls
Open layouts cause the cooling load to spread.
A PTAC that used to cool a 250 sq ft room now has to fight a 450 sq ft space.
📚 ✔️ Adding Built-in Shelving or Closets
Reduces square footage but often interferes with airflow.
Poor airflow = increased runtime.
🧰 ✔️ Finishing a Basement or Attic
Both are notorious for extreme temperature swings:
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basements absorb cold
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attics absorb heat
Expect major BTU adjustments.
🚗 ✔️ Converting a Garage
Garages have:
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poor insulation
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concrete floors
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unconditioned air
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high heat gain and loss
You almost always need 25–40% more heating and cooling capacity.
🧼 ✔️ Upgrading Flooring
Swapping carpet for tile or hardwood affects:
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floor temperature
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radiant heat loss
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winter comfort
Tile floors often require higher heating wattage for winter performance.
You get the idea.
Your room changes — your HVAC needs change.
This is why you never size a PTAC for only what the room looks like today.
🌞 3️⃣ How Sun Exposure Changes After a Remodel (and Why It Matters Even More Than Square Footage)
One of the biggest mistakes homeowners make is ignoring solar gain.
Hotels NEVER do.
Sun exposure can change drastically after a remodel:
✔️ You add bigger windows
More sun = more cooling load.
✔️ You cut back shading trees
Suddenly that “cool room” becomes a sun oven.
✔️ You add a skylight
That’s direct heat from above.
✔️ You change the room orientation
Removing or moving walls changes where the sunlight hits.
✔️ You finish a room above a garage
These rooms experience harsh heat gain and loss.
See EnergyStar’s guidance on solar exposure here
If you know your remodel will increase sun exposure, you MUST size with this in mind.
🧮 4️⃣ Tony’s Future-Proof Sizing Formula (The Simplest You'll Ever Use)
This is the formula I use for homeowners planning remodels.
🧩 STEP 1: Start with today’s BTU calculation
20 BTUs per square foot is the baseline.
🧩 STEP 2: Add your future project buffer
If remodel involves:
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new windows → +15–25%
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wall removal → +20–40%
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room expansion → +10–30%
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increased electronics → +500–1,000 BTUs
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higher ceilings → +20%
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conversion to office / guest suite → +10–15%
🧩 STEP 3: Subtract efficiency gains
If remodel includes:
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insulation upgrades → subtract 10–20%
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window replacement (low-e) → subtract 5–15%
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vapor sealing → subtract 5–10%
🧩 STEP 4: Evaluate future humidity
More windows → more humidity.
More people → more humidity.
More sunshine → more humidity.
Humidity load matters — see Energy.gov:
🔗 https://www.energy.gov/energysaver/air-conditioning
🧩 STEP 5: Choose a PTAC with flexible settings
These give you wiggle room:
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multiple fan speeds
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adjustable humidity modes
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modulating heat pump
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optional heat kit upgrades
🧩 STEP 6: Never size more than one class up
This avoids dangerous oversizing:
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9k → 12k (OK)
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9k → 15k (too big)
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12k → 15k (OK)
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12k → 18k (too big)
This rule alone prevents short-cycling.
🌀 5️⃣ How to Predict the Room’s Future Load Before You Remodel
When I walk into a home, I ask the homeowner one simple question:
“What will this room be in 5 years?”
Most people have an answer:
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“It’ll become my office.”
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“This will be the nursery.”
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“We want to open the wall next year.”
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“We’re adding windows.”
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“Planning to finish the attic.”
Here’s what I want you to consider.
🧠 Ask yourself:
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Will more people use this room?
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Will electronics increase?
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Will the sun exposure change?
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Will windows be added or removed?
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Will the room size increase?
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Will insulation improve?
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Will the room take on a second-purpose (office/guest)?
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Will you raise ceilings?
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Will doors remain open or closed?
Every answer affects BTUs.
If ANY of those impacts your room, then sizing only for today is a mistake.
🌀 6️⃣ When You Should Step Up One BTU Size — and When You Should NOT
This is where homeowners blow it — not intentionally, but because they use old sizing rules that don’t apply to evolving homes.
Here’s my rule:
✔️ Step up one BTU class if you're planning ANY of these remodels:
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adding windows
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removing walls
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finishing attics
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finishing basements
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converting a garage
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adding electronics
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turning room into office or gym
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raising ceilings
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opening up the layout
❌ DO NOT step up more than one size:
Oversizing causes:
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short-cycling
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uneven cooling
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humidity issues
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noise complaints
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temperature swings
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higher energy use
Hotels rarely oversize.
You shouldn’t either.
🔥 7️⃣ Choosing the Right Electric Heat Kit for the Room’s Future Winters
Remodeling changes heat loss dramatically.
Adding windows?
Heat loss increases.
Removing interior walls?
Heat loss increases.
Raising ceilings?
Heat loss increases.
Improving insulation?
Heat loss decreases.
Here’s a simple rule:
✔️ Choose 3.5 kW if:
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you're in a warm climate
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room is well insulated
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no new windows planned
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room is under 350 sq ft
✔️ Choose 5 kW if:
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adding windows
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room is over 350 sq ft
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room is north-facing
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remodeling near an exterior wall
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room is above a garage
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you live in a mixed climate
✔️ Choose 7 kW if:
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cold climate zone
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severe winter area
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large remodel increases heat loss
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multi-use room needs quick morning warm-ups
Reference: 🔗 https://www.energy.gov/energysaver/heat-pump-systems
🧱 8️⃣ Insulation Upgrades That Can Reduce Your BTU Demand
Not all remodel changes increase your load.
Some remodels reduce the BTU requirement — which means your PTAC doesn’t have to work as hard.
🧊 Insulation upgrades that lower load:
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spray foam walls
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insulated ducts
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attic insulation
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basement vapor barriers
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insulated garage ceilings
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low-E window upgrades
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airtight sealing and caulking
This is why I ask homeowners:
“Will you be upgrading insulation in the next 3–5 years?”
Sometimes a smaller unit — paired with a quality heat kit — is better long-term.
🌀 9️⃣ Features That Make a PTAC Future-Proof
Hotels use these features because they expect remodels, furniture layout changes, and occupancy swings.
Homeowners can benefit from them too.
✔️ Variable-speed fans
Allows the unit to adapt to higher or lower future loads.
✔️ Smart thermostats
Adjust for lifestyle, usage, and remodel changes.
✔️ Modulating or inverter heat pumps
Provide smoother, longer cycles whether the room is small or large.
✔️ Upgradable heat kits
A future remodel might require 5 kW instead of 3.5 kW.
✔️ Enhanced dehumidification modes
Critical if new windows are added.
✔️ Low-noise fans
Great if the room becomes a bedroom later.
These features extend the lifespan of your comfort by adapting to change.
🧠 🔟 Tony’s Future-Proof Sizing Cheat Sheet
Here’s the quick-reference list I give all remodel clients:
✔️ Add 10–20% BTU buffer for future expansions
Never more than 20%, never less than 10%.
✔️ Add 600 BTUs per future occupant
Kids, guests, home offices — it all counts.
✔️ Add 15–25% if adding windows
The largest single BTU changer.
✔️ Subtract 10–20% for insulation upgrades
Better insulation holds conditioned air.
✔️ Step up one BTU class only
Never jump multiple sizes.
✔️ Choose a PTAC with flexible cooling and heat options
Future-proofing means adjustability.
✔️ If converting attic/garage → go up 20–40%
Those spaces are thermal nightmares.
🏁 11️⃣ Tony’s Final Take: Size for Where Your Home Is Going — Not Where It Is Today
A PTAC should last you 10 to 15 years.
Your room will not look the same 10 to 15 years from now.
The biggest mistake homeowners make is buying a unit for the room they have today — not for the space it will become.
If you’re planning ANY remodel, expansion, layout change, or future repurposing of a room, future-proof sizing is not optional — it’s essential.
⭐ Future-proof PTACs last longer.
⭐ They perform better after remodels.
⭐ They handle shifting loads.
⭐ They save money long-term.
⭐ They give you peace of mind.
And in this business, nothing beats a system that stays comfortable through every season — and every remodel.
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In the next topic we will know more about: Voltage, Amperage, and BTUs: How Electrical Specs Shape Your System Size Choices
