🛏️🖥️ How to Size for Dual Spaces: Getting the Right PTAC for a Bedroom-Plus-Office Setup

If there’s one type of space that keeps tripping homeowners up when choosing a PTAC, it’s the bedroom + home office combo.

Amana J-Series PTAC Model 17,000 BTU PTAC Unit with 5 kW Electric Heat

Maybe you’ve merged two small rooms into one.
Maybe you converted a spare bedroom into a workspace.
Maybe you’re heating and cooling two adjoining rooms through one system because adding ductwork is a pain.

Whatever the case, the same problem always comes up:

❗ Sizing for one load gives you a system too small… but sizing for both often overshoots the mark.

And oversizing causes short-cycling, humidity issues, uneven comfort, and higher energy bills — especially in small or medium rooms.

Today, I’m breaking down exactly how to size the perfect PTAC for a bedroom-office dual space — the kind that keeps you comfortable while you work during the day and helps you sleep at night.

No guessing.
No oversizing.
No underpowered units.

Just clear, real-world math and Tony’s simple rules.

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🧱 1️⃣ Why Dual Spaces Are Tricky to Size

Dual rooms — or a single room serving two functions — behave completely differently from standard square-footage spaces.

Let’s break down why they’re complicated.

👤 Different Comfort Expectations

• Bedrooms run colder and quieter.

• Offices run warmer because of electronics and daytime activity.

A PTAC sized for sleeping comfort often can’t keep up with office heat loads.
A PTAC sized for your busy daytime office becomes too strong at night, cycling on/off and making noise.

🧠 Different Occupancy Patterns

• Office peak load: 10 AM–4 PM

• Bedroom peak load: 10 PM–6 AM

Most rooms have one peak. Dual spaces have two opposite peaks.

🔌 Different Internal Heat Loads

Offices add extra heat from:

• laptops

• monitors

• printers

• routers

• lighting

• your body heat

Bedrooms add almost none of that.

🌞 Different Sun Exposure

Maybe:

• the office gets afternoon sun

• the bedroom faces north

• or only one room has windows

Sun exposure can change BTU demand by 10–25% — which throws off standard sizing.

For reference:
Rooms with heavy sun exposure may require 10–20% more BTUs (EnergyStar).

🚪 Different Airflow Paths

Airflow is the biggest overlooked factor.

If your PTAC sits on one side of the space — say the office — the bedroom may not receive enough conditioned air unless:

• the door stays open

• there’s a transfer grille

• airflow can circulate freely

All these small variables complicate sizing.

This is why dual spaces can’t be treated like a big rectangle.

They’re more like two smaller rooms sharing one system — and that requires careful BTU balancing.

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🔍 2️⃣ The Problem With Treating Two Rooms as One Big Room

Most homeowners add up the square footage:

Bedroom: 150 sq ft
Office: 120 sq ft
Total: 270 sq ft

Then they say:

“I’ll just size for 270 sq ft.”

That’s a mistake.

Here’s why:

🚫 Square footage alone ignores shape

Air doesn’t flow evenly into corners, around walls, or through doorways.
A 270 sq ft open studio behaves differently from two boxed rooms sharing a door.

🚫 Only one of the rooms may have the PTAC

Even if both rooms total 270 sq ft, the load isn’t distributed evenly.
The PTAC might be cooling one room perfectly and the other only partially.

🚫 Heat loads differ dramatically

Offices run hotter.
Bedrooms run cooler.
This means the combined load is not equal to the sum of both floors areas.

🚫 One room may be shaded

If the office has windows and the bedroom doesn’t, the rooms have different thermal behavior.

🚫 Doors change everything

Open doors?
Closed doors?
Half-closed at night?

Airflow might be limited when you actually need comfort the most.

You must size for two distinct behaviors — not one.

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🧮 3️⃣ Tony’s Dual-Space Sizing Formula (The Simple Way)

I’ve tested dozens of methods over the years, and this one just works. It avoids oversizing while accounting for real room behavior.

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🔢 STEP 1: Calculate each room individually using the 20 BTU per sq ft baseline

Office:
120 sq ft × 20 BTU = 2,400 BTU

Bedroom:
150 sq ft × 20 BTU = 3,000 BTU

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👥 STEP 2: Add internal heat loads for the office

Offices add between 400–1,200 BTUs depending on usage.

Use this table:

Example office load: 800 BTUs

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🌞 STEP 3: Adjust for sun exposure

• North-facing or shaded → 0%

• Moderate sun → +10%

• Heavy afternoon sun → +15–20%

Say the office has a west-facing window → add 15%

Office load after sun adjustment:
(2,400 + 800) × 1.15 = 3,680 BTU

Bedroom load:
3,000 BTU (no adjustments)

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➕ STEP 4: Combine the loads

3,680 BTU + 3,000 BTU = 6,680 BTU

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🔽 STEP 5: Apply the balancing factor (subtract 10–15%)

Why subtract?

Because:

• Both rooms won’t peak at the same time

• Airflow limitations reduce “effective load” on the PTAC

• Real-world diversity lowers the true combined load

Use 10% for airflow-friendly layouts.
Use 15% if the doorway is small or offset.

Let’s use 10%:

6,680 × 0.90 = 6,012 BTU

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🔼 STEP 6: Round to nearest available PTAC size

PTACs are commonly:

• 7,000 BTU

• 9,000 BTU

• 12,000 BTU

• 15,000 BTU

A 6,012 BTU load → choose a 7,000 BTU PTAC.

Perfect for a 270 sq ft dual room.

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🌡️ 4️⃣ Day–Night Load Shifts: The Factor Most People Miss

Dual rooms behave differently depending on the time of day.

🕘 Daytime (Office Peak)

• Computers: on

• Lighting: on

• Sun exposure: active

• Occupancy: high

• Internal load: high

🌙 Nighttime (Bedroom Peak)

• Computers: off

• Sun: gone

• Occupancy: resting

• Internal load: low

• Cooler outdoor temps

Most of the time, nighttime loads drop by 15–40% compared to daytime.

So if you size for the OFFICE only, you absolutely oversize for the bedroom.
That’s why people complain that the room feels too cold, cycles too fast, or stays humid at night.

Removing heat and removing humidity are two different tasks — and nighttime humidity is a killer for oversized units 

Dual-room PTAC sizing must balance:

Office daytime comfort

AND

Bedroom nighttime comfort

You cannot size for only one.

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🪟 5️⃣ Sun, Windows, and Insulation — How Each Room Affects the Other

Windows dramatically change heat gain.

🌞 If the office has windows and the bedroom doesn’t:

The office will run hotter.
Bedroom will lag behind the office’s thermal load.

You MUST add office heat load (10–20% is common).

🧱 If insulation differs between rooms:

One room might lose heat faster in winter.
One might retain heat longer in summer.

You’ll need to adjust the BTUs accordingly.

💨 If one room is interior while the other is exterior:

Interior room → stable temp
Exterior room → fluctuates wildly

Dual usage magnifies those differences.

They recommend adding 10–20% BTUs for sunny rooms and using shading factors for window-heavy layouts.

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🖥️ 6️⃣ Electronics, Occupancy & Hidden Heat Loads

Offices generate heat. Bedrooms don’t.

Let’s break down common hidden loads:

💻 Electronics

• Computer tower: +400–600 BTUs

• Gaming laptop: +250–350 BTUs

• Dual monitors: +300–600 BTUs

• Printers/scanners: +100–200 BTUs

• LED desk lamps: +50–100 BTUs

• A/V gear or ring lights: +200–800 BTUs

Offices add 25–45% more heat than bedrooms.

👥 People

Each extra occupant beyond the first → add 600 BTUs.
(EnergyStar occupant load guidance)

🛌 Bedroom loads

Bedrooms usually just have:

• 1–2 people

• minimal electronics

• low daytime load

Their BTU requirement is far more predictable.

This is why you calculate each room separately before combining.

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🌀 7️⃣ Airflow Strategies: Making One PTAC Support Two Zones

A PTAC can heat/cool multiple spaces IF you help it.

Here’s how.

🚪 1. Keep the connecting door open

This lets cool air drift and warm air return.

➡️ 2. Use a transfer fan

Doorway fans:

• cost $35–$60

• improve airflow by 15–30%

• eliminate hot/cold pockets

🧱 3. Don’t block the PTAC with furniture

Air needs space to spread.

🖼️ 4. Avoid “dead corner zones”

If the bedroom is far from the PTAC, aim airflow toward the doorway.

🔁 5. Use ceiling fans

Helps push conditioned air into secondary areas.

🪟 6. Add blackout curtains in the office (if sunny)

Reduces load by 10–20%.

🔧 7. Level the thermostat behavior

Many PTACs use return-air sensing.
If the return is in the office, it may shut off early.

Choose a model with:

• remote thermostats

• wireless sensors

• or wall thermostats

This gives even readings.

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📏 8️⃣ Tony’s Recommended PTAC BTU Ranges for Common Dual-Space Layouts

These examples assume moderate insulation, 8–9 ft ceilings, and standard occupancy.

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🛏️🖥️ Example A: 200 sq ft Bedroom + 100 sq ft Office

Office load:
(100 × 20) + electronics (800) + sun (+10%)
= 2,200–2,600 BTUs

Bedroom load:
200 × 20
= 4,000 BTUs

Combined: ~6,200 BTUs
Balanced: 6,200 × 0.90 = 5,580 BTUs
PTAC size: 7,000 BTUs

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🛏️🖥️ Example B: 150 sq ft Bedroom + 150 sq ft Office (Shaded)

Office: (150 × 20) + 600 equipment = 3,600
Bedroom: 150 × 20 = 3,000
Total: 6,600
Balanced: 5,900
PTAC size: 7,000 BTUs

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🛏️🖥️ Example C: 300 sq ft Loft Divided by Furniture

Furniture partitions add resistance & raise effective load by 10%.

(300 × 20) = 6,000
+10% partition factor = 6,600
+400–600 equipment load = 7,000–7,200
Balanced: ~6,300
PTAC size: 9,000 BTUs

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🛏️🖥️ Example D: 220 sq ft Studio with Two Distinct Zones

220 × 20 = 4,400
+600 equipment = 5,000
Balanced = 4,500
PTAC size: 5,000–7,000 BTUs depending on sun exposure

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🔌 9️⃣ When You Actually Need Two Units Instead of One

A single PTAC is NOT always the right answer.

Here’s when you add a second system.

🚫 1. L-shaped or U-shaped floorplans

Airflow cannot turn corners efficiently.

🚫 2. Rooms separated by solid doors

If you prefer closed-door sleeping, you need zoned heating/cooling.

🚫 3. Office with heavy equipment

Gaming rigs, servers, or A/V setups = high thermal load.

🚫 4. High-sun office + shaded bedroom

The difference in loads becomes too big for one system.

🚫 5. Rooms differ by more than 20–25% temperature needs

For example:

• You like your office cool (68°F)

• You like your bedroom warm (72°F)

One system cannot satisfy incompatible setpoints.

If any of these describe your layout — use two smaller PTACs instead of one big one.
Oversizing is always the wrong move.

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🧠 🔟 Tony’s Final Takeaway

Here’s what I tell every homeowner with a bedroom-office combo:

⭐ “Dual spaces are all about balance — not brute force.”

You can’t size one PTAC based on square footage alone.
You can’t size for just the bedroom.
You can’t size for just the office.

You need:

1. Individual room BTU calculations

2. Heat load adjustments from electronics & sun

3. Diversity correction (10–15% reduction)

4. Proper airflow strategies

Follow that, and you’ll get:

• smoother cycles

• better humidity control

• lower energy bills

• consistent temperature day and night

• longer equipment life

• real comfort in BOTH spaces

And trust me — when a PTAC fits the load like a glove, you can feel it immediately.

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In the next topic we will know more about: Climate Zone Secrets: Why the Same PTAC Performs Differently in Miami vs. Minneapolis