Mike Sanders has a rule he repeats on every job, whether he’s installing a through-the-wall AC, a PTAC, a heat pump, or a multi-zone ductless system:
“Air doesn’t care about your thermostat. It follows pressure.”
Rooms that feel:
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muggy
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stagnant
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uneven
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stuffy
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or oddly warm in one corner and cold in another
…are almost never suffering from a BTU shortage.
Instead, they’re experiencing pressure imbalance—the invisible force that dictates how air moves, where it gathers, and why some rooms can’t “breathe.”
This long-form guide breaks down how Mike designs pressure-neutral rooms, eliminating stale pockets and restoring circulation using building-science principles and decades of field experience.
📘 1. What “Pressure-Neutral” Really Means
A pressure-neutral room has:
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equalized intake and exhaust paths,
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no significant air accumulation or depletion zones,
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predictable circulation loops,
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stable comfort without hot/cold stratification,
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and free-flowing return paths.
In a pressure-neutral environment, the room’s air behaves like a looping river, not a blocked pond.
A pressure-neutral room:
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pulls in only as much air as it can exhaust
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exhausts only as much air as it can recirculate
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avoids pushing conditioned air into dead zones
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stays stable even when doors open or close
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distributes cooling/heating evenly
Mike designs every layout—especially with through-the-wall units—to maintain this balance.
🧭 2. Why Imbalance Happens (And Why Most Homes Have It)
Most rooms are not pressure neutral due to:
🚪 2.1. Door Position & Undercuts
When a door closes tightly, the room becomes a sealed container.
That traps:
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supply air
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humidity
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heat
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pollutants
It also prevents stale air from returning to the unit.
🛋️ 2.2. Furniture Blocking Return Paths
Return airflow doesn't just move backward in a straight line.
It crawls along:
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floors
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walls
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room edges
Furniture blocking edges or corners kills circulation.
🪟 2.3. Window & Exterior Wall Pressures
Windows cause radiant heating and cooling, altering air density.
Warm air rises, piling up near ceilings and drifting into corners.
🌀 2.4. Missing Secondary Air Pathways
Rooms with:
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closets
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alcoves
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partitions
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corner bump-outs
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stair connections
…often trap pockets of air simply because there is no return vector.
🔌 2.5. The AC Unit Itself
Many through-the-wall and PTAC units:
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push air forward strongly
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but draw return air only from a small intake area
This creates an imbalance that homes were never designed to handle.
📐 3. Mike’s First Step: Mapping the Room’s Air Pressure Zones
Mike begins with a pressure scan, a process any homeowner can copy with simple tools.
🧪 3.1. The Tissue Test
He holds a tissue near:
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baseboards
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door gaps
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corners
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vents
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return grille
Movement indicates active pressure flow—or leaks.
🕯️ 3.2. The Candle/Incense Drift Test
Air movement becomes visible:
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toward a dead pocket
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along walls
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under doorways
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around the AC
Mike learns which direction the room “breathes.”
🌡️ 3.3. Temperature Stratification Scan
With an IR thermometer, he measures:
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ceiling
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mid-wall
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floor
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corner temps
Differences larger than 4°F suggest imbalance-induced stagnation.
🔵 3.4. Humidity Hotspot Check
Humidity collects where air slows.
Mike uses a handheld hygrometer to identify microzones above 55% RH—the giveaway of stale air.
Reference:
EPA Indoor Air Quality & Humidity: https://www.epa.gov/indoor-air-quality-iaq
📊 4. The Pressure Loop: How Air Should Behave in a Healthy Room
Mike’s ideal airflow loop consists of:
➤ 4.1. Primary Throw Path
The cool/ warm air exits the unit and travels into the room.
🔄 4.2. Cross-Flow Spread
Air wraps around objects and diffuses laterally.
↩️ 4.3. Return Loop
Air loses energy, drops lower, and flows back toward the intake.
♻️ 4.4. Exhaust Equalization
Air is pulled into the unit at the same rate that conditioned air is pushed out.
If any of these four stages fail, the room becomes pressure-imbalanced.
🪄 5. Mike’s 6 “Pressure Laws” for Perfect Balance
These are the fundamentals Mike uses on every job.
🪟 Law 1: Air Always Seeks the Lowest Resistance Path
Air flows where it is easiest—not where you want it to.
Mike structures rooms so the easiest path is the correct path.
🧱 Law 2: A Closed Door Is a Pressure Wall
Unless there’s a ¾" undercut or alternative return path,
closing the door instantly unbalances the room.
↕️ Law 3: Warm Air Piles Up Vertically
A room with 10–12° temperature difference between floor and ceiling has a pressure imbalance—not an equipment issue.
♻️ Law 4: Stagnant Air Is a Symptom of Poor Return Flow
If the return path is blocked, the AC will keep blowing air forward but nothing will cycle back.
🪶 Law 5: Light Objects Reveal Real Air Behavior
Mike uses:
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feathers
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incense
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tissue paper
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micron-light strips
These show how pressure behaves better than any gadget.
📦 Law 6: Corners Are Where Air Goes to Die
Every stagnant pocket Mike ever fixed began in a corner.
Corners require deliberate airflow design.
🔧 6. Mike’s Pressure-Neutral Design System (Step-by-Step)
This is the full method he uses to create equilibrium in any room.
1️⃣ Step One: Identify the Return Path (The Heart of Pressure Balance)
Mike asks:
“If air leaves the AC and travels forward…
how does it find its way back?”
He ensures the return path:
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travels along walls
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has zero furniture blockages
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avoids “dead legs” created by doorways
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has enough space for air to circulate
He often shifts furniture by as little as 3–6 inches to re-open a return loop.
2️⃣ Step Two: Evaluate Door & Hallway Pressures
If the room’s door shuts tightly:
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the AC becomes a pressurizing device
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the room loads with conditioned air
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stale air accumulates
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humidity stabilizes in corners
Mike fixes this by:
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increasing the undercut
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adding a jump duct
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installing a transfer grille
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modifying the return path
Reference:
Door undercut & airflow research – https://www.energy.gov/energysaver/whole-house-ventilation
3️⃣ Step Three: Redirect the Primary Throw
Through-the-wall units often blast air forward.
Mike adjusts:
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vane angles
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output direction
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fan speeds
He aims the throw toward:
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the room’s longest open path
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the area opposite the return loop
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high-load zones (windows, hot walls)
This enforces a healthy circulation cycle.
4️⃣ Step Four: Activate the Cross-Flow Zone
Mike ensures:
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no tall furniture interrupts the lateral spread
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air flows behind sofas, not into them
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circulation reaches every corner
He may install:
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small, silent cross-flow fans
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doorway circulators
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low-floor “assist fans”
These maintain lateral movement.
5️⃣ Step Five: Neutralize Pressure Pockets
Stale pockets appear where air stagnates.
Mike fixes these by:
🪑 Adjusting furniture geometry
Opening a 2–3 inch gap behind couches can restore circulation entirely.
📦 Clearing floor edges
Air travels along walls like a rail track.
🌀 Installing a micro-return booster
Small 4–6 inch whisper fans help pull air from pockets back into circulation.
🪟 Addressing window convection
Warm window air flows upward and disrupts the loop; Mike may redirect AC throw toward that wall.
6️⃣ Step Six: Balance Intake vs. Exhaust
The AC must inhale as much as it exhales.
Mike ensures:
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intake grille is never blocked
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room volume matches unit output
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fan speed matches room impedance
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no competing air sources exist (vents, fans, open windows)
If the unit is “starved,” pressure imbalance is guaranteed.
🧩 7. Mike’s Fixes for Common Room Layout Problems
🪟 Problem 1: Window-Loaded Rooms
Hot window air creates vertical convection.
Mike’s Fix:
Aim the AC output across the windowed wall to flatten convection loops.
🛋️ Problem 2: Big Sectional Sofa Blocks Return Path
This is the #1 cause of stale air behind couches.
Fix:
Move the sectional 3 inches forward to open the floor return pathway.
👕 Problem 3: Walk-In Closet Attached
Closet becomes a negative-pressure trap.
Fix:
Install a grille between the closet and room to equalize pressures.
🔁 Problem 4: L-Shaped Rooms
Air doesn’t turn corners naturally.
Fix:
Add a helper fan at the corner transition to force a loop.
🚪 Problem 5: Door Closes Too Tight
Cuts off return airflow.
Fix:
Add a ¾" undercut or a jump duct.
📉 8. Diagnostics: How Mike Proves a Room Is Pressure-Neutral
He runs:
🔵 8.1. RH Stabilization Test
Humidity stabilizes below 50–55%.
🌡️ 8.2. Stratification Reading
Ceiling and floor temps differ by less than 4°F.
🕯️ 8.3. Drift Test
Smoke or incense follows a smooth loop, not chaotic swirls.
🧻 8.4. Tissue Test
No fluttering in corners or near baseboards.
📊 8.5. ΔT Efficiency Check
The AC gains 1–2 degrees of performance simply from pressure neutrality.
⚙️ 9. Performance Gains from Pressure-Neutral Design
When Mike re-balances a room, homeowners typically gain:
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20–40% faster cooling
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10–18% energy savings
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60–90% reduction in stale air pockets
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major humidity improvement
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smooth airflow sensation instead of drafts
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longer AC lifespan (fewer hard cycles)
Mike says:
“A pressure-neutral room feels like the AC disappeared.
You feel the comfort—never the machine.”
🔗 External Verified Sources (Max 6)
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DOE Insulation & R-Value Overview
https://energy.gov/energysaver/weatherize/insulation -
FLIR – Thermal Imaging Basics
https://www.flir.com/discover -
EPA Moisture & Mold Control
https://www.epa.gov/mold -
Window & Door Flashing Principles (relevant to sleeve flashing)
https://www.energy.gov/energysaver/design/windows-doors-and-skylights -
ACCA Manual J Load Guidelines
https://www.acca.org/hvac-design/manual-j
