Jake’s No-Math Method for Perfect Airflow in Every Room — Even With a Single-Stage Furnace
🧰 Introduction: Every Comfort Problem Jake Sees Comes Down to One Thing — CFM
Jake has a simple rule:
“Airflow is the whole game.”
Not furnace BTUs.
Not SEER2 ratings.
Not heat exchanger design.
If a system has the wrong CFM (Cubic Feet per Minute of airflow), the home will never be comfortable — no matter how good the equipment is.
Jake explains it this way:
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Too little CFM → furnace overheats, coil freezes, rooms starve
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Too much CFM → drafts, noise, poor dehumidification
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Uneven CFM → hot/cold rooms, pressure imbalance, high bills
This is why he teaches the CFM Triangle — the foundational relationship between:
1️⃣ Furnace Blower Speed
2️⃣ Duct Size
3️⃣ Room-by-Room Airflow Requirements
Get these three right, and any HVAC system can deliver “even-room comfort” — even a basic single-stage furnace like the Goodman GR9S960803BN 96% AFUE, 80,000 BTU model
Let’s break down Jake’s field-tested method for controlling airflow without complicated calculations.
🔺 1. The CFM Triangle: Jake’s Core Airflow Framework
Jake says:
“Airflow is never one thing. It’s always three things arguing with each other.”
Those three things form the CFM Triangle:
🔷 1. Furnace Blower Speed (How Hard It Pushes)
Your blower can run:
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faster
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slower
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temperature-based
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pressure-based
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continuous low
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staged
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or full speed (single-stage units)
🔷 2. Duct Size (How Much Air It Can Carry)
Ducts are airflow highways. Their size determines:
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airflow potential
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friction losses
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static pressure
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noise
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throw distance
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temperature delivery
🔷 3. Room-by-Room Design (Where the Air Needs to Go)
Every room has a target CFM.
If the duct delivers too little or too much, comfort disappears.
Jake’s rule:
“CFM = Blower + Duct + Room. If one fails, the other two suffer.”
⚙️ 2. Component One: Furnace Blower Speed (Your System’s Engine)
Jake starts every airflow job by identifying blower capability.
Modern ECM blowers (like the one in the Goodman 96% furnace above) can deliver:
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steady airflow
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self-adjusting CFM
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quiet ramp-up
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zoning capability
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smart thermostat compatibility
PSC blowers (older systems):
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lose airflow under pressure
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can’t support zoning
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struggle with restrictive filters
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lose heating/cooling performance
Blower speed affects:
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CFM
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static pressure
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noise
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temperature rise
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humidity removal
Jake’s target airflow:
400 CFM per ton (cooling)
350–450 depending on climate
110–130 CFM per 1,000 BTUs (heating)
But blower speed isn’t enough — the ducts must support it.
📏 3. Component Two: Duct Size (Your Airflow Highway)
Ducts determine:
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actual CFM delivered
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static pressure
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airflow noise
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room comfort
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system efficiency
Jake’s duct sizing cheat sheet:
🌬️ Round Supply Duct Sizing (Jake’s Real-World Numbers)
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6-inch → 100 CFM
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7-inch → 130 CFM
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8-inch → 180 CFM
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9-inch → 225 CFM
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10-inch → 325 CFM
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12-inch → 475 CFM
🌪️ Main Trunk Size
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2–2.5 tons → 14-inch
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3 tons → 16-inch
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3.5–4 tons → 18-inch
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5 tons → 20-inch
🔄 Return Air Rules
Jake’s golden rule:
“Return must be 20–30% bigger than supply, no exceptions.”
Why?
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return air is denser
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it experiences more friction
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more static occurs on the return side
Return sizing chart:
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2 tons → 16-inch return
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3 tons → 18-inch return
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4 tons → 20-inch return
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5 tons → 24-inch return
Jake’s favorite trick:
“A starved return kills more systems than any heat exchanger ever will.”
🏠 4. Component Three: Room-by-Room Design (The Most Ignored Step)
Jake performs room airflow calculations in seconds.
Here’s his method:
🛏️ Bedroom CFM Targets
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Small bedroom: 80–100 CFM
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Master bedroom: 120–180 CFM
🍳 Kitchen CFM Target
+20% more to compensate for:
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appliances
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sunlight
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heat load
🛁 Bathroom CFM Target
30–50 CFM (too much causes uncomfortable drafts)
🛋️ Living Room CFM Target
150–250 CFM depending on layout.
🧱 Basements
Downsize airflow by 10–30%.
Basements naturally feel cooler.
Once Jake knows room CFM needs, he matches register size + duct size.
💡 5. The “Static Pressure Link” — How the Triangle Harmonizes
Jake calls static pressure “the referee of the CFM Triangle.”
Static pressure governs:
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how much airflow the blower can push
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how much the ducts restrict
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how balanced the rooms feel
Jake checks static using a manometer (his preferred tool)
Target total external static pressure (TESP)
0.30–0.50 in-WC (ideal)
0.60–0.80 in-WC (problems coming)
0.80+ in-WC (system choking)
Static pressure reveals:
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undersized ducts
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bad filter racks
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dirty filters
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choked returns
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poor plenum transitions
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restrictive registers
Jake says:
“Static pressure lets you see every airflow mistake at once.”
🔧 6. Jake’s Field Method for Designing a Perfect CFM Triangle
Here’s Jake’s real job-site sequence:
Step 1 — Measure Static Pressure
High static = start with returns and filter rack.
Step 2 — Verify Blower Speed
Adjust heating and cooling speeds based on duct capacity.
Step 3 — Inspect Plenums & Transitions
Jake checks:
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plenum size
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elbow radius
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filter cabinet size
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coil orientation
Step 4 — Check Register Type
Decorative registers? Replace them.
Use high-flow where needed, low-flow in oversupplied rooms.
Step 5 — Room-by-Room CFM Testing
Jake checks:
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airflow delivery
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balancing dampers
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duct length
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CFM losses
Step 6 — Balance the System
He opens and closes dampers to match room targets.
Step 7 — Re-Test Static Pressure
Goal:
0.3–0.5 in-WC under normal heating speed.
Step 8 — Confirm Temperature Spread
Rooms should vary no more than 1–2°F.
❄️ 7. Why Most Homes Fail the CFM Triangle Test
Jake sees six common reasons:
❌ Undersized returns
95% of homes fail here.
❌ Restrictive 1-inch pleated filters
Jake replaces them with 4–5 inch media cabinets.
❌ Long ducts with no boosting
Rooms far from the furnace get starved.
❌ Short ducts that blast air
Rooms closest to furnace become overheated.
❌ No balancing dampers
Impossible to tune the home.
❌ Decorative registers
Airflow killers.
🛠️ 8. The Goodman Furnace Example: How CFM Triangle Optimizes It
The Goodman GR9S960803BN 96% AFUE furnace has:
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9-speed ECM blower
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single-stage heat
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narrow 17.5-inch cabinet
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excellent duct flexibility
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perfect match for heat pumps
This furnace performs beautifully when:
✔️ return ducts are oversized
✔️ blower speed matches the CFM target
✔️ coil is properly sized
✔️ plenum transitions are correct
✔️ filter rack is unrestricted
✔️ room-by-room CFM is balanced
Jake says:
“This furnace is only as good as the duct system you give it.”
🎯 9. Case Study: When the CFM Triangle Fixed a “Bad Furnace”
A homeowner complained:
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main floor overheats
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upstairs freezing
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vents noisy
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furnace short-cycles
Jake tested static pressure: 0.91 in-WC — far too high.
He checked CFM:
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upstairs rooms: 40–60 CFM
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downstairs rooms: 180–220 CFM
The furnace wasn’t the issue.
The CFM Triangle was broken.
Jake fixed it by:
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enlarging return duct
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adding a second return
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installing high-flow registers upstairs
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slowing blower speed
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adding balancing dampers
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replacing 1-inch filter with 5-inch media cabinet
Final static: 0.41 in-WC
Upstairs airflow doubled.
Home balanced within 1.5°F.
Homeowner:
“It feels like a whole new system.”
🧊 10. Jake’s CFM Triangle Cheat Sheet
To fix any airflow issue, check these:
✔️ Is blower speed correct?
✔️ Are ducts sized properly?
✔️ Are returns oversized?
✔️ Are registers restrictive?
✔️ Is static pressure below 0.5 in-WC?
✔️ Does each room get target CFM?
✔️ Are transitions smooth?
✔️ Is the filter rack unrestricted?
Jake says:
“Comfort isn’t magic — it’s airflow math done the simple way.”
🚀 Conclusion: The CFM Triangle Is the Foundation of All HVAC Comfort
When blower speed, duct size, and room airflow work together:
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rooms stay balanced
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system runs quieter
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furnace lasts longer
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coil stays clean
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energy bills drop
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comfort becomes predictable
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efficiency increases
When they don’t?
You get:
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noise
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overheating
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freezing coils
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uneven rooms
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high energy bills
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homeowner frustration
Jake’s CFM Triangle ensures:
the system you install today will still feel perfect tomorrow — and for the next 20 years.
Buy this on Amazon at: https://amzn.to/48HGh2g
In the next topic we will know more about: The 17.5-Inch Rule: Why Furnace Cabinet Width Decides Your Entire Mechanical Room Layout
