If there’s one number installers cling to like gospel during startup, it’s heat rise.
They fire the furnace.
Wait a couple minutes.
Stick a probe in the supply and return.
Do the math.
Then they say the words I hear all the time:
“Heat rise looks perfect.”
Most of the time? That reading is lying to you.
Not maliciously. Not intentionally.
But startup heat rise almost always lies — and trusting the first reading is one of the fastest ways to miss airflow problems, limit issues, and comfort complaints that show up weeks later.
That’s why I never trust the first number.
80,000 BTU 96% AFUE Upflow/Horizontal Single Stage Goodman Gas Furnace - GR9S960803BN
🧠 What Heat Rise Is Supposed to Tell You
Heat rise is meant to confirm one thing:
Is the furnace moving the right amount of air across the heat exchanger?
Too high:
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Not enough airflow
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Risk of limit trips
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Long-term exchanger stress
Too low:
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Too much airflow
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Reduced comfort
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Possible noise and short cycling
On paper, it’s simple.
In the real world — especially during startup — it’s anything but.
⏱️ The Big Lie: Early Heat Rise Is Not Stabilized Heat Rise
Here’s the truth most manuals don’t emphasize enough:
The furnace is still lying to you in the first few minutes.
During early startup:
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The heat exchanger is cold
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The cabinet hasn’t expanded
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Ducts haven’t warmed
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Blower ramps aren’t settled
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Static pressure hasn’t stabilized
You’re measuring a moving target.
So when someone grabs a reading at minute three and locks it in, they’re not confirming airflow — they’re guessing.
🔄 Why the GR9S Makes This Even Trickier
On furnaces like the Goodman GR9S960803BN, the blower behavior matters.
That 9-speed ECM-style blower:
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Adjusts as temperature changes
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Responds to duct resistance dynamically
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Alters airflow slightly as the system warms
So the airflow you see early isn’t the airflow you’ll have at steady state.
If you don’t wait, you’re measuring a temporary condition — not real operating behavior.
🌬️ Blower Ramp Timing Changes Everything
One of the most common startup mistakes is ignoring blower timing.
Early heat rise readings are often taken:
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Before full airflow is established
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During partial ramp-up
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Before torque compensation finishes
That inflates heat rise.
Then later, when airflow increases naturally, the real heat rise drops — sometimes out of spec — and nobody knows why.
Except the furnace knew the whole time.
🔥 Cabinet Expansion: The Silent Variable
Metal moves when it heats.
During startup:
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Panels relax
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Gaskets seat
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Internal air paths change
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Tiny leaks seal or open
That affects airflow and temperature readings.
So if your heat rise looks “perfect” before the cabinet finishes expanding, don’t celebrate yet.
You haven’t seen the final system.
📉 Why First Readings Usually Look Too Good
Here’s the pattern I see all the time:
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Startup heat rise: dead center of the range
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20 minutes later: creeping higher
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After an hour: flirting with the upper limit
Why?
Because early airflow is often higher than steady airflow.
As the system warms:
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Filters start restricting more
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Duct resistance increases slightly
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Air density changes
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Blower compensation settles
That’s when the truth shows up.
🧪 Jake’s Heat Rise Reality Test
Here’s how I actually verify heat rise during startup:
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Let the furnace run at least 15–20 minutes
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Confirm blower is at full heating airflow
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Re-check return and supply temps
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Walk away for a few minutes
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Re-check again
If the number is stable across multiple readings, then I trust it.
One reading is a snapshot. Patterns tell the truth.
🌡️ Probe Placement Lies Too
Even good numbers lie if your probes are wrong.
Common mistakes:
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Measuring too close to the furnace
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Reading stratified air
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Probes touching metal
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Using registers instead of trunk lines
I always measure:
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Supply in the main plenum
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Return where air is well-mixed
The Air Conditioning Contractors of America (ACCA) explains why proper temperature measurement location matters for accurate diagnostics:
🔊 When Heat Rise and Noise Disagree
Here’s a red flag I never ignore:
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Heat rise looks “fine”
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Blower sounds stressed
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Cabinet noise increases as it runs
When sound and numbers disagree, I trust the sound.
Airflow problems reveal themselves audibly before they violate heat rise specs.
That’s why I never isolate heat rise from startup airflow behavior.
🧯 Limits Don’t Care About Your First Reading
Limits react to real conditions — not early guesses.
I’ve seen systems where:
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Startup heat rise was in range
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Limit tripped later that night
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Installer swore the numbers were good
They probably were — for the first five minutes.
The U.S. Department of Energy has documented that improper airflow and delayed heat rise issues are a major contributor to premature limit cycling and equipment wear:
🧰 Tools Matter — Timing Matters More
Yes, I use good instruments.
But no tool fixes impatience.
If you rush the reading, the tool just helps you be wrong faster.
🧾 What I Actually Write Down
My startup notes don’t say:
“Heat rise = 45°F”
They say:
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“Heat rise stabilized after 20 min”
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“Multiple readings consistent”
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“No upward creep”
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“Blower steady”
Because six months later, when someone says:
“It’s tripping the limit sometimes,”
I know whether this is new — or something we never waited long enough to see.
🧠 Jake’s Rule on Heat Rise
If the first reading makes you feel good, be suspicious.
If the second and third readings agree, now you’re learning something.
Heat rise isn’t a checkbox. It’s a trend.
And startup is where trends begin.
🔚 Final Thought: Patience Is Part of the Install
The furnace doesn’t rush. The heat exchanger doesn’t rush. Airflow doesn’t rush.
So why do installers?
If you slow down during startup, heat rise stops lying to you.
And when heat rise tells the truth, everything else tends to behave.
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In the next topic we will know more about: Condensate on Day One: Jake’s Startup Trap That Saves Secondary Heat Exchangers
