The 120k Furnace Exhaust Reality — Why Tony Rebuilds Every Vent Instea

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By Tony — because the exhaust is not a suggestion. It’s the lifeline of the furnace.

🔥 1. Introduction — A 120,000 BTU Furnace Isn’t “Just Another Swap”

When most installers replace an old 80% or 90% furnace with a brand-new 120,000 BTU high-efficiency Goodman, they think:

“The old exhaust is fine.”
“It’s already PVC.”
“It’s venting now, so it’ll vent the new unit.”
“It’ll save time to reuse it.”
“It’s only vent pipe — not rocket science.”

But here’s Tony’s truth:

**The exhaust on a 120k furnace is the single most dangerous, performance-critical, and code-sensitive part of the entire installation.

That’s why I rebuild every vent. Every job. No exceptions.**

And if you saw what I’ve seen — melted pipe, backdrafting, CO spikes, induced-draft failures, draft limit trips, and homeowners unknowingly inhaling exhaust — you’d rebuild every vent too.

A 120k BTU furnace is NOT the same as a smaller system.

It:

dumps more heat
creates more condensate
builds more pressure
requires stricter slope
demands precise sizing
reacts harder to resistance
punishes tight elbows
produces more acidic drainage
requires exact termination placement

That’s why old venting — even PVC — is NEVER good enough for a new furnace.

Let’s break down exactly why Tony never reuses old vent piping.

4 Ton 14.5 SEER2 120,000 BTU 80% AFUE Goodman Upflow Air Conditioner System with Models GLXS4BA4810, CAPTA6030D3, GR9S801205DN

🔧 2. Old PVC Isn’t Rated for Today’s High-Efficiency Furnace Temperatures

Most older PVC vent installations were:

Schedule 40 (fine)
But NOT rated for sustained high exhaust temperatures
Or NOT certified to ASTM standards required by new furnaces

Newer furnaces — especially 120k 96%+ AFUE units — can produce higher flue gas temperatures when:

the inducer ramps up
condensate temporarily stalls
staging changes
the vent length is near maximum
the pressure switch compensates
a cold start occurs

Older PVC becomes:

brittle
yellowed
micro-cracked
chemically weakened by years of condensate
structurally compromised

✔️ Tony’s First Rule

If the pipe is older than the furnace going in, it’s coming out.

PVC weakens from the inside out.
You can’t see the damage — but the furnace will find it.

🧪 3. Condensate Acid Erodes Old Venting From the Inside

High-efficiency furnaces generate acidic condensate.
That water flows through:

elbows
joints
sags
backflows

PVC doesn’t dissolve instantly.
But acidic condensate weakens the pipe wall over years.

Especially:

horizontal runs
long sweeps
low spots
poorly sloped sections
areas with dried residue inside

When a 120k furnace fires at full input?

Weak PVC flexes under pressure, and that leads to:

hairline cracks
leakage
pressure switch trips
CO risk
moisture damage
eventual pipe failure

✔️ Tony’s Rule

If condensate ever touched the pipe, the pipe is suspect.

This is why reusing vent pipe is a gamble — and I don’t gamble with exhaust.

📏 4. Old Exhaust Runs Were Not Designed for the New Furnace’s Required Vent Sizing

Here’s the part rookies never check:

Your old furnace may have needed:

2" vent
short runs
fewer fittings
lower BTU input

But your 120,000 BTU furnace likely requires:

2.5" or 3" vent
precise maximum length
specific fitting allowances
pressure-switch-tuned pipe diameter
matched intake/exhaust lengths

Old piping simply does not meet the new sizing requirements.

Running a 120k furnace on undersized or long existing PVC leads to:

flame rollout issues
ignition delay
pressure switch failure
inducer motor overwork
reduced efficiency
noisy exhaust
condensate pooling
furnace cycling off on safeties

✔️ Tony’s Vent Sizing Truth

BTU input decides vent diameter — not what was already on the wall.

This alone eliminates 80% of reused vent systems.

🌡️ 5. Thermal Expansion Breaks Old Joints — Especially on 120k Units

PVC expands when hot exhaust runs through it.

Old PVC installations have:

glued joints weakened by age
poorly primed connections
unbraced sections
sagging pipe
brittle elbows
sloppy slopes
dried glue failure

When the new furnace runs its first full cycle?

Expansion stresses these joints — HARD.

And that’s when Tony sees:

joint separation
micro leaks
condensate seepage
CO odor near the furnace
pressure drops in the vent
inducer ramp-up noise
vibration through piping

A 120k furnace simply puts too much stress on old pipe.

✔️ Tony’s Expansion Rule

If I didn’t glue it, I don’t trust it.

🧭 6. Code Changes Mean Old PVC Installs Rarely Meet Today’s Safety Requirements

Modern high-efficiency furnace installations must meet updated codes, including:

correct support spacing
proper slope (¼” per foot)
no sags or bellies
specific clearances to windows & corners
exhaust termination orientation
fresh air intake positioning
frost considerations
UL- or ASTM-rated vent material
sealed, solvent-welded fittings

Old vent installations almost NEVER comply with all updated mechanical and fuel gas codes.

Especially spacing and slope.

✔️ Tony’s Code Rule

Every new furnace deserves a new vent that meets TODAY’S code, not last decade’s.

🔍 7. Reusing PVC Hides Existing Problems That the New Furnace Will Magnify

Old vent systems often have:

hidden low spots
partial blockages
glue globs
insect nests
internal debris
elbow pooling
cracked fittings
slope reversal
hidden leaks in walls
improperly oriented terminations

A 120,000 BTU furnace will reveal every one of those flaws by:

tripping pressure switches
increasing condensate
running noisy
short cycling
leaking water
backdrafting
stressing the inducer

✔️ Tony’s Logic

If a 120k inducer is going to fight the vent, rebuild the vent so it doesn’t need to fight.

🏗️ 8. What Tony Does Instead — The Full Vent Rebuild Process

When Tony replaces a furnace, he does the following:

Step 1 — Remove 100% of existing vent PVC

No partial reuse.
No “just the straight section.”

Step 2 — Inspect wall penetration for water/CO intrusion

Step 3 — Recalculate vent diameter using manufacturer tables

Step 4 — Build all new venting using:

new ASTM-approved PVC
new primers and cements
new hangers
new termination kits
new supports
new elbows

Step 5 — Install proper slope

¼” per foot minimum.

Step 6 — Balance intake & exhaust lengths

Critical for 120,000 BTU units.

Step 7 — Pressure test (if required)

Step 8 — Run furnace under load for 15 minutes

Step 9 — Verify pressure switch operation

Step 10 — Seal exterior penetration

This ensures performance, safety, and longevity.

💨 9. The Pressure Switch Never Lies — Old Venting Always Fails the Test

A 120k furnace’s pressure switch is calibrated for clean, correctly sized, properly sloped piping.

Old vent runs cause:

slow inducer spin-up
low pressure differential
switch not closing
intermittent lockouts
delayed ignition
error codes

When installers try to “force” the old PVC to work?

They end up misdiagnosing:

inducer failure
heat exchanger issues
blocked vent
faulty switch

When the real problem is the vent.

✔️ Tony’s Diagnostic Rule

If the vent wasn’t built for this furnace, the pressure switch will tell you instantly.

📘 10. Verified External Technical Resources

Here are reputable, verified external resources supporting coil installation, condensate management, and TXV behavior:

ASHRAE Fundamentals – Coil Construction & Airflow (Technical)
https://www.ashrae.org/technical-resources/ashrae-handbook
HVAC Drainage Code Requirements (ICC)
https://codes.iccsafe.org/
EPA HVAC Moisture & Condensate Guidelines
https://www.epa.gov/mold
AHRI Air Coil Performance Standards
https://www.ahrinet.org/standards
RSES TXV Installation & Superheat Guidelines
https://www.rses.org
Goodman (Daikin) Coil & TXV Transition Resources
https://www.daikincomfort.com

🏁 11. Final Word — The Vent Is the Furnace’s Lifeline, Not an Afterthought

Here’s the truth every installer must accept:

**A 120,000 BTU furnace pushes the limits of vent material, pressure, heat, and condensate.

Reusing old PVC is gambling with the furnace’s life — and the homeowner’s safety.**

Tony rebuilds every vent system because: