By Tony — the guy who’s seen more garages sweating, rusting, and molding because the installer didn’t understand venting physics
🛠️ Introduction: When Your Heater Turns Your Garage Into a Rainforest
If your garage feels damp, clammy, or “sweats” every time you run your heater, I’ve got news for you:
Your heater isn’t broken.
Your venting is.
Propane combustion produces a TON of water vapor — and if you vent it wrong, that moisture ends up inside your garage instead of outside.
Reznor UDX 60,000 BTU Propane Unit Heater
Most homeowners don’t know this.
A surprising number of installers don’t either.
Today, I’m breaking down exactly:
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why propane creates moisture
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how venting errors trap humidity indoors
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signs you’re soaking your garage without knowing it
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why rust, mold, and fogged windows point to venting failure
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how to fix the problem permanently
Let’s cut straight to the truth.
🔥 1. Propane Produces Water — A LOT of It
Here’s the chemistry nobody talks about:
When propane burns, it creates:
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heat
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carbon dioxide
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carbon monoxide (if combustion is poor)
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and WATER VAPOR — tons of it
The actual math?
Every gallon of propane burned = 1.6 pounds of water vapor.
Yeah.
Not ounces.
Pounds.
In a typical 60,000 BTU heater running for one hour, you can easily generate the moisture equivalent of:
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a hot shower
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a boiling pot on the stove
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or a humidifier running on high
If that moisture can’t escape, it builds and builds and builds inside your garage.
🚫 2. The #1 Problem: Using Indoor Air for Combustion in a Tight Garage
Most garages today are sealed up tight:
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new gasket-style garage doors
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insulated exterior walls
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spray foam
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weatherstripping everywhere
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insulated ceilings
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vinyl windows
Great for energy savings.
Terrible for propane combustion.
When your heater pulls air from inside the garage instead of outdoors:
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It sucks in your indoor air
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Burns propane
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Produces water vapor
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Dumps that vapor BACK INTO THE ROOM
This is the recipe for moisture disaster.
That’s why sealed-combustion (direct-vent) units exist — to prevent this exact issue.
🔄 3. Wrong Venting = Moisture Recirculation = Hidden Humidity Trap
Let’s talk about the ways venting goes wrong and traps humidity.
❌ Backdrafting
Moist exhaust air gets pulled back into the garage through:
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vents
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flue pipes
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furnace cabinets
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poorly sealed seams
❌ Improper vent slope
A flat or reversed slope lets condensation pool in the vent and drip back into the garage.
❌ Unsealed joints
Moist exhaust escapes into the room instead of outdoors.
❌ Using a single-paneled louvered exhaust vent
Wind can push moisture back inside.
❌ Short vent pipes
Not enough rise = weak draft = humid exhaust rolls back indoors.
❌ Incorrect vent termination height
Too low = exhaust curls back against the house or garage opening.
❌ Shared combustion & ventilation paths
Makes drafting unpredictable — often sucking moist air inside instead of pushing it out.
Every one of these errors makes your garage wetter and colder — not warmer.
☁️ 4. Where Does All This Moisture Go? Onto Everything You Care About
Moisture always finds a landing spot.
In garages with bad venting, you’ll notice:
Condensation on:
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garage door panels
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windows
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tools
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electrical equipment
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stored lumber
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vehicles
Rust forming on:
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saws
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drill bits
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table saw tops
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mower decks
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car brake rotors
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metal hand tools
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the heater itself
Musty smell or mold on:
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drywall
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exposed insulation
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cardboard boxes
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stored clothes
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old furniture
Water dripping from:
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the ceiling
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the heater
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the vent pipe
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the garage door track
If your garage feels like a sauna when the heater runs — this article is describing your life.
🌡️ 5. The Physics Problem: Warm Air Can Hold More Moisture
As temperature rises, the air’s ability to hold water vapor increases dramatically.
That means:
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warm propane-heated air grabs moisture
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then that warm humid air hits cold surfaces
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condensation happens instantly
Cold wall? Water.
Cold tools? Water.
Cold car? Water.
Cold concrete slab? Water.
Cold garage door? Water.
This is why garages “sweat” in winter when propane heaters run with incorrect venting.
🌬️ 6. Why Direct-Vent (Sealed Combustion) Solves Everything
A direct-vent heater uses:
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one pipe to pull air from outdoors
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one pipe to exhaust combustion gases outdoors
What this does:
✔️ no indoor air used for burning
✔️ no moisture released into garage
✔️ no negative pressure
✔️ no backdrafting
✔️ more efficient burn
✔️ cleaner combustion
✔️ safer CO levels
✔️ reduced rust & mold risk
This is why I recommend direct-vent for:
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garages
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workshops
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barns
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spray foam buildings
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metal outbuildings
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tight spaces
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any insulated room with a heater
It eliminates the moisture problem entirely.
🔧 7. The Most Common Mistakes That Create Moisture Problems (Tony’s List)
❌ Using a vent-free heater
(These pump all combustion moisture directly inside)
❌ Using a natural-draft heater in a sealed garage
(No airflow = moisture + CO risk)
❌ Installing a power-vent heater with no combustion air makeup
(The heater has no air to burn — so it backdrafts)
❌ Direct-vent unit installed with:
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wrong termination height
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too much horizontal run
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no upward pitch
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low-quality vent hoods
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elbows every two feet
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poorly sealed seams
❌ Running the heater with garage door closed and no fresh air path
Even if combustion air is from indoors, it MUST be replaced.
If your setup violates any of these — humidity is guaranteed.
🚨 8. Signs You Have a Moisture Problem (Before Damage Happens)
Subtle Early Signs:
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windows fog after heater starts
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metallic smell in air
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warm humid “swampy” feel
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you see “breath fog” in lower parts of garage
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garage feels warmer than the thermostat reads
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stuff rusts faster than expected
Serious Signs:
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water dripping off metal
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mold on drywall or insulation
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rust on heater cabinet
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water pooling near entrance
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ceiling dripping or sweating
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flue pipe sweating or staining
If you see ANY of these, stop using the heater until venting is inspected.
📘 9. Verified External Sources
These reputable references confirm everything Tony explains about moisture, combustion, and venting:
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Modine Hot Dawg Installation Manual (Mounting & Throw Charts)
https://modinehvac.com/ -
Reznor UDX Engineering Specifications
https://www.reznorhvac.com -
Building Science Corporation – Air Mixing & Comfort Control
https://buildingscience.com -
HVAC Ventilation & Throw Distance Principles (Titus HVAC)
https://www.titus-hvac.com/
These resources all support one core idea: moisture problems are system design problems — not heater problems.
🧰 10. How To Fix Moisture Problems — Tony’s Battle-Tested Solutions
✔️ 1. Convert to sealed combustion (direct-vent)
This is the BEST solution.
✔️ 2. Add combustion air intake
If direct-vent isn’t possible, add a fresh air makeup vent.
✔️ 3. Fix vent slope (rise ≥ 1/4 inch per foot)
Prevents condensation from pooling.
✔️ 4. Seal vent joints with high-temp tape or silicone
Stops moisture leaks back into garage.
✔️ 5. Use a better vent hood
Choose one that resists wind-driven backdrafts.
✔️ 6. Increase heater angle downward
Improves air circulation; reduces stratification.
✔️ 7. Add a small mixing fan
Prevents warm moist air from accumulating near ceiling.
✔️ 8. Maintain correct gas pressure
Weak burn = extra moisture + CO
✔️ 9. Avoid vent-free heaters in garages
They are humidity machines.
✔️ 10. Check for exhaust obstructions
Bird nests, snow, leaves, ice — all moisture traps.
🚀 Conclusion: Moisture Isn’t a Heater Problem — It’s a Venting Problem
Propane and natural gas heaters are NOT supposed to make your garage damp.
They only do that when:
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combustion air comes from inside
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venting is wrong
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exhaust comes back indoors
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you use vent-free equipment
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the heater can’t draft properly
But here’s the good news:
Fix the venting and your moisture problem disappears — instantly.
Your garage becomes:
✔️ dry
✔️ warm
✔️ rust-free
✔️ safer
✔️ more efficient
And your tools, vehicles, and projects stop getting soaked.
Buy this on Amazon at: https://amzn.to/4oCrGcV
In the next topic we will know more about: Tony’s 2-Vent Rule: When You Must Separate Intake and Exhaust (and When You Can’t Cheat It)
