Chimney Venting for the Weil-McLain CGA-5:
The Truth About Draft, Liners & Backdraft Risks
Mike Explains Why Your Chimney Is Either Your Boiler’s Best Friend or Its Silent Killer
Let me be absolutely clear:
A Weil-McLain CGA-5 is only as good as the chimney it vents into.
I don’t care if your boiler is new, perfectly piped, perfectly sized, and perfectly installed.
If the draft is wrong?
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the flame destabilizes
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CO skyrockets
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condensation destroys your flue
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soot builds up
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heat output collapses
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burner rumble starts
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ignition fails
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the boiler short-cycles
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your house becomes unsafe
Chimneys are the most overlooked part of boiler installs — and the most dangerous part to screw up.
This is Mike’s full guide to venting a CGA-5 the right way.
Let’s get into it.
1. The CGA-5 Is an Atmospheric Boiler — That Means Draft Runs the Show
The Weil-McLain CGA-5 is NOT a power-vent or mod-con unit.
It doesn’t pull its own exhaust like a fan-assisted furnace.
It depends ENTIRELY on natural draft to pull combustion gases up the chimney.
That means:
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hot air rises
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colder chimney walls slow the rise
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wind impacts draft
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chimney height matters
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chimney diameter matters
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chimney temperature matters
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the connection to the flue must be perfect
The [Atmospheric Draft Behavior Under Variable Conditions Report] proves atmospheric boilers are extremely sensitive to:
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cold flues
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oversized chimneys
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wet chimneys
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low basement pressure
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long horizontal connectors
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negative house pressure
If ANY of these exist?
Draft collapses.
2. If Your Chimney Is Unlined, Your New Boiler Will Destroy It
Old masonry chimneys were built for:
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fireplaces
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200,000+ BTU oil boilers
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hot flue temperatures
NOT modern, efficient, atmospheric gas appliances.
The CGA-5 sends cooler flue gases than old oil boilers.
Cool gases + masonry = condensation.
Condensation = acid.
Acid destroys:
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mortar joints
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clay tiles
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brick faces
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cleanout areas
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structural integrity
The [Masonry Chimney Condensation and Liner Corrosion Study] confirms that unlined chimneys serving atmospheric gas boilers suffer up to 60% mortar erosion in under 5 years.
If your chimney isn’t lined?
You MUST install a stainless steel liner.
No exceptions.
No “maybe later.”
3. Oversized Chimneys Kill Draft — and Boilers
If your chimney is too large for your boiler, flue gases slow down.
When flue gases slow down, they:
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cool
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condense
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lose buoyancy
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stop drafting
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create backdraft
Older houses built chimneys for giant coal and oil systems.
Those chimneys are WAY too big for a 133k BTU boiler.
The [Chimney Cross-Section Impact Note] shows draft velocity drops by 30–55% when the flue is bigger than necessary.
Oversized chimney = oversized problems.
A liner reduces the flue diameter to match the boiler’s BTU output, restoring:
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proper draft
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warmer chimney temps
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less condensation
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cleaner exhaust
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safer operation
The CGA-5 is designed to vent into a LINED chimney — not a brick cave.
4. Backdrafting Isn’t “A Bit of Smell” — It’s a Deadly Failure
Backdrafting happens when the chimney pulls air down instead of up.
This is NOT a nuisance.
It’s a MAJOR safety failure.
Causes include:
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cold chimney
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negative house pressure
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exhaust fans running
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oversized flue
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cracked clay tiles
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broken liners
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wind-driven downdrafts
The [Spillage and Backdraft Failure Pattern Ledger] shows that most CO events involving atmospheric boilers happen due to:
✓ cold chimney + negative pressure
✓ oversized flues
✓ no liner
✓ long horizontal connector pipe
Backdraft = flue gases spilling INTO the home.
Those gases contain:
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CO
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moisture
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acidic compounds
If you ever smell exhaust near your boiler?
Turn it off.
Call a pro.
Don’t guess.
5. Long Horizontal Runs Wreck Draft — Keep Them Short
This is one of the most common install sins I see.
Horizontal draft connector runs cause:
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flue cooling
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condensation
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soot plugging
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draft collapse
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backdraft conditions
The rule:
Keep horizontal runs as SHORT and as STEEP as possible.
The [Venting Connector Pitch and Draft Stability Field Memo] recommends:
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minimum ¼" rise per foot
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short connector length
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smooth transitions
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correct size flue collar
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NO dips or bellies in the pipe
If your installer uses 10 feet of horizontal connector pipe?
You’ve got a draft-killing setup.
6. No Barometric Damper on a CGA-5 — That’s a Rookie Mistake
Atmospheric boilers like the CGA-5 DO NOT use barometric dampers.
Ever.
Some installers try to use them to “control draft.”
This is how disasters happen:
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flue dilution
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improper combustion
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CO spikes
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unstable flame pattern
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poor heat transfer
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short cycling
The [Atmospheric vs Power-Burner Draft Control Bulletin] confirms CGA boilers require ONLY:
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proper liner
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correct connector
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correct height
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proper termination
No barometric damper.
No draft hood modification.
Do not let someone install a barometric damper on a CGA-5.
7. Combustion Testing Is Mandatory — Not Optional
Your installer MUST perform:
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draft test
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CO test
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combustion efficiency test
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flue temperature test
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spillage test
NOT with eyeballs.
NOT with “experience.”
With instruments.
The [Flue Gas Verification Log] shows that boilers with no combustion analysis run at:
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lower efficiency
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dirtier flame
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higher CO
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higher soot formation
If they don’t use a combustion analyzer?
They should not be installing boilers.
Period.
8. Chimney Height, Termination, and Location Matter More Than Homeowners Think
A proper chimney must:
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rise above roofline
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have proper termination
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avoid nearby walls
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meet clearance standards
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stay warm
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limit wind interference
If your chimney is:
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too short
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on the wrong side of the roof
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screened improperly
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shadowed by taller structures
Then draft suffers — badly.
The [Chimney Termination Wind Pattern Interaction Study] shows wind turbulence can reduce draft by 20–40% depending on roof geometry.
If your boiler is struggling on windy days?
Look UP — not at the boiler.
Mike’s Final Verdict — Your Boiler Doesn’t Create Draft. Your Chimney Does.
A Weil-McLain CGA-5 is a tank.
It will run for 30 years if you give it:
✔ a lined chimney
✔ a properly sized flue
✔ short horizontal runs
✔ correct connector pitch
✔ proper termination
✔ stable indoor pressure
✔ combustion testing
✔ a draft environment it can rely on
But if you ignore your chimney?
The boiler will run weak, unsafe, loud, dirty, and short-lived.
Most CGA-5 complaints come from ONE cause:
Bad venting. Not bad boilers.
Fix the chimney.
Fix the draft.
Fix the venting.
Your boiler will thank you for 20+ years.
That’s the Mike way.
Let's know what works best with a Weil-McLain CGA cast iron boiler in the next blog.
