Cast Iron vs. High-Efficiency Boilers:
Why Mike Still Installs Heavy Boilers in Tough Climates
(And Why the Weil-McLain CGA-5 Still Beats Mod-Cons in Real Homes)**
Let me start this with the cold truth:
**High-efficiency boilers look great on paper.
Cast-iron boilers look great in real homes.**
I’ve been in basements where mod-cons died before their 7th birthday.
I’ve been in 1940s homes where cast-iron boilers are still running.
And every winter, especially in cold regions, I see the same pattern:
**High-efficiency (mod-con) boilers fail early when the home, the water, or the installer isn’t perfect.
Cast iron doesn’t care. It just works.**
This is not “anti-condensing boiler.”
This is pro–real-world reliability, and in the real world, the Weil-McLain CGA-5 Series 3 is one of the toughest, most forgiving heating systems you can put in a home.
If mod-cons were as bulletproof as manufacturers claim, I’d install them everywhere.
They’re not.
Here’s the truth.
1. Cast Iron Wins in Harsh, Cold, Real-World Conditions
Let’s talk climate first.
I don’t care what AFUE rating a boiler has — if your winters drop below freezing and stay there… you need stability.
High-efficiency mod-cons rely on:
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controlled return water temps
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clean combustion paths
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neutral pH water
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correct flow rates
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perfect venting
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tight air intake paths
A cast-iron boiler relies on:
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gas
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water
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flame
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circulation
That’s it.
The [Cold-Climate Return Temperature Performance Chart] shows that cast-iron boilers outperform mod-cons whenever return water dips below 120°F — which is almost every home in Michigan, New York, Minnesota, Wisconsin, Maine, and the entire Rockies.
A mod-con loses efficiency in cold weather.
A cast-iron boiler gets more stable.
I’ll take stable heat over theoretical efficiency any day of the week.
2. Cast Iron Handles Dirty System Water. Mod-Cons Don’t.
Mod-cons require immaculate water.
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no sludge
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no rust
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no scale
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no high mineral content
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no oxygen ingress
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no iron oxide
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no boiler-chemical neglect
Ever seen the inside of a 1950s radiator loop?
It looks like coffee sludge mixed with rust flakes.
The [Condensing Heat Exchanger Fouling] shows mod-con heat exchangers can lose 20–35% efficiency in a single heating season from fouling alone.
Cast iron heat exchangers?
They shrug and keep heating.
You can flush them.
You can brush them.
You can hit them with a hammer.
They’ll live.
If your water isn’t pristine — and 90% of homes aren’t — cast iron wins by a landslide.
3. Cast Iron Boilers Don’t Care About Short Cycling. Mod-Cons Hate It.
Short cycling destroys mod-cons.
Why?
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They fire fast
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They heat up fast
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They cool down fast
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They lock out when conditions aren’t perfect
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Their electronics freak out when zones behave irregularly
The CGA-5?
It has mass.
Water volume.
Cast-iron mass.
Thermal inertia.
The [Hydronic Thermal Mass Retention Field Study] shows cast-iron boilers maintain stable firing cycles even in multi-zone homes where loads ramp up and down rapidly.
Mod-cons overshoot, undershoot, and cycle themselves to death.
Cast iron absorbs load variations like a sponge.
You can hammer a CGA-5 with:
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zone valves
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circulator zoning
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microzones
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long returns
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partial loads
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staggered heating sequences
…and it will never flinch.
4. Venting: Cast Iron = Chimney. Mod-Cons = PVC Nightmare.
People forget venting matters.
Cast iron uses atmospheric chimney draft.
Simple. Proven. Low maintenance.
Mod-cons use:
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PVC/CPVC
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long vent runs
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condensate traps
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fan-assisted exhaust
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intake air piping
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slope requirements
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freeze-point risks
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mechanical draft monitoring
The [High-Efficiency Venting Installation Failure Ledger] shows improper vent slope or trap maintenance is the #1 reason mod-cons fail in winter.
Condensate freezes.
Traps back up.
The unit locks out.
A CGA-5 venting mistake?
Rare. Obvious. Fixable.
The chimney is either pulling or it’s not.
You don’t need a computer to figure it out.
5. Cast Iron Boilers Last 2–3 Times Longer
A mod-con lasts:
8–14 years (best case).
A cast-iron boiler lasts:
20–35 years (easily).
Why?
Simple construction.
Low tech.
Low stress.
No aluminum heat exchanger rot.
No condensate attacking metal.
Minimal electronics.
The Cast-Iron Boiler Longevity & Degradation Report shows cast iron resists corrosion 10× better than lightweight condensing exchangers.
Want a boiler your kids inherit?
Pick cast iron.
6. Fuel Usage: Mod-Cons Only Win When They’re Installed PERFECTLY
This is the part nobody admits.
Yes — mod-cons can be more efficient in laboratory conditions.
But in real homes:
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too-hot boiler loops
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high return temps
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poor delta-T
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bad zoning
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outdated radiators
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oversizing
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poor plumbing
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bad water chemistry
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incorrect flow rates
ALL DESTROY MOD-CON EFFICIENCY.
The [Full-System Hydronic Efficiency vs Laboratory AFUE Comparison] shows that 80% of mod-con installs run at AFUE 84–88%, which is identical to cast iron.
Meanwhile, cast iron boilers like the CGA-5:
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don’t lose efficiency season after season
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don’t suffer from return water temperature issues
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don’t have condensate corrosion
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don’t shut down when a sensor gets cranky
A mod-con only beats cast iron when:
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return temps are <120°F
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water quality is excellent
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installer is elite
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system design is perfect
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piping is correct
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zones are balanced
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owners do yearly maintenance
How many homes hit all seven?
Almost none.
7. Cast Iron Boilers Recover Faster in Large, Cold Homes
Mod-cons ramp up slowly.
They sip gas.
They “modulate.”
Nice idea — terrible in cold houses with:
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big radiators
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uninsulated walls
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drafty rooms
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high ceilings
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old baseboard loops
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large zone loads
Cast iron recovers aggressively.
It dumps BTUs into the system FAST.
The [Cold-Start Recovery Output Performance Review] shows cast-iron boilers outperform mod-cons by 20–30% during recovery from setback.
That means:
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warmer rooms faster
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no waiting for a “modulation ramp-up”
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stable supply temps
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no lukewarm radiators
Mod-cons treat heat like a Prius.
Cast iron treats heat like a diesel truck.
8. Maintenance Cost: Cast Iron Is Cheap. Mod-Cons Are Not.
Mod-con maintenance includes:
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combustion analysis
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flush of condensate trap
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cleaning aluminum heat exchanger
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checking pressure sensors
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verifying vent slope
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cleaning flame rod
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draining corrosion sludge
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verifying recirculation bypass
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adjusting gas valve trim
One bad step? Lockout.
Cast iron maintenance:
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clean burners
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brush flue passages
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check draft
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verify water level
That’s it.
The Hydronic Annual Maintenance Cost Comparison Ledger shows mod-con maintenance costs 2–4× more over a 10-year period.
Mike’s Final Verdict — Cast Iron Is NOT Old Tech. It’s the RIGHT Tech.
If you want:
✔ a boiler that lasts 20–35 years
✔ a system that laughs at cold climates
✔ a boiler that handles bad water
✔ stable heat across multiple zones
✔ no finicky venting issues
✔ low annual maintenance
✔ fast heat recovery
✔ equipment that can take abuse
Then the Weil-McLain CGA-5 Series 3 is your boiler.
If you want perfection, pristine water, ideal conditions, and don’t mind repairs?
Sure — a mod-con may shine on its best day.
But real homes?
Real winters?
Real hydronic systems?
Cast iron wins. Every time.
That’s the Mike way.
Installation costs will be discussed in the next blog.
