Efficiency Breakdown: How Much the Weil-McLain CGI-4 Really Costs to Run

If you’re here because you want the real, data-backed, BTU-crunched, fuel-usage-modeled truth about what the Weil-McLain CGI-4 actually costs to operate through a heating season, welcome to the right breakdown.

This isn’t a salesman’s “trust me, it’s efficient” pitch.
This is a full operating-cost audit, based on:

• Realistic gas consumption modeling

• Seasonal heating loads

• Outdoor temperature behavior

• Aquastat optimization

• Comparison to older boilers

• Efficiency translation (AFUE → real money)

And because you deserve more than napkin math, I’ll reference 6–7 legitimate external sources — the same ones engineers, tech trainers, and energy-modeling nerds use:

• AHRI Directory

• Energy.gov

• Energy Star

• DOE Climate Zones

• NREL heating research

• The Furnace Outlet boiler resources

These aren’t fluff links — they’re the backbone of accurate hydronic cost modeling.

Let’s get into the numbers.

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1. What the CGI-4 Actually Is (Savvy’s Technical Reality Check)

The Weil-McLain CGI-4 is a cast-iron, atmospheric-draft, gas boiler designed for hot-water heating systems. It’s built under the principle that heavy iron + simple combustion = reliability that outlives appliances, contractors, and sometimes entire remodels.

Key specs that matter when calculating operating costs:

• Input: ~105,000 BTU/h

• DOE Heating Capacity: ~88,000 BTU/h (varies by series revision)

• AFUE: ~84%

• Water content: high compared to mod-cons

• Steady-state thermal behavior: extremely stable

The CGI series doesn’t condense, doesn’t modulate, and doesn’t chase ultra-low return temperatures. What it does is deliver consistent thermal output, low maintenance, and predictable gas consumption.

Want to verify these specs yourself? Go straight to the AHRI certification listings:
AHRI Directory – https://www.ahridirectory.org

The CGI-4 sits in the performance band where AFUE is honest, not theoretical. That makes it great for modeling actual fuel costs.

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2. How Gas Usage Is Actually Calculated (and Why Most Homeowners Do It Wrong)

Every heating system’s operating cost is determined by three variables:

1. Annual Heating Load (BTUs needed per season)

2. Boiler Efficiency (AFUE)

3. Fuel Price (cost per therm or per CCF)

Most homeowners only look at AFUE.
That’s like predicting gas mileage by reading the sticker on a pickup truck while ignoring how you actually drive.

2.1 The Formula

To estimate annual fuel use:

Seasonal Gas Use (therms) = Annual Heating Load (Btu) ÷ (100,000 × AFUE)

Where:

• 100,000 BTU = 1 therm

• AFUE is expressed as a decimal (0.84 for 84%)

Energy.gov explains AFUE vs real-world efficiency in detail:
Energy.gov – Furnaces & Boilers Guide – https://www.energy.gov/energysaver/furnaces-and-boilers

Example Heating Load

A typical 2,000 sq-ft home in a cold climate (DOE Zone 5) consumes around:

55 – 75 million BTUs per season

This varies widely by insulation, window quality, and outdoor temperature.

For DOE climate zone reference (so you can adjust your calculations), use the official map:
DOE Climate Zones – https://www.energy.gov/eere/buildings/climate-zones

We’re going to use 65 million BTU/year as the modeling baseline.

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3. Gas Usage Chart for the CGI-4 (Savvy’s Field-Verified Model)

Here is the realistic gas usage for a Weil-McLain CGI-4 in three climate categories, based on heating load scaling from NREL residential heating data:

NREL Residential Heating Study

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3.1 Gas Usage Chart (Therms per Season)

Why the range? Because outdoor temp isn’t constant. Your boiler is fighting heat loss proportional to the temperature delta between indoors and outdoors.

The CGI-4’s AFUE of 84% is predictable, so these numbers don’t swing wildly like mod-con systems that depend on water temperatures.

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4. Seasonal Cost Breakdown (Savvy’s Month-by-Month Reality)

Let’s model a 2,000 sq-ft Zone 5 home (Chicago, Detroit, Cleveland, Minneapolis) with a 65 million BTU annual heating load.

We break down the heating season into:

• Shoulder months

• Mild winter months

• Deep winter months

Here’s the month-by-month gas usage:

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4.1 Seasonal Cost Breakdown Table

Total annual cost: ~$1,160
(Assuming natural gas at $1.50/therm — adjust for your local rate.)

Want fuel-cost comparison references?
Energy Star’s boiler listings include documented efficiency data:
Energy Star – Boilers – https://www.energystar.gov/products/heating_cooling/boilers

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5. Aquastat Optimization: The Hidden Efficiency Lever That Most CGI-4 Owners Ignore

Aquastats determine boiler supply temperature.
The default setting on many boilers? 180°F.

That’s a design-day temperature — the coldest day of the year.

You should NOT run 180°F water all season.
That’s how fuel bills blow up.

5.1 The Logic Behind Aquastat Optimization

The goal is to reduce boiler water temperature whenever possible so the boiler runs longer, steadier cycles with less standby loss.

For cast-iron boilers like the CGI-4, you typically set:

• High Limit: 170–180°F

• Low Limit (if tankless coil): 140–160°F

• Differential: 10–15°F

If no tankless coil is present (most modern installs), set the aquastat to:

• 160–170°F for mild weather

• 170–180°F for deep winter

This is not an outdoor reset (that’s for mod-cons).
This is manual seasonal tuning, and yes — it works.

Energy.gov explains how temperature management affects operating costs:
Energy.gov – Heating Controls Overview – https://www.energy.gov/energysaver/thermostats-and-control-systems

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6. Comparison to Old Boilers (Savvy’s “Stop Guessing, Here’s the Math” Section)

Old boilers — especially ones built before 1990 — often have:

• AFUE of 60–72%

• Poorly insulated jackets

• Heavy standby losses

• Bad draft behavior

• Inefficient burners

Let’s compare gas usage for a 65 million BTU annual load.

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6.1 Fuel Usage Comparison Table

*Assumes system can run low enough return temps to allow condensing, which many fin-tube baseboard systems cannot.

The Furnace Outlet’s hydronic heating myth guide explains why cast-iron boilers remain competitively efficient in real homes:

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7. Why the CGI-4 Performs More Efficiently Than Its AFUE Suggests (Savvy’s Field Notes)

Here’s what AFUE doesn’t account for:

7.1 System Mass & Cycling

The CGI-4 has enough water mass to:

• Reduce short cycling

• Distribute heat more evenly

• Hold heat between cycles

Systems with high-mass radiators or cast-iron baseboard pair especially well with it.

7.2 Distribution Efficiency

Hydronic distribution losses are far lower than duct systems.
Even Energy.gov notes that duct-based systems lose significant heat in basements and attics.

7.3 Standby Losses Are Lower Than Old Boilers

Better insulation, better sealing, and consistent performance keep seasonal efficiency stable.

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8. Real-World Example Homes (So You Can Compare Your Own)

Home A: Zone 4, 1,800 sq ft, good insulation

Annual load: 50M BTU
CGI-4 cost: ~$895/year

Home B: Zone 5, 2,200 sq ft, average insulation

Annual load: 75M BTU
CGI-4 cost: ~$1,340/year

Home C: Zone 6, 2,800 sq ft, old windows

Annual load: 105M BTU
CGI-4 cost: ~$1,875/year

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9. The CGI-4 vs Mod-Con Boilers: The Money Reality

Mod-cons achieve higher efficiency only when return temps drop below 130°F, as documented thoroughly in industry resources like NREL and Energy.gov.

Most baseboard systems run too hot to condense for most of the season.

Therefore:

• Mod-con theoretical advantage = YES

• Mod-con practical advantage in high temp systems = SMALL

If your system is traditional baseboard and not oversized, the CGI-4 may cost only $100–$250 more per year to run than a mod-con — while costing far less in maintenance and lasting twice as long.

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10. Savvy’s Final Verdict: What the CGI-4 Really Costs to Run (No Guessing Allowed)

Here’s the distilled, Data-Savvy truth:

✔ Annual gas cost for most homes: $900–$1,400

Depends on climate, insulation, and fuel price.

✔ CGI-4 efficiency is stable and predictable

Actual seasonal efficiency closely matches the AFUE.

✔ Aquastat optimization can save 5–12%

Especially in shoulder seasons.

✔ Compared to old boilers, the CGI-4 saves $300–$500 per year

Even more if your existing boiler is pre-1970.

✔ Compared to mod-cons, cost difference is smaller than people think

Unless your system supports low-temperature return water.

✔ If you want durability, predictable bills, and low maintenance

The CGI-4 is one of the most cost-stable boilers in the cast-iron category.

This has been Savvy — the Data-First, No-Nonsense Boiler Guy — keeping you out of bad assumptions and showing you exactly what the fuel bill math looks like in the real world.

In the next blog, you will learn about Troubleshooting Guide: Common Weil-McLain Boiler Errors & Fixes