When I bought my Weil-McLain CGI-4 Series 4 cast iron boiler, the spec sheet said 90,000 BTUs. But I kept asking myself: Is that really enough for my home? Or could it be oversized?
Sizing a boiler is one of the most important decisions you’ll make. An undersized unit struggles in winter. An oversized one short-cycles, wastes fuel, and wears out faster. In this guide, I’ll break down how I figured out whether 90,000 BTUs was right for my 1,600 sq. ft. Midwest home — and how you can do the same.
🧮 Understanding Boiler Sizing Basics
What does “90,000 BTUs” mean?
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A BTU (British Thermal Unit) is the energy required to heat one pound of water by one degree Fahrenheit.
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A boiler rated at 90,000 BTUs per hour can produce that much heating output each hour.
Efficiency matters too
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The Weil-McLain CGI-4 runs at 82–84% AFUE (Annual Fuel Utilization Efficiency).
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That means only about 74,000–76,000 BTUs/hour actually reach your radiators.
🔗 Energy.gov – Boilers & Heating Systems
💡 Mike’s Note: Don’t just look at the nameplate BTUs. Look at net output (after efficiency losses) to know what your home will really feel.
📏 Rule of Thumb: BTUs Per Square Foot
A quick way to estimate boiler size is using square footage and climate.
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Mild climate: 30–35 BTUs per sq. ft.
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Moderate climate: 40–45 BTUs per sq. ft.
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Cold climate: 50–60 BTUs per sq. ft.
Examples
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1,500 sq. ft. home × 50 BTUs = 75,000 BTUs (cold climate).
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1,800 sq. ft. home × 35 BTUs = 63,000 BTUs (mild climate).
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2,000 sq. ft. home × 45 BTUs = 90,000 BTUs (moderate climate).
👉 That’s why 90,000 BTUs is typically right for 1,500–2,000 sq. ft. homes, depending on climate.
🔗 HVAC.com – Boiler Sizing Guide
❄️ Climate Zone Makes All the Difference
Where you live determines how many BTUs you need.
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Cold Climates (Midwest, Northeast, Mountain States):
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50–60 BTUs per sq. ft.
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A 90k boiler covers ~1,500–1,800 sq. ft.
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Moderate Climates (Mid-Atlantic, Pacific Northwest):
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40–45 BTUs per sq. ft.
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A 90k boiler covers ~1,800–2,000 sq. ft.
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Mild Climates (South, coastal California):
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30–35 BTUs per sq. ft.
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A 90k boiler would be oversized for most homes under 3,000 sq. ft.
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💡 Mike’s Note: My Midwest winters drop below zero, so 90k was perfect. If I lived in Virginia, it would’ve been oversized.
🧱 Insulation & Construction Factors
Sizing isn’t just about square footage. The way your house is built makes a huge difference.
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Older homes with poor insulation: Need more BTUs.
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Newer homes with tight construction: Need fewer BTUs.
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Upgrades like attic insulation & new windows: Can reduce heating load by 10–20%.
🔗 EPA – Energy Efficiency at Home
💡 Mike’s Note: When I added R-38 attic insulation, my heat demand dropped enough that the CGI-4 cycles less often.
📊 When 90,000 BTUs Is Enough
✅ Mid-sized homes (1,500–2,000 sq. ft.) in cold climates.
✅ Larger homes (up to 2,200 sq. ft.) in moderate climates.
✅ Homes with decent insulation and modern windows.
👉 For most single-family homes in the Midwest or Northeast, 90,000 BTUs is a great fit.
🛑 When 90,000 BTUs Isn’t Enough
❌ Homes over 2,500 sq. ft. in cold climates.
❌ Drafty old houses with little insulation.
❌ Multi-family properties with multiple zones.
❌ Homes in extreme northern regions (where design temps hit –30°F).
🔗 DOE – Heating Load Calculation
📋 Manual J: The Pro’s Way to Size
Contractors don’t rely on rules of thumb — they use Manual J load calculations.
What Manual J factors in:
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Square footage.
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Insulation levels.
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Window type and orientation.
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Air leakage.
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Local climate data.
👉 If you want precision, ask your HVAC contractor for a Manual J report.
💡 Mike’s Note: I had a Manual J done. My home’s load came in at 72,000 BTUs at design temp. That meant the CGI-4 was slightly oversized, which is fine for a cast iron unit.
⚡ Efficiency vs. Sizing Tradeoffs
One mistake homeowners make: chasing the highest efficiency without checking sizing.
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Oversized boiler: Short-cycles, wastes gas, uneven heating.
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Undersized boiler: Runs constantly, can’t keep up in extreme cold.
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Properly sized boiler: Runs steady, efficient cycles, longest lifespan.
🔗 Energy Star – Boiler Efficiency
🧰 Mike’s Real-World Example
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Home: 1,600 sq. ft., 1960s construction, attic insulation upgraded.
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Climate: Midwest, design temp –5°F.
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Manual J heat loss: ~72,000 BTUs.
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Boiler: Weil-McLain CGI-4 (90,000 BTUs input, ~74,000 BTUs output).
👉 Result: Runs steady cycles, keeps the house at 70°F even in subzero temps.
📝 Mike’s Final Word
So, is 90,000 BTUs enough for your home?
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Yes if you have a mid-sized home (1,500–2,000 sq. ft.) in a cold climate or a slightly larger home in a moderate climate.
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Maybe too much if you live in the South or in a super-insulated new build.
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Not enough if your house is very large, drafty, or poorly insulated.
👉 My advice: Don’t guess. Do a quick square-foot calculation, consider your climate, and — if you want accuracy — get a Manual J load calculation. If your numbers land in the 70k–90k BTU range, the Weil-McLain CGI-4 is a rock-solid choice that’ll keep you warm for decades.
In the next topic we will know more about: Installation Guide: What Mike Learned Setting Up the CGI-4 Boiler
