How to Size a Weil-McLain Boiler: BTUs, Zones, and Mike’s Pro Tips

Why Proper Sizing Is a Big Deal

Let’s get something out of the way: bigger isn’t better when it comes to boilers.

An oversized boiler short cycles, meaning it turns on and off frequently. That wastes fuel, increases wear on components, and makes it hard to maintain even heat throughout the house. An undersized boiler runs constantly and still can’t keep up when the temps drop.

The key is to size the boiler to your home’s actual heat demand—no more, no less.

Too many homeowners (and even a few contractors) make the mistake of simply matching the BTU rating of the old unit. But if the old one was mis-sized, you’re just repeating the problem. Today’s homes have better insulation, tighter windows, and more efficient distribution systems—so it pays to do the math.

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What Is BTU and Why Does It Matter?

BTU, or British Thermal Unit, is a measure of how much heat your boiler can produce per hour. When someone says they have a “100,000 BTU boiler,” that means the unit can deliver up to 100,000 BTUs of heat per hour—under optimal conditions.

Most residential boilers fall somewhere between 50,000 and 250,000 BTUs, depending on the square footage and regional climate.

But here’s where it gets tricky: not all BTUs are created equal.

A 100,000 BTU input rating means that’s how much fuel it burns. If the boiler is only 80% efficient, then you’re only getting 80,000 BTUs of usable heat. That’s why AFUE ratings matter too (we’ll talk more about that later).

To size your boiler correctly, we start by figuring out the heating load of your house.

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Step 1: Start With Square Footage and Climate

This gives you a ballpark BTU requirement before we refine it.

Use this rough guideline:

So let’s say you’ve got a 2,000 sq ft home in upstate New York. That’s 60 BTUs per square foot, or a 120,000 BTU heat load. Now, that’s before adjustments for insulation, layout, windows, etc.—but it’s a start.

To go deeper, use this HVAC BTU calculator and select “Heating” mode. You can plug in variables like ceiling height and insulation quality.

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Step 2: Adjust for Insulation, Windows, and Age of Home

If your house has:

• Great insulation and new windows → Subtract 10–20%

• Poor insulation or drafty windows → Add 10–20%

• Unfinished basement being heated → Add 5–10%

• Open floor plan → May require slightly less due to better air circulation

Say your home from 1985 has single-pane windows and moderate insulation. Instead of 120,000 BTUs, you might bump it to 130,000–135,000 to stay warm without overworking the system.

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Step 3: Factor In Zones (This One’s Important)

A zoned system divides your home into separate areas with their own thermostats—like basement, main floor, second floor, etc.

Each zone needs:

• A circulator pump (or zone valve)

• Enough BTUs to handle that area’s load

• A boiler that can deliver heat to all zones as needed

The key here is that your total BTU capacity must cover all zones simultaneously, even if only one is running at a time. Don’t size your boiler by zone. Size it for the total heating demand, then divide control among zones.

This is especially important if you’re running radiant heating or adding heated floors in a basement or bathroom. Each zone will pull demand from the boiler, and you want to avoid low water temperature or delay during peak usage.

For a technical dive into zone calculations, HeatingHelp’s boiler zone sizing guide is a fantastic resource.

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Step 4: Match It to a Weil-McLain Model

Now that you know your heat load, it’s time to pick the right Weil-McLain boiler.

Here’s a quick rundown of their most popular models:

Let’s say your home needs 135,000 BTUs and runs on natural gas. You could choose the CGa-5 or CGa-6 depending on how close your needs are to the output. Want better efficiency and modulating performance? Look at the Evergreen 110 or 155 models.

Check the exact BTU input and output ratings in the model spec sheet. You’ll also want to make sure your model includes the right number of tapping ports, flue outlets, and zone capabilities for your setup.

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Step 5: Don’t Forget AFUE

We touched on this earlier, but it’s worth repeating.

If your heat load is 120,000 BTUs and you’re using an 80% AFUE boiler, you’ll need a 150,000 BTU input to hit your heat target. But a 95% AFUE boiler needs only 126,300 BTU input to give you the same usable heat.

That’s why high-efficiency condensing boilers like the Evergreen and AquaBalance can save you serious money over time, even if they cost more upfront. The U.S. Department of Energy recommends them for homes in cold climates with high annual fuel usage.

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Step 6: Consider Future Add-Ons or Expansions

If you're planning to:

• Finish a basement

• Add radiant floor heating

• Install a garage heater

• Build an addition

…then you’ll want to size up slightly. Don’t go crazy—just ensure your boiler has a buffer of 10–15% extra capacity, and consider using a buffer tank if you’re worried about short cycling.

You’ll also want to choose a boiler with multiple tapping points or easy expansion features. Weil-McLain is great at this—they design their manifolds and piping layouts with real-world growth in mind.

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Step 7: Work With a Pro Installer

I’ll always advocate for working with a licensed contractor—especially when it comes to gas line connections, flue venting, and zoning.

But even if you’re managing the job yourself, the most important thing is to use quality components and buy from a supplier that understands boilers, not just HVAC equipment in general.

If you’re looking for authentic Weil-McLain boilers and accessories, shipped fast and priced fairly, check out The Furnace Outlet’s full Weil-McLain collection. They carry everything from high-efficiency models to NG-to-LP conversion kits, and they know their stuff when it comes to heating equipment.

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Bonus Pro Tips from Mike

1. Don’t Skimp on Circulator Pumps
   Undersized or cheap pumps can bottleneck a perfect boiler setup. Always match pump capacity to your zone length and head pressure.

2. Use Outdoor Reset Controls
   These automatically adjust boiler water temp based on outdoor conditions. You’ll save fuel and increase comfort without touching your thermostat.

3. Check Your Venting Setup
   High-efficiency boilers require proper PVC venting and condensate drainage. Make sure your installer understands the difference between atmospheric and sealed combustion.

4. Install a Pressure Relief Valve and Expansion Tank
   Always. No exceptions. A properly sized expansion tank prevents leaks, system strain, and premature wear on your circulators.

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Final Word from Mike

Sizing a Weil-McLain boiler isn’t about guesswork—it’s about knowing your home, understanding heat loss, and respecting how hydronic systems work. If you take the time to get your BTUs right, your zones balanced, and your model selected with care, that boiler will pay you back for decades in comfort, efficiency, and reliability.

And trust me—when that January cold snap hits and your house is warm, even in the corners, you’ll be glad you got the right size the first time.