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Is the Weil-McLain CGA-5 Big Enough for Your Home?

Mike’s Sizing Rules for Radiators, Baseboards & Multi-Zone Hydronic Systems**

Let’s settle this once and for all:

Your boiler isn’t sized by square footage.

Your boiler isn’t sized by dead-installer guesses.

Your boiler isn’t sized by “what was there before.”

Hydronic heat doesn’t care about rules written for forced-air furnaces.
It runs on:

water volume
emitter capacity
thermal mass
system layout
radiation load
piping flow rates
circulation power
and real-world heat loss

That’s why the Weil-McLain CGA-5 Series 3 — 133,000 BTU Cast Iron Gas Boiler is either:

the perfect size for your home

a catastrophic mismatch waiting to short-cycle itself into an early grave

It all depends on what you actually have connected to it.

Let’s find out if your house deserves the CGA-5 — or if you’re about to oversize yourself into a comfort disaster.

1. What 133,000 BTU REALLY Means in a Hydronic System

Contractors love throwing BTU numbers around.

But the CGA-5’s 133,000 input BTU only matters when you translate it into:

✔ NET heat into the water

✔ NET heat delivered to emitters

✔ NET heat delivered to the rooms

Cast-iron boilers like the CGA-5 have:

thick water jackets
long heat retention curves
high mass
slow temperature swings
stable output even under uneven loads

According to the [Cast-Iron Boiler Water Volume & Heat Retention Map], systems with high water volume benefit MASSIVELY from cast iron because the boiler stays stable, efficient, and quiet even when multiple zones turn on and off.

In plain English:

133k BTU in cast-iron behaves bigger, smoother, and more stable than 133k in a lightweight boiler.

This is the biggest mistake homeowners and “air guys” make — hydronics is a different world.

2. Step One: Determine Your Hydronic Emission Capacity (EDR, Baseboard, Panels)

Before deciding if the CGA-5 is big enough, you MUST calculate:

How much radiation your home actually has
How much heat your emitters can release
Whether your zones are balanced
Whether your baseboard or radiators can handle the boiler’s output

Each emitter type has different math:

Cast Iron Radiators

Measured in EDR (sq. ft. of equivalent direct radiation).
1 sq ft EDR = 150 BTU/hr (at 215°F steam)
OR
1 sq ft EDR = 110–140 BTU/hr (hot water)

Baseboard Finned Tubing

Typically 500–600 BTU per linear foot at 180°F.

Steel Panel Radiators

Varies — typically 200–600 BTU per panel depending on size and water temp.

The [Hydronic Radiation Output Capacity Evaluation] gives these exact conversion ranges.

If your house has:

50,000 BTU worth of radiators → CGA-5 is too big
80,000 BTU worth of radiation → borderline
100k–130k BTU worth of radiation → CGA-5 may be perfect
140,000+ BTU of radiation → consider CGA-6

Your boiler should NEVER be more than 15–20% larger than your radiators.

Oversize = short-cycling = wasted fuel = early failure.

3. Step Two: Evaluate Your Real Heat Loss (Not the Fake HVAC Calculator Numbers)

Forced-air installers size furnaces by:

square footage
climate zone
old furnace size
rule-of-thumb charts

Hydronic sizing is different.

Heat loss is driven by:

wall insulation
window infiltration
air tightness
number of exterior walls
foundation losses
attic insulation
room loads vs circulation

The [Residential Heat Loss and Envelope Load Field Analysis] shows actual heat loss often comes in lower than expected in real hydronic homes, especially older ones with massive cast-iron radiators.

Here’s the punchline:

**Most homes with old radiators don’t actually need 133,000 BTU…

but DO need the water volume and stability of a cast-iron boiler.**

That’s why cast-iron often “feels” more powerful than high-efficiency units.

It’s not raw BTU — it’s controlled thermal mass.

4. Step Three: Check Multi-Zone Layout & Circulator Load

Multi-zone hydronics complicates everything.

Zones create:

uneven flow
intermittent demand
micro-loads
temperature swings
cycling stress
pump head variations

The [Multi-Zone Circulator Load Distribution Sheet] shows that cast-iron boilers handle multi-zone abuse better than almost any modern boiler because:

they tolerate cold returns
they don’t overreact to small loads
they avoid rapid cycling
they buffer temperature change

Zones don’t break cast-iron boilers.

Zones break lightweight mod-cons.

With the CGA-5, the key questions are:

✔ Do all zones combined exceed 100k BTU?

✔ Is the circulator sized for longest, highest-head loop?

✔ Are zone valves or circulators balanced properly?

✔ Is the system piped correctly with purge valves?

If your total emitter load is 90k-130k BTU and the piping is correct?

The CGA-5 is a perfect multi-zone workhorse.

If your total load is under 70k?

You’re oversized.

5. Step Four: Check Radiator Count & Water Volume

The CGA-5 shines in homes with:

lots of cast-iron radiators
large water volume systems
long loops
cold basements
second-floor radiator runs
big panel radiators
multiple zones with staggered demand

Because cast-iron boilers LOVE:

stable flow
high mass
slow temperature swings
gradual BTU extraction
continuous circulation

The [High-Mass Hydronic Performance Stability Report] shows cast-iron boilers outperform mod-cons every time when the system contains 10+ gallons of water.

The more water you have?

The better a CGA-5 performs.

Oversized water volume = stability
Undersized water volume = cycling

6. Step Five: Flue and Gas Supply Must Match the Boiler’s Appetite

Most homeowners forget:

A boiler is only as strong as its gas pipe and chimney draft.

Gas pipe must be:

¾" minimum
1" preferred for longer runs
properly sized to avoid pressure drop

The Residential Gas Supply Pressure & BTU Flow Audit confirms 133k BTU units need proper upstream pressure or the flame will starve.

Chimney must:

be lined
drafted correctly
have proper height
not be cold or oversized

The [Atmospheric Boiler Draft and Spill Condition Field Note] shows atmospheric boilers lose 15–25% efficiency in chimneys that are cold, oversized, or unlined.

If the gas and chimney are ready?