Radiators, Baseboards & Hydronic Emitters:
What Works Best With a Weil-McLain Cast-Iron Boiler?**
Mike Explains How to Match the CGA-5 to Your Emitters for Maximum Heat Output and Perfect Comfort
Let me start with the hard truth:
Your boiler doesn’t decide your home’s comfort — your EMITTERS do.
And the type of emitters you have absolutely determines whether a CGA-5 performs like a champion or a choked-out mess.
You can buy the best boiler in the world — and the Weil-McLain CGA-5 Series 3 (133,000 BTU) is one of them —
but if your radiators or baseboards can’t release the boiler’s heat at the RIGHT rate, your system will:
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short-cycle
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overheat water
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leave rooms cold
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waste gas
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run noisy
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stress the boiler
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and fail early
Hydronics is a marriage between:
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boiler output
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emitter capacity
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water volume
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flow rate
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return temperature
Let’s break down EXACTLY which emitters pair best with a Weil-McLain cast-iron system.
1. Cast-Iron Radiators: The PERFECT Match for a CGA-5
Let’s get this straight:
✔ Cast-iron boiler + cast-iron radiators = the most stable, comfortable heat ever created.
WHY?
Because cast-iron radiators deliver:
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massive thermal mass
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slow, even heat distribution
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wide temperature tolerance
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low flow requirements
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perfect pairing with high-mass boilers
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minimal cycling
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unmatched comfort
Each radiator is rated in EDR (Equivalent Direct Radiation).
The [Radiator EDR Output and Temperature Curve Ledger] shows:
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1 sq ft of EDR gives 110–140 BTU/hr at typical hot-water temperatures
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slow, steady output matches cast-iron boilers perfectly
A home with:
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LOTS of radiators
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BIG radiators
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MULTIPLE floors
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LONG piping runs
…is exactly what the CGA-5 was designed to dominate.
Cold climate?
Old home?
Radiators everywhere?
CGA-5 is a bull in its natural environment.
2. Copper Baseboard (Fin Tube): A Good Match — But Only When Designed Correctly
Baseboard fin-tube emitters work great with cast iron —
but they’re more sensitive to:
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water temperature
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flow rate
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long loop runs
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return temperature
The [Baseboard Heat Transfer Coefficient Field Analysis] confirms fin-tube output is heavily dependent on entering water temperature:
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180°F supply → 500–600 BTU/ft
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160°F supply → 350–450 BTU/ft
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140°F supply → 200–300 BTU/ft
That means:
The CGA-5 works BEST with baseboard loops that keep water temps above 160°F.
If your baseboard is too short for the room’s heat loss?
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the boiler cycles rapidly
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rooms heat unevenly
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return water may be too cold
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the boiler risks thermal shock
Signs your baseboard isn’t long enough:
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boiler fires often
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long recovery times
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rooms fall behind on windy days
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return piping feels too cool
Baseboard can work beautifully —
but ONLY if it’s sized for the load.
3. Steel Panel Radiators: The Goldilocks Option for Modern Hydronics
Steel panel radiators (Runtal, Buderus, Myson, etc.) are the middle ground between baseboard and cast iron.
They offer:
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fast response
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decent mass
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excellent output
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compact sizes
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silent operation
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zone compatibility
The [Convective Output Ratio Study] shows steel panels deliver extremely stable output across a wide water temperature range.
They’re especially perfect for:
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mixed emitter homes
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multi-level zoning
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remodels
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additions
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retrofits where radiators were removed
And the CGA-5 works GREAT with them because steel panels:
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maintain stable delta-T
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don’t shock the boiler
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maintain consistent return temps
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handle both 160–180°F loops and mid-temp loops
If you want modern looks with cast-iron comfort?
Steel panels are the way.
4. Radiant Floor Heat: Works with CGA-5 ONLY with Proper Mixing
Radiant floor systems use:
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90°F–130°F supply temperatures
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low flow resistance
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high-mass delivery
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extremely low return water temps
This is DANGEROUS territory for a cast-iron boiler.
The CGA-5 MUST NOT receive cold radiant returns without protection.
The Low-Temperature Loop Mixing & Return Protection Bulletin demonstrates that boilers see metal stress when return temps drop below 130°F.
Radiant floors can be paired with a CGA-5 ONLY IF:
✔ A mixing valve is installed
✔ OR primary/secondary piping is used
✔ OR a hydraulic separator is used
✔ OR a boiler bypass is used
Get this wrong?
You will crack the boiler.
Radiant + cast iron works,
but only with engineered protection.
5. Fan-Coil Units: POSSIBLE, but Not Ideal for Cast-Iron Systems
Fan-coil units (air handlers with hydronic coils) demand:
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high flow
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high temperature
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consistent circulation
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fast recovery
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minimal thermal lag
Cast-iron boilers CAN feed them — but not ideally.
The [Hydronic Coil High-Load Delta-T Response Ledger] shows fan-coils produce temperature swings that create:
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rapid cycling
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unstable return temps
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noisy piping
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poor comfort in small zones
They’re fine in:
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garages
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additions
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single-zone setups
They’re bad in:
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multi-zone homes
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long loop systems
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areas with intermittent demand
Fan-coils work better with mod-cons.
Stick to radiators or baseboard for a CGA-5.
6. Snowmelt Loops: DO NOT DIRECTLY CONNECT TO A CGA-5
Snowmelt uses:
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80–120°F water
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anti-freeze (glycol)
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extended run times
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low delta-T
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low return temps
This will destroy a cast-iron boiler if piped incorrectly.
The [Outdoor Hydronic Loop Temperature Protection Report] confirms:
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glycol loops rob boilers of efficiency
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cold water shock destroys cast iron
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snowmelt loops MUST be isolated
If you want snowmelt with a CGA-5:
✔ Use a heat exchanger
✔ Use a dedicated pump module
✔ Keep radiant separated from boiler water
Direct connection = boiler death sentence.
7. Mixed Emitter Systems: The CGA-5 Can Handle Them — IF Piped Right
Many older homes now have:
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cast-iron radiators in old rooms
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baseboard in additions
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radiant in bathrooms
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panel radiators in remodeled spaces
This is actually where cast iron shines.
High-mass boilers handle mixed temperature loads better than mod-cons BECAUSE:
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they buffer temperature
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they stabilize return temps
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they minimize cycling
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they tolerate varying flow
The Thermal Mass Distribution & Loop Stability Map notes that cast-iron systems outperform mod-cons in homes with more than three emitter types.
BUT:
Mixed systems require proper zoning and mixing.
Hire someone who understands hydronics —
not someone who thinks “a zone is a zone.”
8. Mike’s Recommendations — Which Emitters Work BEST With the CGA-5?
BEST MATCH:
✔ Cast-iron radiators
✔ Steel panel radiators
✔ Properly sized baseboard
GREAT with protection:
✔ Radiant floor heat (with mixing)
✔ Low-temp zones (with bypass or P/S piping)
ACCEPTABLE but not ideal:
✔ Fan-coils (only in limited applications)
NEVER directly connect:
✘ Snowmelt loops
✘ Glycol-heavy systems
✘ Low-temp manifolds without mixing
Mike’s Final Verdict — The Boiler Isn’t the Boss. The Emitters Are.
Here’s the real truth:
✔ The Weil-McLain CGA-5 is a monster…
✔ But ONLY if the emitters can release the heat correctly.
✔ Radiators = perfect match
✔ Baseboard = great match when sized right
✔ Steel panels = extremely compatible
✔ Radiant = safe ONLY with mixing
✔ Fan coils = possible but not preferred
✔ Snowmelt = absolutely not without isolation
Hydronics is emitter-driven.
If your emitters align with the CGA-5’s strengths, you get:
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stable heat
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quiet rooms
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perfect comfort
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long cycles
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low maintenance
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decades of reliability
If not?
You’ll fight your system every winter.
Pick the right emitters.
Pipe them correctly.
Protect the boiler.
That’s the Mike way.
Troubleshooting Guide will be provided in the next blog.
