0 comments

Troubleshooting Guide:

If Your Weil-McLain CGA-5 Isn’t Heating Right, Here’s What’s Actually Wrong**
Mike Breaks Down Every Real-World Cause of “No Heat,” “Weak Heat,” and “Lukewarm Radiators” in Cast-Iron Boiler Systems

Let me tell you something most technicians are too scared to admit:

90% of “boiler problems” aren’t boiler problems at all.

They’re:
— air problems
— flow problems
— thermostat problems
— circulator problems
— venting problems
— safety-lockout problems
— piping mistakes

The Weil-McLain CGA-5 Series 3 (133,000 BTU) is one of the most reliable cast-iron boilers ever made.
If it’s not heating right, something around the boiler is choking it.

This is the real troubleshooting guide — not the soft, incomplete junk you find in manuals.

Let’s fix your heat the RIGHT way.

**1. Radiators Cold on One Floor?

You’ve Got an Airlock — Not a Boiler Failure**

Air is the #1 enemy of hydronics.
It causes:

cold radiators
uneven heat
noisy pipes
dead zones
circulator cavitation

The [Radiator Flow Interruption Log] shows that nearly 70% of cold-radiator calls come from trapped air — not from the boiler.

Check for:

upper radiators cold
baseboards gurgling
water not flowing on multi-floor home
circulator whining

The fix:

purge each loop
confirm auto-vent is working
verify expansion tank isn’t waterlogged
check supply-side air separator
verify system pressure (12–18 psi cold)

If you just added water recently, expect air.
Bleed it out and watch the magic happen.

**2. Boiler Fires but Rooms Stay Cold?

Your Circulator Pump Isn’t Moving Water**

If the boiler heats up but radiators don’t?

That’s flow — NOT temperature.

Circulator failures happen because of:

bad bearings
seized rotor
clogged impeller
low system pressure
zone valve motor failure
air in pump housing

The [Hydronic Circulator Failure Pattern Matrix] shows circulators fail 10× more often than cast-iron boilers.

Signs of pump failure:

boiler is hot but pipes are cold
pump is humming but not pumping
pump is silent but should be running
one zone is dead while others work

Test:

feel pump body vibration
check discharge temp
manually open zone valve
confirm call for heat at control board

Fix the flow, and the heat returns immediately.

**3. Boiler Short-Cycling?

Your Emitters Can’t Release the Heat**

Short cycling means:

boiler fires
overheats quickly
shuts down
fires again
repeats

This is a design or flow imbalance.

Causes include:

baseboard too short for room
radiators undersized
low flow through loops
faulty bypass or missing bypass
mixing valves stuck
primary/secondary piping missing
low return temperatures spiking boiler temp

According to the [Heat Emitter BTU Output Stability Ledger], emitters that release fewer BTUs than the boiler produces cause rapid cycling.

The CGA-5 is a hardcore, high-output unit — if the system can’t absorb the heat, it shuts down to protect itself.

Solutions:

increase baseboard length
adjust flow
add radiator sections
add bypass valve
verify system delta-T

**4. Pilot Won’t Stay Lit?

You’ve Got a Combustion or Draft Problem**

Pilot outages are caused by:

bad thermocouple
weak pilot flame
dirty pilot orifice
failed gas valve
low inlet gas pressure
backdrafting chimney
inadequate combustion air

The [Draft Stability Snapshot Report] highlights that atmospheric boilers like the CGA-5 are extremely sensitive to:

wind-induced downdrafts
cold chimney startup
negative pressure basements

If your pilot goes out:

FIRST: Check draft

SECOND: Check gas pressure

THIRD: Clean pilot

THEN: Replace thermocouple

Do NOT skip the draft test.
Do NOT replace parts blindly.

**5. Boiler Fires But Water Temps Stay Low?

You’re Either Under-Fired or Gas Starved**

If your boiler’s burners sound weak or soft:

gas pressure is low
burner orifices are blocked
utility supply is inadequate
shared gas appliances are stealing flow

The [Gas Train Combustion Pressure Drop Field Note] confirms firing rate collapses when dynamic gas pressure drops below 5–7" WC.

Check:

utility supply pressure
CSST runs longer than 30 ft
undersized ½" line (common culprit)
pressure drop when water heater fires
burner cleanliness

Weak flame = weak BTUs = cold house.

**6. Rooms Heat Slowly?

Your Delta-T Is Wrong**

Correct hydronic delta-T (temp difference between supply and return) should be 15–25°F.

If delta-T is too small:

flow is too high
water moves too fast
heat isn’t transferred

If delta-T is too large:

flow is too low
circulator is undersized
zone valves aren’t opening fully
air in system
blocked radiators

The [Delta-T Hydraulic Performance Diagnostic Guide] shows that improper delta-T reduces system efficiency and causes uneven room temperatures.

Solve by:

adjusting pump speed
checking air elimination
verifying zone valve operation
measuring real pipe flow

**7. Relief Valve Dripping?

Your Expansion Tank Is Waterlogged**

If your 30-psi relief valve drips or dumps water,
the cause is almost ALWAYS the expansion tank.

When the tank loses its air cushion:

system pressure spikes
relief valve opens
boiler temp swings badly

The Hydronic Expansion Tank Charge & Pressure Response Memo shows over 80% of relief valve calls involve failed expansion tanks.

Test:

tap tank → solid = bad
check Schrader valve → water = bad
check pressure charge → should be 12 psi

Replace the tank and pressure stabilizes instantly.

**8. Radiators Only Heat Halfway?

Flow Is Restricted or Emitters Are Air-Bound**

Symptoms:

bottom hot, top cold = air
one end hot, one end cold = poor flow
halfway warm = sludge buildup

The Old-System Sludge & Radiator Flow Impedance Study reveals that older homes suffer from internal radiator restrictions caused by:

rust
iron oxide
sediment
hard water scale

Fixes:

bleed radiators
flush loops
reverse flush problem radiators
clean or replace balancing valves

Hydronic emitters MUST be clean — or you’ll fight cold rooms forever.

**9. Boiler Runs Hot But House Stays Cold?

Your Thermostat or Controls Are Lying to You**

Sometimes the boiler is fine.
The controls aren’t giving it permission to heat the right zone.

Causes:

bad thermostat
loose thermostat wire
stuck zone valve
open-circuit end switch
bad transformer
miswired circulator relay
stuck relay contacts

The Hydronic Zone Control & Low-Voltage Fault Pattern Sheet documents the high failure rate of aging zone valves in older homes.

Fix:

check thermostat voltage
confirm call-for-heat at control board
manually open zone valves
test end switches
check transformer output (24V nominal)

If the boiler runs but the circulator doesn’t?
You’ve got a control problem — NOT a boiler problem.

**10. Boiler Makes Banging or Rumbling Sounds?

It’s Either Kettling or Flow Turbulence**

Boiler rumble (kettling) comes from:

scale on cast-iron sections
microbubbles flashing into steam
uneven heat transfer
low flow

The Thermal Shock & Heat Exchanger Scale Accumulation Journal shows kettling reduces lifespan dramatically.

Fixes:

flush boiler
add treatment
check bypass
verify adequate flow

Flow turbulence banging?

bad check valve
backward circulator
incorrect pump speed
unbalanced zones

Identify the noise source — then fix the hydronics.

**11. Boiler Satisfied Quickly But Rooms Still Cold?

Your System Is Short-Cycling From Oversizing**

If your boiler:

fires
shuts off within 3 minutes
fires again
repeats

You’re oversized.
The CGA-5 might be too big for your radiator or baseboard load.

Solutions:

add more baseboard
increase radiator EDR
add a buffer tank
tighten delta-T
reduce water temperature
adjust circulator strategy

Oversizing is the silent killer of comfort.

**12. Boiler Won’t Fire at All?

Check These Before Blaming the Boiler**

Most "dead boilers" are caused by:

blown fuse
tripped switch
locked-out safety limit
no call for heat
bad transformer
pressure too low
thermostat wire broken

And yes —
someone flipping the boiler switch thinking it’s a light switch is extremely common.

Before calling a pro, verify:

power switch ON
breaker ON
system pressure 12–18 psi
thermostat calling
zone valves open
low-water cutoff not tripped

**Mike’s Final Verdict — The CGA-5 Rarely Fails.

Everything Around It Does.**

Here’s the honest truth:

✔ Circulators fail

✔ Zone valves fail

✔ Air traps the system

✔ Radiators sludge up

✔ Gas pressure drops

✔ Controls misfire

✔ Thermostats lie

✔ Chimneys backdraft

✔ Emitters undershoot

These issues make it LOOK like the boiler is bad —
but the boiler is usually the LAST thing to blame.

The Weil-McLain CGA-5 is a workhorse.
If it’s not heating right?

The hydronic system is screaming for attention.

Fix the air.
Fix the flow.
Fix the controls.
Fix the draft.
Fix the emitters.
Fix the piping.

Your boiler will reward you with decades of perfect heat.

That’s the Mike way.

Maintenance checklist is provided in the next blog.

Tags: Cooling it with mike

WRITTEN BY

Mike Sanders

Previous article Maintenance Checklist: How Mike Keeps Weil-McLain Cast-Iron Boilers Running for 25+ Years Next article Radiators, Baseboards & Hydronic Emitters: What Works Best With a Weil-McLain Cast-Iron Boiler?