There’s nothing worse than a boiler that quits on the coldest day of the year. I’ve been there. My Weil-McLain CGI-4 Series 4 had been running smooth, but one January morning, the radiators were stone cold. Instead of panicking, I went down to the utility room, grabbed my flashlight and notebook, and got to work.
👉 Here’s the step-by-step troubleshooting guide I wish I had that day. I’ve broken it down into common symptoms, what usually causes them, and the exact steps I’ve taken to fix them. Some are quick DIY tasks; others are best left to a pro.
❄️ Symptom 1: Boiler Won’t Fire Up
What’s happening: The thermostat calls for heat, but the boiler doesn’t turn on at all.
Possible causes
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Thermostat not working
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Power supply issue
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Gas valve closed
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Pilot or ignition problem
Step-by-step fix
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Check the thermostat
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Replace batteries if it’s a wireless unit.
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Confirm it’s set to “Heat” mode and temperature is above room temp.
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For smart thermostats, check Wi-Fi and software updates.
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Check the breaker & switch
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Boilers require 120V power. Make sure the breaker isn’t tripped.
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Flip the boiler’s service switch near the unit (looks like a light switch).
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Check the gas valve
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Locate the gas shutoff near the boiler. Handle should be parallel to the pipe.
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If perpendicular, the gas is off. Turn it slowly back on.
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Check the pilot/ignition
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If your boiler uses spark ignition, listen for clicking.
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For standing pilot models, make sure the pilot flame is lit.
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🔗 Resource: Energy.gov – Gas Heating System Basics
💡 Mike’s Tip: 70% of the time, when my neighbor’s boiler wouldn’t start, it turned out to be the thermostat — not the boiler itself.
🔥 Symptom 2: Burner Lights, Then Shuts Off
What’s happening: Boiler fires up for a few seconds, then shuts down.
Possible causes
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Dirty flame sensor
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Faulty thermocouple
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Blocked vent or flue gases backing up
Step-by-step fix
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Clean the flame sensor
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Shut power & gas off.
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Remove flame sensor (thin metal rod near burner).
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Lightly rub with fine sandpaper or steel wool. Reinstall.
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Check thermocouple (for pilot systems)
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If the pilot lights but won’t stay lit, the thermocouple may be weak.
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Replacement is inexpensive and straightforward.
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Inspect the venting
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Make sure vent pipe slopes upward to the chimney.
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Look for obstructions (bird nests, debris).
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🔗 Resource: HVAC.com – Why Your Boiler Shuts Off
💡 Mike’s Tip: My CGI-4 once shut off because a wasp had nested in the vent screen outside. A quick sweep with a brush fixed it.
💧 Symptom 3: Boiler Runs, But No Heat in Radiators
What’s happening: The boiler is on, but the house is still cold.
Possible causes
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Air trapped in radiators
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Circulator pump failure
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Zone valve stuck closed
Step-by-step fix
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Bleed radiators/baseboard loops
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Start with top-floor radiators.
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Use radiator key/screwdriver to release air until water flows.
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Recheck boiler pressure afterward (may need refill).
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Check circulator pump
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Put your hand on it — you should feel vibration or hear humming.
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If silent, it may be seized or the relay failed.
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Test zone valves
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Move the manual lever on the valve.
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If heat flows after, the motor inside is faulty.
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🔗 Resource: Family Handyman – How to Bleed Radiators
💡 Mike’s Tip: A noisy circulator pump is usually on its way out. Replacing mine early saved a mid-winter breakdown.
⚖️ Symptom 4: Boiler Pressure Too High or Too Low
What’s happening: Pressure gauge shows outside the normal 12–15 psi range.
Possible causes
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Expansion tank waterlogged
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Relief valve stuck
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Fill valve malfunction
Step-by-step fix
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Tap the expansion tank
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One end should sound hollow (air). Other should be solid (water).
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If full of water, drain/replace or recharge with air.
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Check relief valve
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Look for dripping or hissing.
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If leaking constantly, replace.
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Check automatic fill valve
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Should maintain steady pressure.
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If stuck, pressure can spike or drop.
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🔗 Resource: Weil-McLain Support – Expansion Tanks
💡 Mike’s Tip: After I ignored a waterlogged expansion tank, my relief valve kept dumping water. $40 for a new tank fixed it.
🌬️ Symptom 5: Strange Noises (Banging, Gurgling, Whistling)
What’s happening: The system makes odd sounds while running.
Possible causes
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Scale buildup (“kettling”)
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Air trapped in system
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Pump cavitation (low water pressure)
Step-by-step fix
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Flush the boiler
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Attach hose to drain valve.
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Run water until clear.
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Helps remove sediment and scale.
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Bleed air
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Air in the lines makes gurgling sounds.
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Use radiator bleeds and automatic air vents.
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Check water pressure
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Too low (<12 psi) = cavitation.
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Adjust fill valve to bring up pressure.
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🔗 Resource: This Old House – Boiler Noises Explained
💡 Mike’s Tip: A whistling sound in my CGI-4 turned out to be lime scale. One flush with fresh water solved it.
🛑 When to Call a Pro
Here’s where I draw the line as a DIYer:
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Persistent yellow flame or soot buildup → CO risk.
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Smell of natural gas → Shut off and call utility immediately.
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Relief valve keeps opening → Could be dangerous overpressure.
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Frequent error codes/shutdowns → Needs diagnostic equipment.
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Annual combustion efficiency test → Specialized tool required.
🔗 Resource: EPA – Carbon Monoxide Safety
💡 Mike’s Tip: I do 80% of troubleshooting myself, but I never mess with gas leaks or combustion tuning.
🧰 Tools I Keep in My Boiler Troubleshooting Kit
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Radiator key / flat screwdriver
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Small shop vac
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Flashlight & mirror
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Multimeter (for thermostat checks)
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Leak-detection spray/soap
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Replacement gaskets & thermocouple
💲 How Troubleshooting Saves Money
By catching issues early, here’s what I’ve avoided:
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$200 service call for a tripped breaker (DIY fix in 5 minutes).
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$400 circulator replacement caught before winter.
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10–15% lower gas bills from cleaning flame sensor & burner (Energy Star).
📅 Mike’s Troubleshooting Flow (At a Glance)
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Check thermostat & power 🔌
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Check gas supply 🔥
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Observe flame & burner cycle 👀
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Inspect pressure & expansion tank ⚖️
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Bleed air & flush if needed 💧
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Call a pro if safety is in question 🛑
📝 Mike’s Final Word
When your Weil-McLain CGI-4 boiler isn’t heating right, don’t panic. Most problems have simple causes. With a flashlight, a logbook, and a bit of patience, you can fix many issues yourself.
But remember: safety comes first. If there’s gas, CO, or high pressure involved, it’s time to call a pro.
The best part? Every time you troubleshoot, you learn more about your system — and that knowledge pays off for decades of reliable heat.
In the next topic we will know more about: Weil-McLain vs. Burnham vs. Utica: Which 90k BTU Boiler Does Mike Recommend?
