Troubleshooting Guide What to Do If Your Gas Boiler Isn’t Heating Right

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

  • Thermostat not working

  • Power supply issue

  • Gas valve closed

  • Pilot or ignition problem

Step-by-step fix

  1. Check the thermostat

    • Replace batteries if it’s a wireless unit.

    • Confirm it’s set to “Heat” mode and temperature is above room temp.

    • For smart thermostats, check Wi-Fi and software updates.

  2. Check the breaker & switch

    • Boilers require 120V power. Make sure the breaker isn’t tripped.

    • Flip the boiler’s service switch near the unit (looks like a light switch).

  3. Check the gas valve

    • Locate the gas shutoff near the boiler. Handle should be parallel to the pipe.

    • If perpendicular, the gas is off. Turn it slowly back on.

  4. Check the pilot/ignition

    • If your boiler uses spark ignition, listen for clicking.

    • For standing pilot models, make sure the pilot flame is lit.

🔗 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

  • Dirty flame sensor

  • Faulty thermocouple

  • Blocked vent or flue gases backing up

Step-by-step fix

  1. Clean the flame sensor

    • Shut power & gas off.

    • Remove flame sensor (thin metal rod near burner).

    • Lightly rub with fine sandpaper or steel wool. Reinstall.

  2. Check thermocouple (for pilot systems)

    • If the pilot lights but won’t stay lit, the thermocouple may be weak.

    • Replacement is inexpensive and straightforward.

  3. Inspect the venting

    • Make sure vent pipe slopes upward to the chimney.

    • Look for obstructions (bird nests, debris).

🔗 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

  • Air trapped in radiators

  • Circulator pump failure

  • Zone valve stuck closed

Step-by-step fix

  1. Bleed radiators/baseboard loops

    • Start with top-floor radiators.

    • Use radiator key/screwdriver to release air until water flows.

    • Recheck boiler pressure afterward (may need refill).

  2. Check circulator pump

    • Put your hand on it — you should feel vibration or hear humming.

    • If silent, it may be seized or the relay failed.

  3. Test zone valves

    • Move the manual lever on the valve.

    • If heat flows after, the motor inside is faulty.

🔗 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

  • Expansion tank waterlogged

  • Relief valve stuck

  • Fill valve malfunction

Step-by-step fix

  1. Tap the expansion tank

    • One end should sound hollow (air). Other should be solid (water).

    • If full of water, drain/replace or recharge with air.

  2. Check relief valve

    • Look for dripping or hissing.

    • If leaking constantly, replace.

  3. Check automatic fill valve

    • Should maintain steady pressure.

    • If stuck, pressure can spike or drop.

🔗 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

  • Scale buildup (“kettling”)

  • Air trapped in system

  • Pump cavitation (low water pressure)

Step-by-step fix

  1. Flush the boiler

    • Attach hose to drain valve.

    • Run water until clear.

    • Helps remove sediment and scale.

  2. Bleed air

    • Air in the lines makes gurgling sounds.

    • Use radiator bleeds and automatic air vents.

  3. Check water pressure

    • Too low (<12 psi) = cavitation.

    • Adjust fill valve to bring up pressure.

🔗 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:

  • Persistent yellow flame or soot buildup → CO risk.

  • Smell of natural gas → Shut off and call utility immediately.

  • Relief valve keeps opening → Could be dangerous overpressure.

  • Frequent error codes/shutdowns → Needs diagnostic equipment.

  • 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

  • Radiator key / flat screwdriver

  • Small shop vac

  • Flashlight & mirror

  • Multimeter (for thermostat checks)

  • Leak-detection spray/soap

  • Replacement gaskets & thermocouple


💲 How Troubleshooting Saves Money

By catching issues early, here’s what I’ve avoided:

  • $200 service call for a tripped breaker (DIY fix in 5 minutes).

  • $400 circulator replacement caught before winter.

  • 10–15% lower gas bills from cleaning flame sensor & burner (Energy Star).


📅 Mike’s Troubleshooting Flow (At a Glance)

  1. Check thermostat & power 🔌

  2. Check gas supply 🔥

  3. Observe flame & burner cycle 👀

  4. Inspect pressure & expansion tank ⚖️

  5. Bleed air & flush if needed 💧

  6. 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?

Cooling it with mike

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