Boiler Troubleshooting Guide

Your boiler is running but the house is cold, or worse — it has gone completely dead with a blinking fault code and no hot water. Most boiler failures trace back to a short list of culprits: lost pressure, a failed igniter or flame sensor, a stuck circulator, air in the lines, or a safety device doing its job. This guide walks you through the checks a technician actually performs, with real US repair costs, so you can decide what to fix yourself and when to call a pro.

Before You Touch Anything: Boiler Safety Basics

A boiler is a pressurized, gas- or oil-fired appliance. Two rules come first:

• Never open the combustion chamber, disconnect a gas line, or bypass a safety switch. The pressure relief valve, low-water cutoff, and rollout sensors exist to prevent explosions and carbon monoxide poisoning.
• Make sure you have working CO alarms on every level of the home and outside sleeping areas. They should be certified to UL 2034. The National Fuel Gas Code (NFPA 54) and ANSI Z21.13 require that gas-fired boilers be installed with proper venting — a blocked flue is a life-safety issue, not a DIY project.

If you smell gas, leave the house and call your utility’s emergency line from outside. Do not flip switches on the way out.

Step 1: The Five-Minute Diagnostic Walk-Through

Do these in order. They resolve a surprising share of “boiler not working” calls before a technician ever leaves the truck.

Check the thermostat and the call for heat

• Set the thermostat 5°F above room temperature. Is the display lit? Batteries die quietly — replace them.
• If you have a smart thermostat, check that the schedule isn’t overriding your manual setting and that the heat mode (not cool/fan) is selected.
• Confirm the boiler’s own aquastat or outdoor reset control isn’t set below the current demand.

Confirm power and gas

• Look for a switched circuit, a tripped breaker, or a blown 3-amp fuse on the boiler’s control board. Most US residential boilers run on a dedicated 120V/15A circuit.
• Verify the gas shutoff valve at the boiler is parallel to the pipe (open), and that the utility hasn’t shut service off.
• If you’re on propane, check the tank level — a 20% tank in January is a classic no-heat cause.

Read the pressure gauge

This is the single most common finding on a cold-weather service call.

Bleed the radiators

Cold at the top, warm at the bottom usually means trapped air. With the system cool, open each radiator’s bleed valve with a radiator key until steady water (not spitting air) appears, then close it. Recheck pressure afterward. For more system-wide checks, our library of HVAC troubleshooting guides covers airflow, thermostat, and electrical diagnostics in the same step-by-step format.

Inspect the obvious

• Is the condensate trap or PVC drain line on a high-efficiency (condensing) boiler frozen or clogged? A blocked condensate line causes a lockout within minutes.
• Is the outdoor combustion air intake buried in snow?
• Are any zone valves manually closed? Check the lever position.

Common Causes of a Boiler Not Working

1. No ignition: pilot light or electronic igniter failure

Standing-pilot boilers (common in older US installations) use a thermocouple that generates about 20–30 millivolts to hold the gas valve open. When it fails, the pilot won’t stay lit.

• Thermocouple replacement: $150–$300 installed.
• Hot surface igniter (HSI) on modern boilers: $200–$400 installed.
• Symptoms: pilot goes out shortly after lighting; clicking without flame; fault code for “no flame detected.”

2. Dirty flame sensor

The flame sensor is a metal rod that proves flame by sensing microamps of current. It oxidizes and stops conducting. Cleaning it with fine steel wool is a legitimate DIY task; replacement runs $150–$300. If you’re seeing repeated lockouts on a boiler that fires for 10 seconds and shuts down, this is your prime suspect.

3. Short cycling

A boiler that fires for two minutes and shuts off repeatedly is short cycling. Causes include:

• Oversized boiler relative to the home’s heat load (common when a 150,000 BTU unit lands in a house that needs 80,000 BTU).
• Failed expansion tank — the most frequent cause. Tap the tank; if it sounds full of water instead of hollow, the bladder has ruptured. Replacement: $250–$500.
• Restricted circulation from a failing circulator pump or a nearly closed zone valve.
• Fouled flame sensor or a dirty heat exchanger causing the high-limit switch to trip.

4. Circulator pump not running

The pump moves hot water out to the zones. A pump with a seized rotor, a failed capacitor, or a dead relay means the boiler heats but nothing reaches the radiators.

• Listen for the pump: quiet hum = running; silence or grinding = problem.
• Replacement cost: $300–$650 installed, higher for a high-efficiency ECM pump ($500–$900).

5. Zone valve or circulator relay failure

If one room stays cold while the rest of the house heats, the zone valve actuator or its end switch has failed. Expect $250–$500 installed per zone.

6. Kettling, banging, or gurgling

Loud banging is usually air or flashing steam in a hot-water system; a low, rumbling “kettle” sound is scale buildup on the heat exchanger. Kettling reduces efficiency and eventually cracks the exchanger. A power flush to remove sludge and scale runs $400–$900 and should be part of maintenance on systems older than 10 years.

7. Boiler leaking water

Where the water appears tells you a lot:

• Pressure relief valve: system overpressure from a bad expansion tank or overfilling.
• Circulator flange or pump gasket: $50–$150 part, $250–$450 installed.
• Heat exchanger: the bad news. Replacement is $1,000–$2,500 on a conventional boiler and often $1,800–$3,500 on a condensing model. That’s the point where you compare repair against a new boiler.

8. Incomplete combustion and yellow flames

A healthy natural gas flame is blue with a slightly lighter inner cone. A yellow, lazy flame or soot on the burners means incomplete combustion — shut the boiler down and call a technician immediately. Gas pressure should be roughly 7 inches water column for natural gas and 11 inches for propane. Never adjust the gas valve yourself.

9. Boiler lockout after a power outage

Many modern boilers require a manual reset after a power interruption or a lockout code. Hold the reset button per the manufacturer’s label — once. If it locks out again, stop resetting it. Repeated resets can flood the combustion chamber with gas. A boiler that reliably needs resetting is a boiler that needs a technician; review our guide on what to do when your boiler is not working for lockout codes by brand.

Boiler Repair Cost Guide (2026, US)

For a full breakdown of what technicians charge by job and region, see our professional boiler repair cost page.

Repair vs. replace: the 50% rule is a reasonable guide. If the repair costs more than half the price of a new boiler and the unit is past 15 years old, replacement usually wins. New condensing boilers run $5,000–$10,000 installed for a combi, or $3,500–$8,000 for a standard unit. Look for the ENERGY STAR label and an AFUE rating of 90% or higher for condensing equipment; conventional atmospheric boilers typically sit at 80–85% AFUE. A 90% AFUE boiler can cut gas use by 15–25% compared with a 30-year-old unit.

When to Call a Licensed Technician

Call a pro right away if you see any of these:

• Gas odor, soot, or a yellow flame.
• Water pooling near the boiler or from the relief valve.
• A boiler that repeatedly locks out after resetting.
• Any work involving the gas valve, gas piping, flue, or combustion chamber.
• Cracked or leaking heat exchanger.

The rest — thermostats, batteries, bleeding radiators, fill pressure, air filters, and clearing snow from the vent — are safe homeowner tasks. Since boilers are pressurized systems, most states require a licensed plumber or HVAC contractor for gas-side work, and many municipalities require a permit for replacements.

Preventive Maintenance That Prevents Most Breakdowns

1. Annual tune-up every fall. A technician cleans the burners, checks combustion with a digital analyzer, tests the relief valve, and verifies the low-water cutoff. $150–$350 is cheap insurance.
2. Check pressure monthly during heating season.
3. Bleed radiators at the start of each season.
4. Keep the vent and intake clear of snow, leaves, and shrubs — 12 inches minimum clearance.
5. Install a water softener if your water is hard; scale is the leading killer of heat exchangers.
6. Replace the filter on the boiler’s air intake (if equipped) and clean the condensate trap annually on condensing units.

Frequently Asked Questions

Why is my boiler not working but the thermostat is on? If the thermostat is calling for heat but the boiler stays cold, the problem is usually downstream of the thermostat: a tripped low-water cutoff, a stuck zone valve, a failed circulator pump, low system pressure (under 12 psi), or a flame-proving fault such as a dirty flame sensor or failed igniter. Check the pressure gauge and the boiler’s error code display first, then verify the gas valve is open.

How much does it cost to fix a boiler that isn’t working? In the US, a diagnostic visit typically runs $89–$250, and most common repairs land between $150 and $650 — for example, a thermocouple or igniter replacement at $150–$300, a circulator pump at $300–$650, or an expansion tank at $250–$500. Control boards ($400–$900) and heat exchangers ($1,000–$2,500) are the most expensive fixes and often push homeowners toward replacement.

What pressure should my boiler be at? Most residential hot-water boilers should sit between 12 and 18 psi cold, rising to roughly 20–25 psi when the system is at full temperature. Below 12 psi, upper-floor radiators go cold and the boiler may lock out on a low-water cutoff. Above 30 psi, the pressure relief valve (set at 30 psi per ASME standards) will discharge water — shut the system down and call a licensed technician.

Conclusion

A boiler that isn’t working almost always fails in one of six predictable places: pressure, ignition, flame sensing, circulation, venting, or a safety switch that tripped for a reason. Start with the five-minute walk-through — thermostat demand, power, gas, gauge pressure, and air in the radiators — and you’ll resolve the easy 30% of calls and gather the diagnostic details a technician needs for the rest. Respect the hard line: gas, flue, combustion chamber, and heat exchanger work belong to a licensed professional, and any sign of a yellow flame, soot, gas odor, or water at the relief valve means shut it down and call today. Repair costs of $150–$650 are far more common than catastrophic failures, so diagnose before you assume the worst — and if your boiler is past 15 years old with a repair bill over half the cost of a new ENERGY STAR condensing unit, put that money toward a replacement that will cut your gas bill by 15–25% for the next two decades.