Boiler not working: where to start
A boiler heats water that circulates through radiators or in-floor piping rather than heating air directly, which means several of the usual furnace checks do not apply: and a few boiler-specific ones do.
Check the water pressure gauge first
Most residential boilers have a pressure gauge on the front. A healthy system typically sits around 12 to 15 psi when cold, though this varies by manufacturer and system height. A reading at or near zero usually means the system has lost water, often through a slow leak in the piping or a relief valve that has been discharging. Many boilers are designed to shut down safely rather than run without enough water.
A pilot light that has gone out
Older standing-pilot boilers can have their pilot light extinguished by a draft, a temporary gas supply interruption, or simply age. Many boilers have a printed relighting procedure on a label inside the access panel. If you smell gas at any point during this, stop immediately, leave the house. And call your gas utility or 911 from outside: do not attempt to relight anything with gas odor present.
Newer boilers with electronic ignition
Boilers with electronic ignition rather than a standing pilot do not have a flame to relight manually. And a failure to ignite points at the ignition control board, a flame sensor, or a gas valve issue. All a technician's territory rather than something to troubleshoot at the panel.
A tripped high-limit or low-water cutoff
Boilers have their own set of safety switches, including a high-limit switch that stops the burner if the water gets too hot. And on many systems, a low-water cutoff that stops the burner if the water level drops too far. These exist specifically to prevent damage or a dangerous condition, and repeatedly resetting them without understanding why they tripped is not a safe substitute for finding the cause.
Kettling: a specific boiler noise worth knowing
A rumbling or banging sound, sometimes compared to a kettle about to boil, often comes from mineral scale on the heat exchanger. The scale restricts water flow at that point, causing localized overheating and steam pockets. It signals reduced efficiency and, over time, potential damage to the heat exchanger. It is worth having addressed rather than treated as background noise.
What not to do
- Do not add water to the system repeatedly to compensate for a leak without having the leak found and fixed. Constantly adding fresh water accelerates mineral scale buildup inside the system.
- Do not attempt to bleed radiators or adjust the pressure-reducing valve without understanding your specific system, since overfilling can trigger the relief valve to discharge.
- Do not ignore a carbon monoxide alarm near the boiler. Leave the house and call your gas utility or 911 from outside; do not investigate it yourself.
When to call a licensed technician
Persistent pressure loss, an ignition system that will not light on an electronic boiler, kettling noise. Or any repeated safety-switch trip are all reasons to call a licensed HVAC or heating technician rather than continue working through it alone.
Frequently asked questions
Why does my boiler's pressure keep dropping slowly over weeks?
A slow, gradual pressure drop over time usually points at a small leak somewhere in the system's piping or at a fitting, rather than a single dramatic failure. It is worth having a technician trace it rather than just topping up pressure repeatedly.
Is a boiler more or less maintenance than a furnace?
They have different maintenance needs rather than simply more or less: boilers benefit from periodic checks of water quality, pressure, and the heat exchanger for scale, while furnaces focus more on airflow, the flame sensor, and the heat exchanger for cracks.
Sources
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