Boiler not working? Reading the symptoms before you call

A boiler not working rarely fails silently. It drops its pressure, or it bangs, or one radiator goes cold while the rest stay hot, or the pilot will not stay lit. Each of those is a different fault with a different answer, and telling them apart takes a couple of minutes.

This is how to read what yours is doing. It will not always let you fix it — some of these you should deliberately not touch — but it will tell you whether a boiler not working means a ten-minute job or a real repair, and that is worth knowing before anyone comes out.

Boiler not working: start with the pressure gauge

Most domestic systems sit around 12 to 15 psi when cold. Below about 10 and a boiler will usually lock out rather than fire, which is the single most common reason a boiler is not working in the first cold week.

• A slow drop over months is normal-ish and usually just needs topping up.
• A drop over days means a leak. It may be a radiator valve, a joint, or the system’s own pressure relief discharging. That is worth finding, not just topping up repeatedly.
• Pressure that climbs when the heating runs and then dumps points at the expansion vessel having lost its charge. Topping up will not fix that one.

Symptom: one radiator is cold, the rest are fine

If the boiler is clearly running and most rooms are warm, the boiler is not the problem — delivery is.

• Cold at the top, warm at the bottom. Air trapped in the radiator. Bleed it. If it comes back within a few weeks, air is getting in somewhere and that is worth tracing.
• Cold at the bottom, warm at the top. Sludge and debris settled in the base. Bleeding does nothing here; it needs flushing, or the radiator has to come off.
• Stone cold all over while the others are hot. Usually a seized valve. In houses around here it is often a valve that has not been turned in twenty years.
• Always the same radiator, furthest from the boiler. That is a balancing problem across the system rather than a fault in that one radiator.

Symptom: banging, knocking or kettling

Noise is information, and the kind of noise matters.

• Banging or hammering on a steam system is usually water sitting where steam needs to travel — a failed vent, a radiator pitched the wrong way, or a sagging return. Very common in the older housing in the river villages and in Yonkers, and fixable rather than terminal. What each kind of bang means, and the fix for it, is in our guide to banging pipes.
• Kettling — a rumble like a kettle coming to the boil — is scale or sludge on the heat exchanger making the water boil locally. It shortens the boiler’s life and should not be left.
• Ticking or clicking as it heats and cools is usually pipe expanding against a joist or a clip. Annoying, not dangerous.
• A new noise that arrives with a drop in performance is the one to act on. It is often the first sign of a boiler not working properly.

Symptom: the pilot light keeps going out

On older gas boilers with a standing pilot, a pilot light out is the classic cause of a boiler not working. Relight it once, carefully, following the instructions on the unit itself.

• If it lights and holds, it may simply have been a draft.
• If it lights and dies within seconds, that is usually the thermocouple — the sensor that proves the flame and shuts the gas off when it cannot. It is a routine replacement.
• If it will not light at all, stop. Do not keep trying.
• Never prop, bypass or tape anything to keep a pilot alight. That device exists to stop unburnt gas entering the house.

Symptom: the boiler fires, then locks out

A boiler that starts and shuts down shortly after is protecting itself. Press the reset once. If it locks out again, that is the fault repeating, and repeated resetting is how a contained problem becomes a dangerous one. Common causes are low pressure, a blocked condensate trap on a high-efficiency unit, poor circulation from a failed pump, or a blocked flue.

Stop and call the gas utility if

• You smell gas. Leave the building first, then call from outside. Do not touch switches on the way.
• A carbon monoxide alarm sounds. Out, then emergency services, and do not go back in.
• There is soot or scorching around the boiler or its flue.
• Water is running from the boiler or a relief valve is discharging continuously.

What this is not

• Nothing happens at all — no noise, no fan, no pilot. Start with the thermostat and the service switch instead: that is covered in heat not working at all.
• Radiators hot, faucets cold. The boiler is running and the hot water side is the fault — see no hot water.
• A forced-air furnace. Different machine, different trade, and not ours.

When to call about a boiler not working

Call if the pressure keeps dropping, if it locks out more than once, if the pilot will not hold, if you can hear kettling, or if radiators stay cold after bleeding. That is boiler repair, and cold radiators with a healthy boiler is usually radiator and valve work. A boiler not working overnight in a freeze, with the house dropping, is an emergency plumber — at that point a heating fault is about to become a plumbing one.

KBD Plumbing & Heating covers gas boiler, steam and radiator heating across lower Westchester — Dobbs Ferry, Hastings-on-Hudson, Irvington, Yonkers, White Plains and the rest. Licensed and insured, price agreed before work starts, nights, weekends and holidays included: (914) 233-3000.

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