Boiler troubleshooting: what to check before you call
Most no-heat boiler faults trace to the thermostat, a breaker, low pressure, or ignition. A few — bleeding a radiator, re-pressurizing the system, thawing a frozen condensate pipe — are homeowner fixes; sealed-system, gas, and circulator faults need a licensed technician.
Which boiler symptom matches yours?
Pick the line that sounds like your boiler. Each row names the symptom, its likeliest cause, and whether it's a homeowner check or a licensed repair — tap through to the fix for that symptom below.
What should the boiler pressure read?
Before anything else, read the pressure gauge on the front of the boiler — low pressure is the single most common no-heat fault, and it's the one thing a homeowner can safely put right. A cold system should sit at 1–1.5 bar; below 1 bar the boiler locks itself out.
How do you work through the common faults?
Take your symptom from the router and work its section below. Each names the DIY check where there is one, and the point where it becomes a licensed-technician job — because gas and sealed-system faults are never a homeowner fix.
No heat or poor heatDIY–PRO
Start simple: confirm the thermostat is in heat mode and set above room temperature, then check for a tripped breaker or blown fuse. Next, look at the pressure gauge — below about 1 bar, the boiler may lock out. A standing pilot that won't stay lit usually means a failing thermocouple; a hot-surface ignitor and the circulator pump are pro-only fixes.
When to call: Pilot, ignitor, gas-valve, and circulator faults need a licensed technician.
Some radiators stay coldDIY: bleed them
If the boiler runs but a few radiators never warm up, they're often air-locked — bleeding the radiator releases the trapped air and restores flow. When whole zones stay cold while others are hot, a faulty zone valve or circulator is the likely cause, and sludge or corrosion in the pipes can block flow too.
When to call: Zone valves, circulators, and a power-flush for sludge are pro jobs.
Low or dropping pressureDIY: re-pressurize once
A boiler needs roughly 1–1.5 bar to run — see the gauge above. If it reads low, top it up using the filling loop, following your boiler's manual. Do it once — if the pressure keeps falling, there's a leak in the system or a failed expansion vessel behind it.
When to call: Repeated pressure loss means a leak or expansion-vessel fault — call a pro.
Frozen condensate pipeDIY: thaw it
On a condensing boiler, the outdoor condensate pipe can ice up in a cold snap, back condensate up into the boiler, and trigger an automatic shutdown. Thaw the pipe with hot — not boiling — water or a heat pack, then reset the boiler per the manual. Boiling water can crack the pipe.
When to call: If it refreezes repeatedly, the pipe may need re-routing or insulating.
Strange noisesPRO look
A rumble or gurgle — kettling — is limescale built up on the heat exchanger, which needs descaling to run efficiently again. Banging or knocking pipes point to trapped water in the lines or a failing circulator pump. These rarely stop the heat, but they're a sign to book service.
When to call: Descaling the heat exchanger and circulator work are pro tasks.
Water leakingCall a pro
Water pooling around the boiler or a radiator is not a DIY job — it may be a faulty pressure-relief valve, a failing pump, or a worn seal. Leaks invite mold and water damage, so shut the system down and call a licensed contractor promptly.
When to call: Any boiler leak is a call-a-pro situation — don't wait.
Checks done, still no heat?
Enter your ZIP — we'll connect you to a licensed local contractor for the diagnosis. The call is free, 24/7.
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When should you stop and call a technician?
If the checks above don't restore heat, it's a service call — a stuck circulator, a failed ignition, a gas-valve fault, or a leak all need a licensed technician. See boiler repair for how the visit works, and if the boiler is old and failing often, weigh boiler replacement. In a no-heat emergency, our emergency line routes after-hours, and the no-heat survival guide covers staying warm while a contractor is on the way. Never DIY a gas or sealed-system fault; if you smell gas, leave and call your gas utility from outside first.
Get a licensed boiler tech on the phone
One call routes you to a licensed local contractor for any boiler problem — 24/7, nationwide. The call is free.
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Frequently asked questions
Why does my boiler keep losing pressure?
A boiler that repeatedly drops below 1 bar has a leak somewhere in the system — a radiator valve, a pipe joint, or the boiler itself — or a failed expansion vessel that can no longer absorb pressure changes. Topping it up is a temporary fix; the underlying leak needs a licensed technician.
Why are some radiators cold when the boiler is working?
The most common cause is trapped air, which you can release by bleeding the radiator. If whole zones stay cold while others heat normally, a faulty zone valve or circulator pump is usually to blame, and sludge buildup in the pipes can block flow — both are pro fixes.
My boiler is on but there's no heat — what's wrong?
Check the thermostat, the breaker, and the pressure gauge first. If those are fine, the fault is likely in the pilot or ignition, the gas valve, or the circulator pump — all of which need a licensed technician. Help is available 24/7 by calling the number on this page.
Is a leaking boiler dangerous?
A water leak itself is mainly a damage-and-mold risk, but it can also signal a pressure or component failure, and any gas smell or signs of combustion trouble are an immediate safety issue. Shut the system down and call a professional; if you smell gas, leave and call your gas utility first.
How do I reset my boiler?
Most boilers have a reset button or lockout indicator; the exact procedure is in your model's manual. Reset once after clearing an obvious cause like a thawed condensate pipe — if it locks out again, stop resetting and call a technician, because repeated lockouts point to a real fault.