No Heat From Your Boiler? 6 Common Causes & Fixes

When the boiler quits, it's usually one of six things

A boiler that won't produce heat is stressful in cold weather, but the cause is often something you can check — or even fix — in a few minutes. Here are the six most common reasons a boiler stops heating, in roughly the order worth checking.

Safety first: if you smell gas, leave the house and call your gas utility's emergency line — don't touch switches. And boilers vent combustion gases, so any soot, scorching, or a carbon-monoxide alarm means shut it down and call a pro immediately.

Quick takeaways:

• Low water pressure is the single most common no-heat cause — and often a DIY fix
• A frozen condensate pipe knocks out high-efficiency boilers in cold snaps
• Press the reset button only once — repeated resets can be dangerous
• Heat but no hot water (or vice versa) usually points to a diverter valve

1. Low boiler pressure

Most modern boilers have a pressure gauge on the front. It should read roughly 1.0–1.5 bar when cold. If the needle has dropped below 1 (often into the red), the boiler may shut down and refuse to fire.

The fix is usually topping up the pressure using the filling loop — the small braided hose or valve(s) under the unit. Open the valve(s) slowly, watch the gauge climb to about 1.2–1.5 bar, then close them. If pressure keeps dropping after you top it up, there's a leak in the system that needs a pro.

2. Thermostat or controls

Same starting point as any no-heat call: confirm the thermostat is set to HEAT, the target is above room temperature, and the batteries are fresh. Check any boiler timer/programmer is set to "on" and hasn't lost its schedule after a power cut. (Our thermostat troubleshooting guide covers this in depth.)

3. The boiler has locked out (reset button)

Many boilers "lock out" after a failed ignition and show a fault light or error code. There's usually a reset button. Press it once and wait for the boiler to attempt a restart.

Important: if it locks out again, don't keep pressing reset. Repeated lockouts mean something is genuinely wrong — ignition, gas supply, or a safety device doing its job — and forcing restarts can be unsafe. Note the error code and call a pro.

4. Frozen condensate pipe (high-efficiency / condensing boilers)

Condensing boilers drain acidic condensate through a pipe that often runs outside. In a hard freeze that pipe can freeze solid, backing up condensate and shutting the boiler down — usually with a specific fault code and sometimes a gurgling sound.

If you can safely reach the external pipe, you can thaw it by pouring warm (not boiling) water over it, or holding a hot-water bottle against it. Once clear, reset the boiler. To prevent repeats, the pipe can be insulated or rerouted.

5. Airlocked or unbalanced radiators

If the boiler runs but some radiators stay cold — especially upstairs — you likely have trapped air. Bleeding the radiators (with a radiator key, boiler off and cool) releases the air so hot water can circulate. Remember to re-check boiler pressure afterward, since bleeding usually drops it and you may need to top up again.

6. Diverter valve or pump (heat or hot water, not both)

On combi boilers, a diverter valve switches flow between heating and hot water. When it sticks, you get one but not the other — common symptoms are "hot water works but no heating," or radiators warm but taps run cold. A failing circulating pump produces similar symptoms (the pump may feel cold or fail to hum). Both are pro repairs.

A note on kettling — a rumbling, boiling-kettle sound — which usually means limescale or sludge buildup on the heat exchanger. It won't fix itself and gets worse; book a service.

What you can safely DIY vs. what needs a pro

Safe to try yourself:

• Topping up pressure via the filling loop
• Checking and resetting the thermostat/timer
• A single reset after a lockout
• Thawing an external condensate pipe with warm water
• Bleeding radiators

Call a licensed pro for:

• Repeated lockouts or recurring fault codes
• Pressure that won't hold (a leak)
• Heat-or-hot-water-only symptoms (diverter valve / pump)
• Kettling, banging, or any burning/soot smell
• Any gas smell or CO alarm — shut down and call immediately

Boilers combine gas, water, and pressure, so anything beyond the basics above is genuinely worth a professional. If your boiler is down and you can't get it back, connect with a licensed boiler pro in your area — and for a cold-weather no-heat emergency, 24/7 emergency dispatch gets someone out fast, with no overtime fees.