Listen Closely: What Your Glow-worm Boiler’s Noises Mean This Winter

Your boiler is part of the background noise of a UK home. In November 2026, with the heating coming on more often, that low hum, click or gurgle suddenly becomes noticeable. Sometimes it is just metal expanding or a pump doing its job. Sometimes it is your Glow-worm system asking for attention.

This is not a repair manual, and it is definitely not a reason to take the cover off. Gas appliance work belongs with a Gas Safe registered engineer. What you can do is learn the vocabulary, note when the sound happens and ask better questions before you book a visit.

First, the sounds that are usually normal

Before you panic, rule out the ordinary stuff. A low, steady hum from the pump or fan is common. A soft ticking as the heat exchanger warms up and cools down is also normal, especially on cold mornings. You may hear the fan whir for a few seconds after the boiler fires, and a single click when the thermostat or programmer calls for heat.

Those sounds become more interesting when they are new, suddenly louder, or paired with a fault code, a pressure drop or radiators that will not heat properly. That is when you move from background noise to diagnosis.

The twelve sounds, decoded

1. A gentle, steady hum. This is usually circulation. If it becomes a loud vibration through the floor or pipes, ask about pump speed, trapped air or a failing pump.

2. Soft ticking at start-up or shutdown. Metal expands and contracts. Normal. If the ticking is rapid, electrical-sounding or continues long after the boiler stops, get it checked.

3. A fan whir on start-up. The fan pulls air in and pushes flue gases out. Normal. Grinding, screeching or a fan that runs far longer than usual is not.

4. A single click when the thermostat calls. That is a switch doing its job. Repeated clicking with no heat, or clicking from inside the boiler casing, needs an engineer.

5. Gurgling or bubbling. Often trapped air or low system pressure. Bleed radiators if you know how, check the pressure gauge when the system is cold, and top up only if your boiler instructions allow it.

6. Kettling. This is the kettle-boiling sound. It usually points to limescale on the heat exchanger or local overheating, especially in hard-water areas. It is a service issue, not a DIY flush.

7. Banging or knocking. Can be water hammer, air in the system, a pump set too high or pipes that are not clipped properly. It can also follow a pressure problem. Do not ignore repeated bangs.

8. Whistling. A restricted flow, sludge, a partly closed valve or a pump speed issue can cause this. If it appears only when hot water runs, note that for the installer.

9. Grinding or screeching. This is one of the sounds you should not sit on. It can mean a pump or fan bearing is failing. Turn the heating off if it is severe and book an engineer.

10. Dripping or trickling. Water inside or near the boiler, a condensate pipe issue or a pressure relief valve discharge all need attention. Check for visible leaks, but do not open the casing.

11. Rumbling or clunking. Sludge, a tired pump or an expansion vessel problem can produce this. A service usually finds the cause before it becomes a winter breakdown.

12. Whooshing or rushing. Some flow noise is normal. Persistent whooshing after the boiler stops, or a rush that comes with pressure loss, is worth investigating.

A simple diagnostic vocabulary

When you call an installer, describe the sound in plain terms: is it intermittent or constant, does it happen on heating or hot water, does it start at cold start-up, and does it come with a pressure change or fault code? Use words like kettling, water hammer, trapped air and pressure drop. You do not need to diagnose the fault. You just need to give the engineer a trail.

What you can safely check

Check the pressure gauge when the system is cold. Many sealed systems sit around 1 to 1.5 bar, but your Glow-worm manual is the authority for your model. Bleed radiators if you are confident, check that TRVs are open in the rooms you want warm, and look for visible leaks around radiator valves. Do not repeatedly reset a lockout, do not ignore a smell of gas, and do not remove the boiler cover.

Questions to ask before booking a Glow-worm repair

• Is the visit a diagnostic call-out or a full service, and what is included?
• What is the call-out fee, hourly rate and VAT situation?
• Are you Gas Safe registered and familiar with this Glow-worm model?
• Will you check system pressure, the expansion vessel, pump, heat exchanger and flue as part of the diagnosis?
• Can you quote repair versus replacement before any work starts?
• What should I do in the meantime if the boiler locks out again?

When not to wait

If you smell gas, hear a very loud bang, see flames or black marks, or your carbon monoxide alarm sounds, stop using the appliance and follow the emergency advice for your area. In the UK, the National Gas Emergency Service number is 0800 111 999. For water leaking near electrics, isolate what you safely can and call for help.

Most boiler noises are boringly normal. The useful skill is knowing which ones stop being boring.