Boiler Making Noise When It Is Not in Use

Normal reasons a boiler makes noise when nothing is on

A modern boiler is never fully asleep while it has power. Several features run on their own, and each has its own sound. Knowing them saves a call-out.

• Preheat or warm start on a combi: the burner fires for a short while every so often to keep the hot water heat exchanger warm, so hot water reaches the tap faster
• Pump anti-seize: if the pump has not run for about 24 hours, many boilers run it for a short spell so it does not stick, which can happen at the same odd hour each night
• Frost protection: when the boiler gets cold, it runs the pump and, if needed, fires at low output to stop the water freezing
• Pump and fan overrun: after the burner stops, the pump and sometimes the fan run on for a few minutes to carry heat away from the heat exchanger
• Pipes ticking and creaking as they cool and shrink against joists and floorboards

How to tell which feature you are hearing

Preheat is the most common cause of a combi firing at random with nobody using water. On many Worcester Bosch Greenstar combis the ECO button on the front turns preheat on and off, and in economy mode the boiler only heats water when you open a tap. Vaillant ecoTEC plus combis have a similar warmstart function that can be switched off at the boiler controls. Baxi, Ideal and others call it preheat or comfort mode. Check your user manual for the exact button. Turning it off costs you a few seconds of waiting at the tap and saves some gas, because preheat fires the burner at intervals all day.

Pump anti-seize sounds like a short hum or whirr with no flame and no heat. Frost protection works the same way on most makes: when the water inside the boiler drops to a few degrees above freezing, the pump runs, and if it keeps cooling the burner fires at low output. It only works while the boiler has power, and the exact temperatures are in your manual. If the noise only happens on cold nights and the boiler is in a garage, loft or outside wall cupboard, frost protection is the likely answer.

Overrun is easy to spot because it only happens straight after the heating or hot water switches off, and it stops within a few minutes. Ticking pipes follow the same pattern, getting quieter as the house cools down.

Do not switch the boiler off at the power to stop these noises in winter. With no power there is no frost protection, and a frozen boiler or pipe costs far more than the gas.

When noise while idle is a real fault

Some noises when the boiler should be doing nothing point to a problem. The most useful test is simple: with every hot tap closed and the heating off at the programmer, does the boiler fire up again and again, several times an hour?

• A combi that keeps firing with preheat off: a hot tap left running slightly, a leaking hot pipe, or a shower or bath mixer letting water through can all make the flow sensor think you want hot water. A slow drip is usually too small to trigger it
• The heating side running when it should not: a faulty programmer or room thermostat, a stuck zone valve or a sticking diverter valve
• A constant hum with no flame: a pump stuck on, or a relay on the control board failing
• Gurgling or dripping: low pressure and air, a frozen condensate pipe (usually heard as the boiler tries to fire, then locks out), or the pressure relief valve passing water outside
• A loud click and then a lockout at random times: a sensor or ignition fault that needs an engineer

What you can safely check

Start with the easy things outside the boiler before booking anyone.

• Go round every hot tap, shower and mixer and make sure nothing is running or trickling. Put your hand under each hot tap spout for a few seconds
• Look along any visible hot pipe runs, and under the boiler, for damp or drips
• Check the pressure gauge. If it is under 1 bar cold, bleed the radiators and then top it up; our boiler pressure too high guide covers the other direction
• Turn the heating off at the programmer and the room thermostat right down. If the boiler still fires, and preheat is off, the controls or a valve are at fault
• Look at the pipe that comes out through the outside wall from the pressure relief valve. Water dripping from it means the pressure is going too high or the valve itself is letting by

Never open the boiler casing to look at the pump, diverter valve or wiring. Anything inside is gas work. If you smell gas, open a window, turn the meter valve off if it is safe to reach, leave switches alone and call 0800 111 999 from outside.

What the engineer does and typical costs

An engineer will first check the settings, because a surprising number of these calls end with preheat switched off and the controls set up properly. If the boiler still fires on its own, they test the flow sensor and diverter valve on a combi, the zone valves and thermostat wiring on a system boiler, and the pump and control board relays.

For a rough idea of typical UK market prices, a diverter valve replacement on a combi is often around £250 to £500 including labour depending on the make, a new circulating pump fitted is commonly £150 to £400, and a replacement room thermostat or programmer is usually cheaper than either. Fixing a tap or mixer that is letting water through may be all it takes. These are market ranges, not our prices.

Our boiler repairs visit starts with the £60 call-out, and the repair price is agreed with you on site before any work starts. Gas work is carried out by Gas Safe registered engineers. If the idle noise has turned into a boiler that keeps cutting out, our guide to a boiler that keeps turning off is the next step.