Boiler Sounds Like an Aeroplane Taking Off

Is a boiler that sounds like a plane dangerous?

The noise on its own is very unlikely to be a danger. Modern boilers have an overheat cut-out and flame supervision that shut the burner down when something goes wrong, and a pressure relief valve that lets water out if the pressure climbs too high. What the roar tells you is that a part is working much harder than it should, and a part working that hard tends to fail soon, often on the coldest night of the year.

The real risk sits with combustion. If the fan that pushes air through the burner and out of the flue is dying, the boiler may not clear its flue gases properly. Most boilers check the fan before they light, either with an air pressure switch on older models or a fan speed signal on newer ones, and they lock out if the fan is not doing its job. That check is good protection, but it is not a reason to ignore warning signs.

• You smell gas, even faintly
• If your boiler has a viewing window, the flame looks lazy, yellow or orange instead of crisp blue
• There are brown or black marks on or around the casing or the flue
• Your carbon monoxide alarm has sounded, or people in the house have headaches, nausea or dizziness that ease when they go out

Any of these: open windows first. For a gas smell, turn the gas off at the meter if it is safe to reach and touch no switches, the boiler’s included. For the other signs, switch the boiler off. Get everyone outside and call the National Gas Emergency Service on 0800 111 999. If anyone feels unwell, seek medical advice. Our page on carbon monoxide signs from a boiler explains what to watch for.

Which part is making the jet-engine noise?

Listen for when the noise starts. The timing points to the part far more reliably than the sound does.

Starts the second the boiler begins its cycle, before the flame lights, and winds down after it stops: that is the fan. A healthy fan gives a steady whoosh. One with dry or worn bearings climbs to a high whine or a rough roar, and some rattle and set the whole casing vibrating as they spin up. If you hear the roar and then the boiler shows an ignition or fan fault, the fan is the prime suspect.

Builds up a minute or two after the flame lights, rises into a rumble and sometimes ends in bangs or a hiss: that is kettling. Limescale or sludge coats the inside of the heat exchanger, water trapped against the hot metal boils into steam, and the bubbles collapse noisily. London has hard water, so scale is a common cause here, especially on older boilers that never had a scale reducer or a magnetic filter.

A constant loud whirr or hum whenever the heating runs, loudest near the pump: that is usually the circulating pump, either set to its highest speed, starved of water because the pressure is low, or churning air.

A roar only at full power, on a boiler that has just been serviced or had a part changed, can mean the gas rate or burner setting is wrong. That is not something to adjust yourself, but it is worth mentioning to whoever did the work.

Safe checks before you call anyone

Everything below is outside the casing. None of it needs a tool beyond a radiator key and a towel.

• Read the pressure gauge with the boiler cold. It should sit between about 1 and 1.5 bar. Low pressure lets the pump draw air and makes kettling worse. If it is low, top it up through the filling loop; our guide to low boiler pressure covers the steps
• Bleed the radiators with the heating off and cool, starting downstairs and finishing upstairs, then recheck the gauge. Air is the easiest cause to rule out
• Check that several radiator valves are open. If every radiator is shut down at once, water has nowhere to go and the pump and heat exchanger both get noisy
• Look at the flue terminal outside. Leaves, a bird nest or a bag caught over it can make the fan labour. Clear only what you can reach safely from the ground, and never push anything into the terminal itself
• Note any fault code on the display and write it down

Never take the front panel off. The fan, gas valve, burner and heat exchanger are all behind it, and any work there is gas work.

Can I keep using the boiler until it is fixed?

If none of the danger signs above are present, the boiler lights, holds pressure and the noise is a roar rather than a grinding or screeching, you can usually keep it running for a day or two while you book a repair. Turn the flow temperature down a notch if you know how, because kettling eases at lower temperatures.

Turn it off and use it no further if the noise is getting worse by the hour, the boiler keeps locking out, you hear metal grinding from the fan, or water is appearing under the boiler. A fan that seizes completely will stop the boiler anyway, and a heat exchanger that kettles hard for weeks can be damaged.

What the engineer does and what it typically costs

An engineer will run the boiler with the case sealed, listen, then isolate it and open it up. On a fan fault they check the bearings by hand, read the fan speed or air pressure switch signal and inspect the flue for blockages. On kettling they check flow and return temperatures, the pump and the pressure, and often find a clogged filter or a plate heat exchanger coated in scale.

Typical UK market prices, parts and labour together, give you a rough idea. A replacement boiler fan often lands around £200 to £400 depending on make. A new circulating pump is commonly £150 to £400 fitted. Clearing kettling can be as simple as a chemical clean and a magnetic filter, while a full power flush on a typical house tends to run from about £300 to £800, depending on the number of radiators. Those are market ranges, not our prices.

Our boiler repairs visit starts with the £60 call-out at any hour, 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 noise started tonight and the house is cold, emergency boiler repair is the quicker route.

Stopping the roar coming back

Many loud boilers are loud because of what is in the water. An annual service, inhibitor at the right strength and a magnetic filter that gets cleaned at each service keep sludge off the heat exchanger and the pump. On hard water, a scale reducer on the cold feed of a combi protects the hot water side too.

If you want to compare the sound you have with other common boiler noises, our guide to a boiler making a loud noise goes through each one, and the banging boiler page covers kettling in more depth.