Why is my boiler kettling?

Kettling is one of the most common boiler noises in London homes. It sounds like a kettle coming to the boil: rumbling, popping, sometimes a high whistle. It often gets worse when the boiler is working hardest.

What causes kettling

Inside the boiler, water passes through a heat exchanger where the burner heats it. If water does not flow through fast enough, part of it gets too hot and starts to boil in small pockets. Those bubbles form and collapse, which makes the noise.

The usual reasons water does not flow well are:

Cause	How it happens	Which part
Limescale	Hard water leaves scale, narrowing the water passages	Main heat exchanger on older systems, plate heat exchanger on combis
Sludge	Black iron sludge from radiators settles in the boiler	Main heat exchanger
Pump problem	A weak or failing pump does not push water round fast enough	Whole heating circuit
Low pressure or air	Low system pressure or trapped air in the boiler	Heating circuit
Thermostat or sensor fault	Boiler overheats because it misreads the temperature	Controls

London’s two big causes: scale and sludge

Scale

Thames Water says all the water in its region is hard. On a combi, if the kettling happens mainly when you run hot water, scale in the plate heat exchanger is a likely cause. On older open vented systems, the ones with a tank in the loft, small amounts of fresh water get drawn into the heating over the years and add scale to the main heat exchanger too.

Sludge

Older heating systems with steel radiators produce rust inside, which turns into fine black sludge called magnetite. It settles at the bottom of radiators and gets carried back to the boiler. Radiators that are cold at the bottom and a boiler that kettles often go together.

Streatham and older systems

In Streatham, many Edwardian and interwar houses have heating that has been altered many times: extra radiators for loft rooms, pipes moved for kitchen extensions, old tanks left in lofts. Each change lets in a little more air and fresh water, which speeds up rust and scale. Kettling is a sound we expect to hear in these homes. Our Streatham area page covers the housing.

Safe checks you can do

1. Check the pressure gauge. For many sealed systems, around 1 to 1.5 bar when cold is normal, but check your boiler manual. If it is low, the manual explains how to top up using the filling loop.
2. Bleed the radiators. If radiators are cold at the top, air may be trapped. Bleeding them may help. See our post on bleeding radiators.
3. Notice when it happens. Only when running hot water, only on heating, or both? That helps the engineer.
4. Check radiators. Cold at the bottom suggests sludge.

Do not take the boiler case off or try to descale the boiler yourself. That is gas work and must be done by a Gas Safe registered engineer.

If the boiler is leaking, smells of burning, or you smell gas, turn it off. If you smell gas, call the National Gas Emergency Service on 0800 111 999.

What an engineer does

The engineer will:

• Listen to when the noise happens and check the pressure
• Check the pump is running correctly
• Check flow and return temperatures, which shows if water is moving well
• On combis, test hot water flow and temperature
• Look at the magnetic filter, if fitted, and the state of the system water
• Inspect the heat exchanger

Depending on what they find, the fix might be:

Problem	Typical fix
Scale in combi plate heat exchanger	Clean or replace the plate heat exchanger
Sludge in system	Filter clean, chemical clean or power flush
Pump weak or failed	Repair or replace pump
Low pressure	Find and fix the leak or replace the expansion vessel
Scale in main heat exchanger	Descale the system, sometimes heat exchanger replacement

Everything is priced after the engineer has found the cause.

Can kettling damage the boiler?

Over time, yes. Repeated overheating strains the heat exchanger and pump, and the boiler may lock out on overheat. A heat exchanger replacement is one of the more expensive repairs, so it is worth dealing with kettling early.

Preventing it

• Yearly service. Catches scale and sludge early. A combi boiler service includes a hot water flow check.
• Magnetic filter. Catches sludge before it reaches the boiler. Clean it at every service.
• Inhibitor. A chemical added to the heating water that slows rust. Top it up after any drain down.
• Scale reducer. On the cold feed to a combi. See our post on combi limescale filters.
• Hot water temperature. Thames Water suggests about 60 degrees to reduce scale.

When to book

If kettling has just started, book a service or repair visit soon. If it is very loud, the boiler keeps locking out, or water is leaking, turn the boiler off and book a repair.

Gas work in the UK must be done by a Gas Safe registered engineer. Check the ID card and the number on the Gas Safe Register.