A faulty boiler heat exchanger usually shows up as kettling or banging noises, the boiler overheating and locking out, weak heating or hot water, or water leaking from inside the case. It is not a DIY repair: the heat exchanger sits inside the sealed combustion part of the boiler, so only a Gas Safe registered engineer should diagnose, clean or replace it, and if you ever suspect fumes or carbon monoxide, turn the boiler off and call the National Gas Emergency Service on 0800 111 999.

What a boiler heat exchanger does

Inside every gas boiler, the burner heats a coiled metal passage full of water: the primary heat exchanger. Hot combustion gases flow around the outside, the water inside picks up the heat, and the pump sends it round your radiators. Condensing boilers use a larger heat exchanger to pull extra heat out of the flue gases, which is why they are more efficient than older models.

A combi boiler has a second one as well: the secondary or plate heat exchanger. When you turn on a hot tap, the boiler diverts its hot heating water through this plate, which heats the cold mains water passing through the other side on its way to your tap. The two waters never mix.

That split matters when you are trying to work out what has gone wrong:

• If the heating and the hot water are both poor, or the boiler is noisy and overheating, the problem is usually the primary heat exchanger or the water flowing through it.
• If the heating is fine but the hot water is weak, fluctuating or slow to get hot, the plate heat exchanger is a common suspect. Our guide to no hot water but the heating works covers that symptom in detail.

Signs of a faulty heat exchanger

What you notice	What it usually points to
Kettling: rumbling, popping or a kettle-like noise	Water boiling in a restricted heat exchanger, often from limescale or sludge
Boiler overheats and locks out with a fault code	Poor water flow through the heat exchanger, or a pump or sensor fault
Radiators slow to heat, or cold at the bottom	Sludge in the system, which also collects in the heat exchanger
Hot water goes hot, then lukewarm, then hot (combi)	Scale or debris in the plate heat exchanger, or a diverter valve fault
Water dripping inside the boiler, or pressure keeps dropping	A leak from the heat exchanger or its seals, or another internal part
Boiler cycles on and off constantly	Overheating from restricted flow, among other causes
Soot, staining or a smell of fumes around the boiler	A combustion problem. Treat this as urgent and turn the boiler off

None of these signs proves the heat exchanger is at fault on its own. Kettling can also come from a failing pump, which is why the engineer tests rather than guesses. If noise is your main symptom, our guide to a boiler making a banging noise goes through the other possibilities.

If you would rather hand the whole thing over, our boiler repair service covers all the major brands, and the phone is answered 24 hours a day, 7 days a week.

Check these first

These checks are safe for a homeowner and none of them involve opening the boiler.

1. Smell gas or suspect fumes? Stop and follow the emergency advice in the box below. Do not try to get the boiler going.
2. Look at the pressure gauge. Most sealed systems want about 1 to 1.5 bar when cold. Low pressure reduces flow through the heat exchanger and can cause overheating. Top it up with the filling loop if your user guide allows it.
3. Bleed the radiators. Air in the system restricts circulation. If you get a lot of air, or the water that comes out is black, that suggests sludge.
4. Feel the radiators. Warm at the top and cold at the bottom usually means sludge is building up. That same sludge ends up in the heat exchanger.
5. Check your carbon monoxide alarm is working. If it sounds, get everyone outside and call 0800 111 999.
6. Reset once. If the boiler has locked out, press reset as your user guide describes. If it locks out again, stop and write down the fault code.

Common causes

Cause	What you might notice	Can you fix it yourself?	What the engineer does
Sludge (magnetite) from corroding radiators and pipes	Cold spots on radiators, dark water when bleeding, kettling	No	Cleans the system chemically or by power flushing, fits a magnetic filter, doses inhibitor
Limescale from hard water	Kettling, hot water that fluctuates on a combi	No	Descales or replaces the plate heat exchanger, recommends scale protection
Low flow from a failing pump	Overheat lockouts, noise, heating slow to warm	No	Tests the pump and replaces it if needed
Low system pressure	Pressure gauge under 1 bar, lockouts	Yes, top up if your user guide allows	Finds out why the pressure is dropping
Corrosion from missing or diluted inhibitor	Leaks inside the case, pressure loss over time	No	Tests the water, repairs or replaces parts, treats the system
Age and repeated overheating	A mix of the above on an older boiler	No	Advises honestly on repair against replacement

Across most of Hertfordshire the water is hard, so limescale in combi plate heat exchangers is common. Sludge is the other big one, especially on older systems that have never had a filter fitted.

What an engineer will check

A Gas Safe engineer will normally:

• Check the system pressure, the pump and whether water is actually moving through the boiler.
• Compare the flow and return temperatures. A big difference, or a boiler that hits its limit very quickly, points to restricted flow.
• Test the water for sludge and inhibitor level.
• Run the hot water and heating separately to tell a primary fault from a plate fault.
• Inspect inside the case for leaks, corrosion and staining around the heat exchanger and its seals.
• Check combustion with a flue gas analyser and make sure the flue is sound.

Repair or replace?

This is the question that matters most with heat exchanger faults, because the answer depends on which heat exchanger it is and what state it is in.

• A blocked primary heat exchanger can often be cleared by cleaning the whole system. There is no point cleaning the boiler alone if the radiators are still full of sludge. Our power flushing service explains how that works.
• A scaled or blocked plate heat exchanger on a combi is a common, replaceable part. It is a standard repair on most boilers.
• A leaking, cracked or corroded primary heat exchanger is one of the most expensive parts in a boiler. On a younger boiler still under the manufacturer’s warranty it may be covered, so check your paperwork. On an older boiler, a new heat exchanger can cost a large share of a new boiler, and the rest of the boiler is still old.

We will tell you plainly which side of that line your boiler is on. Our repair or replace guide sets out how we make that call.

How to protect your heat exchanger

Most heat exchanger failures are preventable.

• Have the boiler serviced every year, so early signs of sludge, scale or leaks are caught.
• Keep the system water treated with inhibitor. BS 7593 is the code of practice for water treatment in domestic heating systems.
• Fit a magnetic system filter to catch sludge. Every new boiler package we install includes one.
• In hard water areas, ask about scale protection on the cold mains feeding a combi.
• Get a boiler that keeps losing pressure looked at rather than topping it up week after week.

What it costs with Vantage

In normal working hours there is no call-out fee. Gas faults start with a fixed-price Gas Safe diagnostic visit, and if we can fix it on the same visit, part of that diagnostic charge comes off the repair. After the first hour, time is charged in half-hour steps, and parts are quoted before we fit them. Evening, weekend and overnight visits are charged at the out-of-hours rates. Every rate is published on our prices page, so you know what you will pay before we arrive.

If the honest answer is a new boiler rather than a new heat exchanger, installations are quoted as a fixed price after a free survey, so the price you agree is the price you pay.

Frequently asked questions

Can a boiler heat exchanger be repaired?

A blocked heat exchanger can often be cleaned, either as part of a full system clean or by descaling. A heat exchanger that is leaking, cracked or badly corroded is replaced rather than repaired. On a combi, the plate heat exchanger is a routine replacement part.

Is a faulty heat exchanger dangerous?

A blocked heat exchanger mainly causes noise, overheating and lockouts, and the boiler’s safety controls are designed to shut it down. Damage on the combustion side of any gas boiler, including the heat exchanger, its seals or the flue, can let fumes escape, so a leaking or corroded heat exchanger should be looked at promptly. Always have a working carbon monoxide alarm near the boiler.

How do I know if my heat exchanger is blocked?

The usual clues are kettling noises, the boiler overheating and locking out, radiators that are cold at the bottom and black water when you bleed them. On a combi, hot water that keeps going hot and cold points to the plate heat exchanger. An engineer confirms it by checking flow, temperatures and the water itself.

Is a heat exchanger covered by the boiler warranty?

Usually, if the boiler is still inside its manufacturer warranty and the conditions have been met, such as annual servicing and a registered warranty. Some manufacturers give the heat exchanger its own, longer cover. Warranties typically exclude damage caused by sludge or poor water quality, which is another reason to keep the system clean.

Will a power flush fix a blocked heat exchanger?

It often helps when the blockage is sludge, because it cleans the whole system rather than just the boiler. It will not fix a heat exchanger that is already leaking or corroded through.

When to call an engineer

If your boiler is kettling, overheating, leaking inside the case or locking out after your checks, it needs a Gas Safe engineer to find out whether the heat exchanger is the cause and whether it is worth repairing. We are Gas Safe registered (956889), based in Stevenage, and cover Hertfordshire, Bedfordshire and Cambridgeshire. Book through our boiler repair page or call 01462 506374.