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Quick answer
The heat exchanger is the part of the boiler that transfers heat from the burning gas into the water circulating to your radiators and taps. Common problems are blockage from sludge or debris, scale build-up in hard water areas, corrosion and leaks. Signs include kettling or rumbling noises, overheating faults, poor heat output, water leaking from inside the boiler and short-cycling. A blocked heat exchanger can sometimes be cleaned; a leaking main heat exchanger is usually one of the most expensive repairs and often tips the balance towards replacement on an older boiler.
Key takeaways
- A modern combi has a main (primary) heat exchanger and a plate (secondary) heat exchanger for hot water.
- Sludge and scale restrict flow and cause overheating, noise and poor performance.
- Leaks from the main heat exchanger usually mean replacing the part or the boiler.
- Correct inhibitor, a magnetic filter and annual servicing are the best protection.
- Some manufacturers give long warranties on heat exchangers, often conditional on servicing and water treatment.
What a heat exchanger does
In a condensing boiler, hot combustion gases pass through the main heat exchanger, usually aluminium or stainless steel, while system water flows through passages inside it. The gases are cooled enough that water vapour condenses, recovering extra heat. That is why condensing boilers are more efficient than older designs.
In a combi, a second plate heat exchanger transfers heat from the primary circuit to mains water when you run a hot tap.
Signs of a problem
Kettling or rumbling. Water boiling locally inside a restricted heat exchanger makes a kettle-like noise. See boiler kettling noise causes.
Overheat lockouts. Restricted flow means the boiler overheats and shuts down.
Short-cycling. Heat cannot get away fast enough, so the boiler turns on and off repeatedly. See boiler short-cycling explained.
Weak or fluctuating hot water. A scaled plate heat exchanger on a combi can cause lukewarm or fluctuating hot water. See combi hot water fluctuating.
Water inside the boiler. Leaks from the heat exchanger may be visible inside the casing or cause pressure loss. See water leaking from a boiler.
Flue side blockage. On the combustion side, debris or deposits can build up. Our job report on a heat exchanger blocked by bees and wasps shows how unusual it can get.
What causes the damage
- Magnetite sludge from corrosion inside radiators and pipes.
- Scale from hard water, particularly on the hot water side.
- Poor water treatment: no or low inhibitor levels.
- Air in the system, which increases corrosion.
- Missed servicing, which lets combustion-side deposits build up.
Repair, clean or replace
A plate heat exchanger on a combi can usually be replaced as a moderate repair. A blocked main heat exchanger may be cleaned, sometimes alongside a system clean. A leaking or corroded main heat exchanger is usually replaced as a part, which is costly. On a boiler more than ten or so years old, or out of warranty, that cost is often a strong argument for replacement. See boiler repair costs.
Protecting the heat exchanger
- Keep inhibitor at the right concentration. See boiler inhibitor explained.
- Fit and clean a magnetic system filter.
- Consider scale protection in hard water areas.
- Service annually.
Book with Gasmaster
Kettling, overheating or leaking inside the boiler? Gasmaster Plumbing & Heating, Gas Safe registered (220932), diagnoses heat exchanger faults across Stoke-on-Trent, Newcastle-under-Lyme and Biddulph. See our boiler repair service or ask about a new boiler if yours is past it.
Frequently asked questions
Can a heat exchanger be repaired?
Blockages can sometimes be cleared. Leaks and corrosion usually mean replacing the heat exchanger.
Is a leaking heat exchanger dangerous?
A leak on the water side is not a gas leak, but water can damage electrical parts. Any suspected leak should be checked by a Gas Safe engineer.
How long do heat exchangers last?
With good water treatment and servicing, they commonly last the life of the boiler. Poor water quality shortens that considerably.
Sources
- Gas Safe Register (gassaferegister.co.uk)
- BS 7593, treatment of water in domestic hot water central heating systems (bsigroup.com)
- Manufacturer warranty terms (Worcester Bosch, Vaillant, Ideal, Baxi)
Related services
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
