Guide: Boilers

Boiler warranty explained: what voids it and what does not

A modern combi boiler can carry a 7 to 10-year manufacturer warranty if it is fitted by an accredited installer. The length is the headline; the conditions in the small print decide whether you can actually claim. A missed annual service, a skipped inhibitor dose or a missing magnetic filter are the most common voids.

A boiler benchmark certificate being completed during installation

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What every major UK warranty requires

  1. An annual service in writing, signed in the boiler manual or on the manufacturer portal. Missing a year voids the rest of the term.
  2. System-protector dosing: the installer must dose the system with an approved inhibitor and log it on the benchmark certificate. Skipping this is the most common void.
  3. A magnetic filter, now required by the major makes to honour the long warranty.
  4. A powerflush on swap: replacing into an existing system requires a powerflush, not just a chemical flush, on most makes.
  5. A completed commissioning (benchmark) certificate at install. No benchmark means no warranty.

The top three real-world voids

  • A missed annual service: a year-two callout, the engineer asks for the boiler manual, no service stamp, warranty closed.
  • No magnetic filter: a brand audit on a later heat-exchanger claim finds no filter, warranty denied.
  • Wrong commissioning: a boiler fitted by a Gas Safe but non-accredited installer, and the long warranty silently reverts to the statutory minimum.

What does not void a warranty

  • Using a different engineer for the service than for the install, as long as both are Gas Safe registered.
  • Selling the property; warranties are transferable on a registration update.
  • Power cuts, RCD trips or external system faults.
  • Re-decorating the boiler casing.

The accredited-installer angle

Manufacturer-accredited installer schemes unlock extended warranties beyond the standard term. The premium on the install over a Gas Safe but non-accredited installer is usually modest, and the extra years of cover are normally worth it on an appliance you keep for a decade.

How to make a claim succeed

Keep the service stamps in the manual, a dated photo of the filter and inhibitor from install day, the benchmark certificate scanned to yourself, and the manufacturer registration completed promptly after install. With those four, parts-and-labour claims run smoothly.

How to choose a vetted trade

  • Use a manufacturer-accredited installer if you want the extended warranty, and confirm the scheme in writing.
  • Check the installer will dose the system with an approved inhibitor, fit a magnetic filter, and complete the benchmark certificate.
  • Get the install evidence (filter, inhibitor, benchmark) photographed and the manufacturer registration done on the day.

Frequently asked questions

What is the most common reason a boiler warranty is refused?

A missed annual service. Every major UK manufacturer requires a documented annual service, and a single missed year typically voids the rest of the term. The next most common are a skipped inhibitor dose and a missing magnetic filter, both checked on a later claim.

Does using a different engineer for servicing void my warranty?

No, as long as the engineer is Gas Safe registered and the service is documented in the boiler manual. You are not tied to the original installer for servicing. What matters is that each annual service is carried out and recorded.

Can I transfer the warranty if I sell the house?

Yes. Major manufacturer warranties are transferable on a registration update, so the cover usually passes to the new owner. Keep the service records and benchmark certificate with the property so the new owner can evidence the history.

Sources

OM

Oliver Mackman

Editor, Sorted Property

Oliver leads Sorted Property's editorial coverage of UK home services. He researches and writes the plain-English guides that help homeowners choose between installers and trades, drawing on the standards set by bodies such as MCS, TrustMark, the Energy Saving Trust and the Property Care Association, and is clear about what to check before any work starts.

Last reviewed: 11 June 2026