Medium Warning
E091 on Vokera Evolve

Your boiler has detected that the main part responsible for heating your water is becoming blocked or dirty and needs a professional service.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description

Clean primary heat exchanger

What causes this fault?

This fault occurs when the boiler detects a build-up of debris or mineral deposits inside the main heat exchanger, which restricts the flow of water. Over time, sludge from the central heating system or limescale from hard water coats the internal surfaces, making the boiler work harder and less efficiently. If left uncleaned, these deposits can prevent the heat from being transferred correctly, eventually leading to a full breakdown.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Find a Gas Safe engineer
Search the official register →

Related fault codes

E020View →

Related guides you might find useful

Banging / kettling / whistling noise

Each noise points to a different fault.

Read →

Radiators cold at the bottom

Sludge build-up — needs flushing, not bleeding.

Read →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →