Emergency
AF on Unical Alkon C 28 HE

Your boiler has detected that waste gases are not escaping correctly through the flue, so it has shut down to prevent them from entering your home.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Act immediately

Turn off your boiler and stop using gas appliances. Call a Gas Safe engineer now. This fault can be dangerous if ignored.

What the manual says

Manufacturer manual

Flue gas anti-overflow thermostat

Community-sourced, unverified
We could not tie this code to a specific manual. Treat it as indicative only.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

Engineer required

This is a emergency severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Find a Gas Safe engineer
Search the official register →
Boiler cover options

Avoid the next call-out bill

A cover plan would handle faults like AF: no call-out fees, no surprise bills.

Approved partner
Domestic Appliance Guard
Unlimited call-outs, 24/7 helpline, no price-hike promise
Get a quote →

Affiliate links. We may earn a commission at no extra cost to you.

Related fault codes

F1View →F10View →

All 47 fault codes on the Alkon C 28 HE

Seeing a different code? Every code this model can display, in one place.

60880910111213141516171819202122232425262728303233384344455355758081BLOCKdbFdFLFRHbHLHtLprbtf

Related guides you might find useful

Banging / kettling / whistling noise

Each noise points to a different fault.

Read →

Boiler won't turn on — 9-step checklist

Work through these before the call-out.

Read →

Radiators cold at the bottom

Sludge build-up — needs flushing, not bleeding.

Read →