BoilerFaultCodes b9 High
Your boiler has shut down because it is overheating or detecting that water isn't flowing through the system properly, so it is taking a short break to prevent damage.
Call an engineer soon
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Lock-out: appears if the maximum temperature rise is exceeded or if the dT between the supply and return temperature ≥ 45°C or a rate of rise in the flow temperature > 1°C/sec. And also at a boiler-start with no through flow or insufficient water. The boiler will try to restart after 10 minutes.
Typical repair cost £120-250 (including labour)
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 for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like b9 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Remeha Avanta 18V is discontinued — time for a new one?
Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.
Affiliate links — we may earn a commission at no extra cost to you.