BoilerFaultCodesYour boiler has shut down because wind or a blockage is pushing harmful exhaust fumes back down the chimney instead of letting them escape outside.
Turn off your boiler and stop using gas appliances. Call a Gas Safe engineer now — this fault can be dangerous if ignored.
Technical description: If this thermostat should operate and switch off the appliance it is because the flue is subject to downdraught, probably as a result of adverse wind conditions. It is an automatic reset thermostat with a built-in reset delay in excess of 10 minutes.
What causes this fault?
This fault occurs when the safety sensor detects that poisonous combustion gases are failing to rise up the chimney and are instead spilling back into your home. The most common reasons are a physical blockage in the flue, such as a bird's nest, or severe wind conditions pushing the air back down the pipe.
This issue is most frequent during autumn and winter when high winds are more common or when debris like fallen leaves and nests block the chimney.
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 TTB Downdraught Thermostat — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Ideal Mexico Super CF3 40-80 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.