Technical description
Air transport. a. Fouling/clogging of the air supply, flue gas discharge or heat exchanger; b. Differential air pressure switch does not function: switch defective, gas/air connections loose/clogged/mixed up, or spring loaded damper does not open.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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