Potterton Performa 24 Installation And Servicing Instructions page 21

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• Hydraulic differential pressure sensor
This pressure sensor, fitted on the hydraulic assembly, allows the
main burner to light provided the pump head is as required and protects
the flue-water exchanger from possible lacks of water or blockings
of the pump (LED 3 flashes slowly).
• Hydraulic safety valve (heating circuit)
This device is set to 3 bar and is used for the heating circuit.
The safety valve should be connected to a siphoned drain. Use a a means
of draining the heating circuit is strictly prohibited.
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