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Mend Saunier Duval SD 623

Saunier Duval SD 623 keeps switching on and off from cold?

The burners on my SD 623 keeps switching on and off, from cold, until the temp gauge is around 40 degrees. the boiler will then work as normal. I have changed the pcb and both thermistors.
The hot water is fine and the burners always work when hot water is drawn.

Anth Mills
May 2011
Saunier DuvalSaunier Duval SD 623Heating, Home Heating Systems
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