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Thanks for the info, always good to learn something newHi Craig, Anti thermosiphon loop is an old school technique to prevent gravitational flow of heated water from rising.
In the case of Intergas boilers, they have a dual back to back hex for heating and hot water and no divertor valve (I think ferolli have similar in their Modena) during hw call, the pump is not powered but water in the heating circuit can thermosiphon through to rads if not checked. Check valves can be used to prevent this in upwards flowing pipework but sending flow pipe downwards for a short distance then looping back upwards also prevents the thermosiphon effect since hot water will not happily follow a downward path. It works and is failure free, no moving parts, the only thing to remember is to include a drain off. It's also used in solar collectors.