The number of loops is largely immaterial to the boiler.
The 'TRV' on the manifold can shut off the flow when the manifold gets up to temp, whatever the number of loops.
I'm sure an external bypass would help, but the facts remain that;
-the system boiler has an internal bypass
-it has apparently worked fine for 5 years
Therefore something has changed recently. It could be due to a number of things, perhaps the pump is not working properly, the internal BV has jammed, the flow temp NTC is defective.
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