DHW supply from Vaillant turbomax 828e plumbing advice

DHW supply from Vaillant turbomax 828e Hi,

We have a combi boiler supplying dhw and c/h to our 4 bed, 2 bath house. The boiler is in a cupboard on the first floor bathroom and supplies shower, bath and sink.
There is an attic bathroom with bath and sink, and a kitchen sink on the ground floor. Recently the dhw does not fire up if kitchen tap or attic hot taps are turned on. If we
leave one of these taps on and turn on a hot supply in the main bathroom then hot water will be supplied. Turning off one of the main bathroom supplies whilst leaving kitchen or attic taps on and this will turn the boiler burner off.

Any ideas on why this has started happening ? Boiler has behaved very well over the years and only problem was the expansion vessel inside the vaillant failed so my plumber fitted remote expansion vessel with gauge and c/h has been fine. Cold water is pressure at 1.4 bar, won't fill up any more. The water pressure incoming at outside tap is just under 2 bar.

Thanks,

Diddlepara

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