The primary flow switch on some boilers when the diaphragm is getting stretched will make for heating but not hot water as the extra resistance to flow in the dhw circuit makes the difference. The way to trick these boilers is to call for heating then turn on a tap, the DV swicthes over without losing the instruction to the burner provided by the primary flow already activated by heating demand.
My diagnosis is new primary diaphragm, possible pump is tired, possible dhw heat exchanger is sludged up, but diaphragm will likely fix it for now.
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