Our Combi boiler will only 'fire up' when the hot water taps are turned up to virtually full. It's almost that the boiler doesn't detect that it it needs to heat the water unless its a strong flow. The central heating is working perfectly, 'firing up' instantly when the thermostat kicks in. Pressure of the system is in the 'Green Zone' so assume it's not related to that. Any help would be much appreciated.
most likely (although not all boilers use this system) would be a worn diaphragm in the diverter valve. it's just rubber and moves a spindle out to make a microwsitch every time you turn a hot tap on as the mains water forces it open. the diaphragm wears with time and use - can stretch or get holes in it so the water struggles to move it enough
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