why My vaillant boiler not making hot water right a way when I open tap ?

Dear sir /madam
in my house I have old vaillant boiler I need to know why every time I open hot tab water I need wait for long time till boiler fire and start making hot water.this boiler working good for radiyator straight a way start fire but when I open hot tab water some times not start or need wait for 5min till boiler start working pipe is next to boiler so is not long way please let me know how can I fix this problem thanks