If the main burner is running at full pressure & the heat input is correct. (This can be gas rated fron the gas meter) then the chances are that it is because of the temperature of the water comming in fron the underground mains. At this time of year the water temperature can be easily 15degC colder than in the summer. This means you will have to reduce the flow of water for the boiler to give you a greater temperature rise.
So slow the flow of the hot tap down by turning the tap off slightly. This should give you much hotter water.
p holland, December 2010
