We have a Healstead Quatro Gold Gas Combination Boiler with a very confusing problem being hot taps upstairs plus shower ignites the burner when turned on, but the downstairs kitchen mixer tap will not fire the burner, hence no hot water downstairs. Have increased the pressure which will then ignite the burner but it is too powerful to heat sufficiently. Also shower will go cold after about 2/3 minutes, which results in some loud screams. Does anyone have any suggestions - Many thanks