can anyone help with this puzzle i have a ravenheat combi it displays a code no. 28 . After a reset it goes through its normal sequence then after approx 3 secs it locks out displaying the above mentioned code .the first thing i checked was the pressure it was at just over 1/2 a bar so i repressured to 1.5 bar reset and the same thing happened then next i changed the low water pressure switch and guess what no joy, checked both fuses in pcb both fine what else can keep the low pressure switch open when the pump is running can it be the c/h - h/w sensor