When I set the timer for the central heating to come on (manual clock) it attempts to ignite but keeps tripping out, and when I turn on the hot water on when the heating is not on it trips out,I have to keep pressing the reset button.

I have had several engineers and they have advised me it needs a new heat exchanger unit, the engineers have already changed several components (circuit board etc) but the problem still exists, please advice.

PAUL SMITH, November 2007

I had the same problem, and after (Like you) few engineers and few hundred pounds still no heating and no hot water, I took matters in my own hand.
1) Switch off the boiler from mains.
2) Emptied the boiler by pulling out the pressure pin on the right side of the boiler (make sure the pressure valve is actually fitted to a pipe taking the water out of the house).Keep the pin pulled until the water stops draining.
3) There are two sensors, one on the flow pipe and one on the hot water pipe; they are both the same type of sensors.
Disconnect the wires and take the pin out of the sensor housing and pull out the sensor.
4) Either replace the sensor or if it looks dirty? Clean and put back (I recommend replacing) and reconnect the wires.
5) Fill the boiler to 2 bars and air the system. (Make sure the sensors are placed in their housing correctly and there is no leak).
6) Put the cover back on and turn the electricity on.

It worked for me.
Hope this will help you too.
Cheers

WizzKidd, November 2007