The 3 way valve sits in the ch position at rest and moves to DHW position when a tap is opened. The valve may be stuck in the DHW position but you should check the Thermisters (temp sensing device) first. If the ch sensor is reading wrong then the boiler will pump but burner may not come on. They work on resistance ie at 20 degrees c they may read 12000 ohms. As the temp increases the resistance decreases. So if you get a reading of 1000 ohms then the boiler thinks the temperature is over 100. Compare the DHW and CH sensors when the boiler is cold.
kenneth
October 2006
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