The Central Heating (CH) side works perfectly but the Domestic Hot Water (DHW) has developed a problem, thus... Turn on hot tap, boiler lights and after a few moments hot water starts coming through tap. Temperature indicator then starts to drop and water temperature falls from very hot all the way down to cold if hot tap left running. Boiler still working (I can hear the burner) but temperature indicator varying between about 39 and 43 although temp knob turned to max. No fault code indicated but burner keeps turning on and off until hot tap turned off. Can get only a few litres of hot water, probably about 5 or 6 Ltrs before temp drops away. Any ideas? Can I fix this or must I call in a technician?

Rick Chattaway, November 2012

yes blocked dhw heat exchanger when removing it will be blocked u can replace it with a new one but it will simply block again you may need to flush it then add a chemical inhibtor to stop it happening in the future

kash, November 2012

blocked heat exchanger at a guess.

A1, November 2012