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while running hot water boiler runs hot & cold for the duration tap is on?

tested gas supply 21 mb flow rate 12 ltr removed heat exchanger dhw clear changed both ch &dhw sensors checked dhw pump & control board by substitution what to look for next ?boiler worcester 35cdi combi about 9 years old
les, March 2009
replace the heat exchanger,clean the system had the same proplem before xmas it is the heat exchanger

stew, March 2009
link Click here to see other fixes for Worcester 35cdi.