Hot water issue?

Halstead ace boiler c/h ok hot water intermitant. Changed diverter diaphram and dhw thermistor still issues. Could the heat exchanger be scaled up?
simpson, February 2012
had this b4 where worked ok on c/h but not on h/w ended up being the fan venturi partially blocked only highlighted itself on hot water because higher burner pressure in h/w mode

hig, February 2012
Hot water heat exchanger scale up need acid clean

ltg, February 2012
link Click here to see other fixes for Halstead.