thay have a min and max gas rate setting, sounds as if its not opening up to full rate, usual burn would be,
hot water - come on full gas within sec and stay high and modulate when temp reached.
ch - come on full and depending on sys temp drop to a lower rate pretty quickly.
if its just a tiny flame from start on hot water demand ie no high fire at all id be thinking gas valve/modulating fault.
maybe temp sensor but id expect it to fire on high and trip out if sensor fault.
ste, December 2009
