hi all, Its my second boiler in 40years.The first was a potterton c55/16,the current one is a potterton kingfisher 2,gravity hot water,pumped heating.The pump was a grundfos super 4-50 single speed.When I tried to adjust the variable adjustment arm it started to leak,so after 40years I fitted a grundfos 15/60 three speed.It dose not seem to matter what speed I use the rads are all hot and plenty of hot water,so I have left it set on one.Now for the basics.If water travels slowly around the system dose it give out more heat,or should I turn the pump up to push as much water through the boiler as possible. Regards bill.