I've just installed 2 digital temperature probes on the boiler flow and return pipes, to try and ensure the boiler is condensing, below the magic number( what ever it is, but that's another discussion). When the system start up from cold in the morning the temperature difference between flow and return is quite large, flow is 50C and the return is about 25C, but after the system has been on for a couple of hours the flow temperature drops a little to 47C and the return rises to above 40C sometimes as high as 45C or 46C. I'm happy that the boiler is condensing, as I did a quick calculation on heating the hot water tank from cold and it turned the boiler off after 50 mins and I worked out how much gas it used and it was over 90% efficiency. My question is should the difference between the flow and return be larger? we have heavy cast iron radiators in all the downstairs rooms 3 of the room have 3 around the bay windows. I'm wondering if slowing the pump speed down will allow more heat to dissipate from the rads before returning to the boiler? Also would a pump which automatically varied the speed be a good investment, as its running for a fair length of time each day during the winter months?