Originally Posted by bourbon
I would think the water circulating would lose heat as it traveled through the piping. Much like a radiator draws heat off water as it passes through. But that's only a guess
Now you are bringing up the whole thermostat thing :-) If the water travels fast enough, the heat will dissipate quicker.
I would go with the turn the pump on during the day, just not all the time.
Of course, I reserve judgment in case an energy engineer comes online with an opinion.