IN floor (concrete) radiant heat low voltage pumps

I have an in-floor concrete radiant heat system. One loop is discharging water at 130 degrees from the boiler. The larger loop is discharging at 101 degrees. Is it possible for low voltage cast iron pumps to work sluggisly and thus reduce the discharge temp from the boiler? It just seems that the fuel consumption has gone down and the larger loop is discharging at a lower temp. I am suspecting the large loop pump is sluggish and the other pump is making up for it and runninging more often. I am a green horn and don't have a lot of experience with in-floor heat.

It could be many reasons. The cooler loop could be longer or run closer to the exterior walls, thus, cooling the heated water off longer, or one pump could be partically clogged up with debris, or not running properly. If a pump is running under a larger load, you can often tell either by the fact it may end up hotter, or is making a different sound than the other one. The real test is to monitor the temperature on the start and end (return) ends of the loop when the circulator starts up or the boiler starts heating the water.