The chart above shows the relationship between the pump and the available head or pressure. Each foot of tubing exerts a certain amount of friction on the fluid moving through it. 500 Ft. of tubing has twice the friction as 250 Ft. the pump must have enough head or pressure to overcome this. But the faster you try to move the fluid the greater the friction. So you reach a point that the available head can not move the fluid fast enough to keep the fluid hot enough to satisfy the heat loss throughout the loop. I HAVE SEEN SYSTEMS DESIGNED WITH SO MANY PUMPS IN THEM,
If you are designing for 500 Ft. loops and it takes 15 Ft. of Head to get .8 Gal. Per Min. in each loop to maintain a temperature drop of less than 15 degrees. You will see from the chart that the blue pump will not work at all. The yellow pump will handle up to eight 500 Ft. loops and the red pump will handle up to twenty loops. The total amount of head does not matter only the available head above the green line counts.
By adding a pressure regulating bypass valve you can design a system using one pump large enough to handle as many loops as most homes need. The practice of using a separate pump for each thermostat is unnecessary and a waste of energy and resources. Designing a radiant system with short loops requires several manifolds scattered throughout the house, each a source of possible leaks and greatly increases the cost of installation and operation.
YOU COULD HEAT A SMALL HOUSE WITH THE ELECTRICITY THEY USE
Using short loops increases the speed of the water going through the heater this decreases heater input and raises the exhaust. This decreases the dwell time in the heater and lowers the heater efficiency. Today's modern modulating /condensing boilers and water heaters will realize a substantial increase in efficiently with a lower return water temperature and lower flow. A single medium or high head pump is more efficient than several low head pumps.