For many years the plumbers have believed that you could not install loops longer than 200 Ft. or 250 Ft. or 300 Ft. this is based on the information they have received from the wholesaler and comes from Wirsbo^®  who designed tubing lengths around the cast iron boilers that are commonly used by plumbers. The cast iron boiler manufacturers ship the least expensive circulating pumps they can  with their boilers. These pumps are normally a cast iron 007 Taco with less than 10 Ft of Head. 10 Ft of Head will only move water through about 250 Ft of 1/2" Pex, any more and the water moves so slow that it gets to cold before it returns. The difference in water temperature between the going out and the return is called the Delta-T or differential. By increasing the size of the pump you can easily lengthen the loops and still maintain a low Delta-T.  The problem is that you do not want the water to get cold before it returns. If the water gets to cold before it returns the output will be uneven and you will not get enough heat to offset the heat loss of the structure.   

    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.   
    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.