There is an upper limit on how much Pex tubing you can use in a radiant heating installation. If the circuit is too long, the solar fluid will cool off before it reaches the end and you will get uneven heating. For standard-sized Pex tubes (half-inch or 5/8-inch internal diameter), the circuit length should not exceed 300 feet – which is conveniently the size of the coil supplied by many manufacturers.
The best way to balance your solar thermal system so that all radiant floors get an equal amount of heat is to cut the circuits to the same length. If they are all roughly as long as each other, the heat will automatically distribute itself. However, if this is not possible, you can install ball valves to adjust the flow.
The goal of this balancing is to get the temperature of all zones the same as the temperature of the solar fluid as it exits. Shorter runs will use less carried heat, so the solar fluid will be hotter when it exits and the building's heating will be unbalanced.
The distance between the tube runs in each section can be very complex to calculate: it's best to consult a professional with experience, so that you get a system which runs at a comfortable heat. As a rule of thumb, 12-inch spacing between the tubes gives a 1:1 ratio between tube length and floor area in square feet, so your 300-foot tube coil will cover 300 square feet of floor space. With 9-inch spacing, the coil will cover only 222 square feet of floor space.
Don't forget that you will lose a little tubing from each coil in going to and from the manifold area to the heating area and that your maximum run length is still 300 feet. If you need 600 feet of tubing for a given area, you'll have to run two separate pipes to two separate sections in the same space.