You don't need a tape measure to know how long your loops are. PEX Tubing is marked every 3 feet, so if you look at the markings going from and coming back to your manifold on the same loop, you can just subtract the lower marked number from the higher and know, within a few feet, the length of your loop. Make sure not to use loops longer than 300 feet in radiant heat applications to avoid heat loss.
PEX Tubing for Radiant Heat can be found at www.supplyhouse.com
