How to lay out radiant floor tubing
Tubing length for a radiant floor is heated area divided by spacing. At 9 in on center, 300 ft² needs about 400 ft of net tubing, and you add roughly 10% for the leader runs back to the manifold and for installation slack. This tool reports that total, the number of circuits it breaks into, and the length each circuit ends up being.
Tighter spacing raises output and evens out floor surface temperature. Use 6 in on center along exterior walls, glass, and bathrooms, 9 in for typical living areas, and 12 in where the slab is well insulated or the floor is only supplemental heat. Keep circuits short enough that head loss stays manageable: 1/2 in PEX is normally limited to about 300 ft per loop, 3/8 in to roughly 200 ft, and 5/8 in can run 400 ft or more.
Flow follows from output and the supply-to-return temperature drop. This tool uses a 10°F drop and the standard water relation GPM = BTU/hr / (500 x delta T), which gives the basis for balancing manifold valves and picking a circulator. Keep loop lengths close to equal, because water takes the path of least resistance and a short loop will starve a long one on the same manifold. Confirm floor covering R-value, supply water temperature, and circuit layout with your design engineer before the pour.
Frequently Asked Questions
At the same water temperature, yes. Closer spacing raises output per square foot and reduces surface temperature striping, at the cost of more tubing and labor, so it is usually reserved for perimeter and high-loss zones.
Loops on one manifold share the same pressure difference, so the shortest loop takes the most flow and long loops run cool. Either balance the lengths or set each loop with a flow meter on the manifold.