Free Radiant Floor Heating Tube Sizing Calculator
Determine PEX tubing length, center-to-center spacing, manifold loop count, and water flow rate (GPM) for in-floor hydronic heating.
🔥 Room & Floor Geometry
📊 Tubing Layout & Manifold Circuits
Recommended Tools & Equipment
Tested hardware and components for high reliability
Principles of Hydronic Radiant Floor Design
Hydronic radiant floor heating transforms your entire room floor into a low-temperature, large-area infrared radiator. Because thermal comfort is delivered via radiant energy directly to occupants rather than blowing hot dry air, radiant systems feel much warmer at air temperatures 3°F to 5°F cooler than forced-air systems, cutting boiler fuel consumption significantly.
1. Tubing Spacing Selection
- 6-Inch Spacing (0.50 ft): Required for exterior perimeter bands (first 3-4 feet along cold outside walls or patio sliders), bathrooms (where bare feet desire maximum warmth and floor space is small), and rooms with floor R-values > 1.5.
- 9-Inch Spacing (0.75 ft): The industry sweet spot for standard residential living rooms, bedrooms, and kitchens on modern insulated slabs.
- 12-Inch Spacing (1.00 ft): Suitable for thick commercial slabs, insulated basements, or high-performance Net-Zero buildings with heat loss under 15 BTU/h/sq ft.
2. The 300-Foot Loop Limit Rule
For standard 1/2" PEX tubing, never exceed 300 linear feet per manifold circuit (including the leader runs from the manifold). Exceeding 300 feet creates excessive hydraulic head friction loss, starving water flow and leaving the end of the loop cold. If a room requires 500 feet of tubing, split it into two equal 250-foot loops connected to a balancing manifold.
3. Hydronic Water Flow Formula (GPM)
For a standard 20°F temperature drop ($Delta T = 20$), each 10,000 BTU/hr requires exactly 1.0 GPM of boiler circulator flow.
Frequently Asked Questions
Can I install radiant heating under plush carpet and thick padding?
Yes, but high-pile carpet with dense foam padding acts as an insulator (R-value 2.5 to 3.5). This traps heat in the subfloor, requiring supply water temperatures as high as 140°F and tighter 6-inch tube spacing to push sufficient BTUs into the room.
What is the difference between oxygen-barrier PEX and standard PEX?
Hydronic heating systems must ALWAYS use Oxygen Barrier PEX (such as PEX-a with EVOH layer). Standard white plumbing PEX allows microscopic oxygen molecules to diffuse through the pipe walls into the water, rapidly rusting iron boiler heat exchangers and circulator impellers.
How do I balance loops of different lengths on the manifold?
Water naturally takes the path of least resistance through shorter loops. Modern hydronic manifolds feature integrated visual flow meters with balancing valves on each port. Turn the balancing screw to throttle shorter circuits until every loop receives its target GPM (typically 0.4 to 0.7 GPM).
Is a counterflow spiral layout better than a serpentine pattern?
Yes! A counterflow spiral lays supply and return tubes adjacent to each other across the entire floor, ensuring the average floor temperature is perfectly uniform from wall to wall. A serpentine pattern creates a hot side of the room where water enters and a cold side where it exits.