How a radiant floor works
In a hydronic radiant floor, warm water circulates through plastic tubing, usually PEX, embedded in a concrete slab, a thin pour or grooved panels under the flooring. The whole floor becomes a low-temperature radiator. Rooms feel comfortable at a lower air temperature, and the low water temperature suits heat pumps and condensing boilers.
Tubing and loops
- Tubing in the floor = heated area ÷ tube spacing. At 9-inch spacing, each square foot needs 12 ÷ 9 = 1.33 ft of tubing.
- Leaders: each loop also runs from the manifold to the room and back.
- Number of loops = floor tubing ÷ (maximum loop length − 2 × leader length), rounded up.
Common maximum loop lengths are about 300 ft for 1/2-inch PEX, 200 ft for 3/8-inch and 400 to 450 ft for 5/8-inch. Longer loops need more pump pressure and deliver uneven heat.
Heat output and flow
- Output needed per square foot = room heat loss ÷ heated area.
- Water flow in gallons per minute = heat in BTU/h ÷ (500 × temperature drop in °F). The factor 500 comes from 8.33 lb per gallon × 60 minutes.
Worked example
A 400 sq ft room with 9-inch spacing, the manifold 15 ft away, 1/2-inch PEX limited to 300 ft per loop, a heat loss of 10,000 BTU/h and a 10 °F drop:
- Tubing in the floor: 400 ÷ 0.75 = 533 ft.
- Loops: 533 ÷ (300 − 30) = 1.97, so 2 loops.
- Total tubing: 533 + 2 × 2 × 15 = 593 ft, or about 297 ft per loop.
- Output needed: 25 BTU/h per sq ft, comfortable for a radiant floor.
- Flow: 10,000 ÷ (500 × 10) = 2 gpm, 1 gpm per loop.
Design limits
- Floor surface temperature: about 85 °F is the usual comfort limit in occupied areas, a bit more in bathrooms and along exterior walls. That caps output at roughly 25 to 30 BTU/h per sq ft. Rooms that need more require better insulation or a supplemental heater.
- Floor coverings: thick carpet and pad add resistance and require warmer water. Tile and stone transfer heat best.
- Insulation under the slab or between joists sends the heat up instead of into the ground.
- Spacing: 6 inches near cold walls and in bathrooms, 9 to 12 inches in well-insulated rooms.
- Balance: keep loops on one manifold close in length, or use balancing valves.
Before you start
Get a room-by-room heat loss calculation, which the heat loss calculator can estimate. Pressure-test the tubing before pouring concrete and keep it under pressure during the pour. Follow the tubing manufacturer’s limits for bend radius, spacing and water temperature.