Radiant Floor Design Calculator
Design hydronic radiant floor heating systems including tube length, flow rate, pump head, and boiler sizing.
About this calculator
This calculator lays out a hydronic radiant floor heating system: how much tubing it needs, how much water must circulate, and what the pump and boiler have to deliver. Total Tube Length is Floor Area divided by Tube Spacing (converted to feet) — tighter loop spacing means more total tubing for the same floor. Water Flow Rate comes from a standard hydronic formula (BTU load divided by 500 times the supply/return temperature drop), so it depends on Heat Loss and the two water temperatures, not on floor area or tube spacing at all. Pump Head Required estimates friction loss along the tubing run, scaled by both total tube length and flow velocity through the tube's inside diameter — smaller tubing at the same flow rate creates more friction and needs a stronger pump.
System Water Volume is the physical volume of water sitting in the tubing loops, based on tube length and diameter. Required Boiler Output simply adds a 15% distribution-loss allowance on top of the raw Heat Loss figure to account for piping and manifold losses between the boiler and the floor. This model assumes a single heating zone with uniform loop lengths; it does not account for multiple manifold zones, individual loop-length balancing, or radiant slab thermal mass and response lag.
Inputs
ASHRAE HVAC Systems: slab radiant 100–130°F SWT; staple-up 120–140°F; use mixing valve for ODR
Results
Total Tube Length
1,333 ft
≈ 4 football fields
Required Boiler Output
34,500 BTU/hr
How to Use This Calculator
- Enter Floor Area, Heat Loss, and Supply Water Temp.
- Set Return Water Temp, Tube Spacing, and Tube Diameter.
- Review Total Tube Length (ft) — driven by Floor Area and Tube Spacing only — and Required Boiler Output (BTU/hr).
- Use Water Flow Rate (GPM), which depends on Heat Loss and the water temperature drop rather than tube spacing, and Pump Head Required (ft) to inform your decision.
How the result changes with Tube Spacing
| Tube Spacing | Total Tube Length | Required Boiler Output |
|---|---|---|
| 4.5 | 2,667 ft | 34,500 BTU/hr |
| 6.75 | 1,778 ft | 34,500 BTU/hr |
| 14 | 857 ft | 34,500 BTU/hr |
| 18 | 667 ft | 34,500 BTU/hr |
What each input means
- Floor Area
- Total floor area to be heated with radiant tubing.
- Heat Loss
- Design heat loss of the space from Manual J or heat loss calculation.
- Supply Water Temp
- Supply water temperature from the boiler or mixing valve per ASHRAE Handbook HVAC Systems. Slab-on-grade: 100–130°F typical; staple-up under floor: 120–140°F; aluminum spreader plates: 100–120°F. Lower SWT = higher efficiency condensing boiler.
- Return Water Temp
- Target return water temperature. 15–30°F drop is typical for radiant systems.
- Tube Spacing
- On-center spacing of tubing loops. 6–9" for high-output, 12" for moderate, 18" for low loads.
- Tube Diameter
- Inside diameter of PEX tubing. Common sizes: 3/8", 1/2", 5/8", 3/4".
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersFloor Area = 1000, Heat Loss = 30000, Supply Water Temp = 120, Return Water Temp = 100 = 6 input(s) provided
- Calculate Total Tube LengthTotal Tube Length1333 = 1333
- Calculate Required Boiler OutputRequired Boiler Output34500 = 34500
- Calculate Water Flow RateWater Flow Rate3 = 3
- Calculate Pump Head RequiredPump Head Required141.8 = 141.8
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why doesn't tube spacing affect the water flow rate?
Water Flow Rate is calculated purely from the heat load (Heat Loss) and the temperature drop between Supply Water Temp and Return Water Temp, using the standard hydronic formula of BTU load divided by 500 times delta-T. Tube Spacing and Floor Area only determine Total Tube Length — a physical layout question — while flow rate is a separate thermal question about how much water needs to move to carry the required heat.
How much more tubing does tighter spacing require?
Total Tube Length is Floor Area divided by Tube Spacing, so it's an inverse relationship — dropping loop spacing from 12" (moderate output) to 6" (high output) roughly doubles the tubing needed for the same floor area. Tighter spacing delivers more heat per square foot, which is why it's specified for high-load spaces, but it comes with proportionally more material and installation labor.
Why does a smaller tube diameter increase pump head?
Pump Head Required estimates friction loss along the tubing based on flow velocity relative to tube diameter — pushing the same GPM of water through a narrower tube forces it to move faster, and friction loss rises steeply (roughly with velocity to the 1.75 power) as tube diameter shrinks. That's why smaller-diameter PEX systems often need a stronger circulator pump even at identical heat loads.
Why is Required Boiler Output higher than my entered Heat Loss?
Required Boiler Output adds a flat 15% allowance on top of Heat Loss to account for distribution losses in the piping and manifolds between the boiler and the radiant floor loops — heat lost in transit that the boiler still has to generate. This is a simplified planning allowance, not a calculation based on your specific pipe insulation or manifold layout.
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