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Calcimator

Radiant Floor Design Calculator

Design hydronic radiant floor heating systems including tube length, flow rate, pump head, and boiler sizing.

Inputs

sq ft
BTU/hr
°F

ASHRAE HVAC Systems: slab radiant 100–130°F SWT; staple-up 120–140°F; use mixing valve for ODR

°F
inches
inches

Results

Total Tube Length

1,333 ft

≈ 4 football fields

Required Boiler Output

34,500 BTU/hr

Water Flow Rate3 GPM
Pump Head Required141.8 ft
System Water Volume13.6 gallons
How to Use This Calculator
  1. Enter Floor Area, Heat Loss, and Supply Water Temp.
  2. Set Return Water Temp, Tube Spacing, and Tube Diameter.
  3. Review Total Tube Length (ft) and Required Boiler Output (BTU/hr).
  4. Use Water Flow Rate (GPM) and Pump Head Required (ft) to inform your decision.

How the result changes with Tube Spacing

Tube SpacingTotal Tube LengthRequired Boiler Output
5.42,222 ft34,500 BTU/hr
8.91,348 ft34,500 BTU/hr
13923 ft34,500 BTU/hr
17706 ft34,500 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

  1. Identify Input Parameters
    4 parameters
    Floor Area = 1000, Heat Loss = 30000, Supply Water Temp = 120, Return Water Temp = 100 = 6 input(s) provided
  2. Calculate Total Tube Length
    Total Tube Length
    1333 = 1333
  3. Calculate Required Boiler Output
    Required Boiler Output
    34500 = 34500
  4. Calculate Water Flow Rate
    Water Flow Rate
    3 = 3
  5. Calculate Pump Head Required
    Pump Head Required
    141.8 = 141.8

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators.

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