How to Calculate Amount of Pex Per Square Foot in Radiant Heat Applications

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Underfloor heating utilizes PEX tubing.
Underfloor heating utilizes PEX tubing. (Image: Floor image by Einar Bog from Fotolia.com)

Cold floors in winter are more than just a chilling thought. They increase heating costs by serving as a drain for the room's heat. Radiant heat applications supply heat directly to the floor of a room, and promote heating from the ground up. PEX, or cross-linked polyethylene tubing, is used to transfer heat to the floor. The amount of PEX used in a radiant heat system depends on the size and insulation of the room, and how the room is to be utilized. Using insufficient PEX may result in inadequately heated rooms and inflated heating costs.

Things You'll Need

  • Measuring tape
  • Calculator

Measure the dimensions of the room in which the PEX tubing is to be installed, and calculate the floor area. For example, a room that is 25 feet wide and 26 feet long will have a floor area of 25 * 26 = 650 square feet.

Multiply the area by 1.5 for 3/8-inch PEX tubing, or by 1.7 for 5/16-inch PEX tubing to obtain the length of tubing required for the room loops. For tubing of other diameters, consult the manufacturer for the multiplication factor. For example, installing 3/8-inch PEX in the 25 * 26 room will require 650 x 1.5 = 975 feet of tubing for the room loops.

Divide the length by the recommended maximum loop length, which is 250 feet for 3/8-inch PEX, 300 feet for 1/2-inch PEX and 400 feet for 5/8-inch PEX. Consult the manufacturer for the maximum recommended loop lengths for other PEX tubing sizes. Round up the quotient to the next whole number to obtain the number of loops that should be installed. Multiply the number of full loops by the recommended length for each loop to obtain the length of tubing needed to construct the heating loops. For example, 975/250 is 3.9, which is rounded to 4 PEX heating loops for a room that is 25 feet wide and 26 feet long. The length of tubing needed for the four loops is 4 * 250 = 1000 feet of PEX.

Calculate the leader length. The leader is the section linking the room loop tubing to the manifold. The leader length = 2 (the horizontal distance from the manifold to the room) + 2 (vertical distance from the floor to the manifold).

Add the leader length and the length required for the room loops to obtain the total length of PEX required to install a PEX heating system.

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