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Underfloor Radiant Heat Systems Instructions

Make a sketch of the floor surface and joists through which piping will be threaded and installed. Identify the manifold location and route to the manifold for each pipe loop.

Measure the length of the floor joist and multiply the joist length by two. This defines the footage of pipe per joist cavity when floor joists are installed on 10" through 18" centers. When floor joists are on 10" through 18" centers, two runs of pipe are installed in each joist space. Three runs of pipe are installed in a joist space when joists are spaced greater than 18" apart.

Calculate the number of joist spaces you can cover with the pipe coil length you are using. For example, if the joist is installed on 18" centers and it is 20 feet long, multiply 20 x 2 to get 40 feet of pipe per cavity. Assuming a 300 foot coil length, 7 joists cavities could be covered. BUT, remember that you need to allow for the length of pipe running from the manifold and back again. In this example and depending on the manifold location perhaps only 6 cavities can be filled.

Pre-drill holes in the floor joists through which pipes will run. Two 1/2" pipes require a 1-1/2" diameter hole, while four 1/2" pipes require a 2" diameter hole. Holes should always be straight and aligned. Holes must be drilled in the center of the floor joist and at least one foot away from the end of the joist support point.

 Step by Step


This process involves a good deal of pipe threading, but it eliminates pipe kinks and reduces stress on the pipe. Two people can work very effectively together with this installation method - one feeding pipe while the other fastens pipe in the joist space.

For joists installed on greater than 18" centers, three runs of pipe are required in each joist space. The pipe handling and installation technique is similar in concept to that described in steps 1 through 8. Create pipe slack and transfer the slack to adjacent bays as before. Note in the following sketch however, that pipe enters the bay at one end of the joist and exists at the opposite end in order to accommodate three runs of pipe.