How to Lay Pex Tubes in a Concrete Slab for Radiant Heating

You are here

Before you begin laying Pex tubes into your heating area, you must prepare the ground. The area should be flat and level, with the insulation in place and a vapor barrier under the insulation. If you are going to enclose the Pex tubes in a slab, you can cover the insulation with reinforcing wire or rebar.

Regardless of whether you use reinforcing wire or rebar, you can lay it so that you can follow the lines with your Pex tubes for simplicity. Reinforcing wire usually comes in big rolls or rectangles measuring 4 feet by 6 feet, both with 6-inch squares defined within them; rebar can be laid to suit your pipe run.

It is generally a good idea to attach the tubing to the reinforcements. You can do this with nylon zip ties or short pieces of wire every 12-18 inches of tube run. Alternatively, you can use barbed staples which hook into the underlying insulation. Reduce the distance between fixings at bends.

Since the tubes will be empty when the concrete is poured into the heating area, they will try to float to the top. It is best to keep the pipe run as deeply buried as possible, so do not scrimp on fixings (they're cheap).

Use a tube holder to organise and label the pipes coming out of the concrete slabWhere the tubes enter and exit the slab, it is a good idea to place a wooden tube grid. This is a piece of wood with holes drilled in it to keep the tubes neatly arranged. Each hole is at least ¾ inch in diameter and they are around 2 inches apart. Remember to leave plenty of extra tubing to reach the manifold area and for any end trimming that will be needed.

Speaking of entering and exiting the slab, you should protect the Pex tubes at this point. A piece of conduit (¾-inch for ½-inch tubing) works very well for this purpose. If your Pex tubes will be entirely contained inside the slab, use a gently curved conduit elbow, with the bottom end horizontal (like the tubing fixed to the reinforcements) and the top pointing straight up, extending at least a few inches beyond the top of the slab. If your Pex tubing will be in a sand bed underneath the slab, you can use straight conduit lengths of about a foot, which should protect the pipe as it passes through the slab.

Laying the tube

Start with the pipe run furthest from the manifold area, with both ends of the tube sticking through your wooden tube tidier. You should mark the ends (on the tape that seals them shut and on the conduit cover, not on the tubing itself) so that you know which one will go to the hot supply and which to the cold return. This will be very important and you don't want to mix them up!

The hot end of the tube runs along the exterior wall (if there is one), following the layout that suits your situation best, with the cold end running out from the middle.

When you have completed all the individual circuits, you will