How to Lay Pex Tubes in a Sand Bed for Radiant Heating

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Structure of a high mass sand bed for solar thermal space heatingIf you're installing your solar thermal radiant floor piping in a concrete block, getting the grid layout as precise as possible is very important. With sand as a heat storage medium, you can be more flexible – sand is more forgiving than solid concrete – but you still need to lay the pex tubing as neatly as possible.

The basic sand bed structure is shown in the first diagram on this page. Precisely how you install your bed depends on the climate, the building and your heating needs – it's best to consult an experienced professional who can assess your particular situation and provide reliable advice.

Two options for laying a high mass thermal sand bedBed design is necessarily very flexible, as it must cater to every possible type of foundation, size and heating need. However, the choice comes down to two layout options, shown to the right.

There are two additional parts to the bed which are not shown on those diagrams: radon mitigation systems and vapor barriers. If you're installing radon mitigation, it should go underneath the sand bed; the vapor barrier can be installed either inside or outside the insulation layer, though most installers prefer to put it outside.

Pipe run layouts

You can find standard pipe run layouts for radiant heating systems on our site. The patterns are the same whether you use a concrete slab or sand as a heat storage medium.

Laying the pex tubing

Before you lay any tubing in the sand bed, make sure you leave the tubes to warm up (so they are more flexible). It will help to have two people work on laying the pipe as, no matter how you prepare, pex has a tendency to curl and try to resist lying flat!

Start by filling the bottom of the sand bed to the level where you will lay the pipe, then compact it down and flatten it out. Then lay the pipe from one end of the run to the other. The best way to do this is to have one person lay the tube flat and the other follow along with a shovel, plopping down a pile of sand every few feet to hold the pipe in place – with dry sand, you may need to cover it continuously.

Compact each layer of sand as it is added. You will find that where you lay tube corners, you need more sand to hold them in place, but if it's too difficult you can always lay a piece of rebar down in the bed and use it as an achor, fixing the tubing to it with zip ties.