Home Installing PEX Tubing in Concrete Slabs

If you’re planning on pouring a concrete slab and wondering if a radiant heat system is right for you, this article may help to make a decision.

 

The most common type of radiant heated slabs is the slab on grade type. This type is suitable for many types of structures, including residential homes and garages, commercial warehouses and storage facilities, workshops, retail stores, manufacturing facilities and many others.

 

A typical concrete slab is usually 4-6” thick, and is reinforced using rebar or wire mesh. PEX tubing is secured on top of the reinforcement using manufacturer approved fastening method(s), pressure tested, and then the concrete is poured.

 

A concrete slab absorbs the heat from warm water that runs through PEX tubing circuits and releases this heat to the area above. In essence, a concrete slab is somewhat similar to a panel radiator, which accumulates heat and then disperses it into the atmosphere. Appropriate insulation should always be used in radiant heated slabs to minimize heat loss.

 

To make such investment worthwhile, some important steps need to be taken into consideration.

 

A typical PEX tubing installation in a concrete slab includes the following:

  1. A key to doing a good radiant heating job is the PEX tubing layout. It will help you to determine the amount of tubing needed, number of circuits (loops) in the system and a manifold size. A well-scaled PEX tubing layout drawing should contain information about any walls, partitions or other areas of the slab that do not require heating.
  2. If there should be are any plumbing or electrical service lines under the slab, they have to be installed and verified first.
  3. Base of the slab should be well compacted and leveled to assure even thickness and stability of the slab.
  4. Next, the vapor barrier should be installed (a typical vapor barrier is a 6-mil thick polyethylene). Similar to a sponge, concrete absorbs moisture. The purpose of the vapor barrier is to prevent this from happening, as moisture in a slab can damage the flooring materials and create excessive humidity in the building. To assure zero vapor permeability, it should be overlapped over the slab edges. Note, that if using bubble type reflective or blanket type insulation, no vapor barrier is needed, since it’s an insulation and vapor barrier in one.
  5. The following step is to install radiant insulation. There are many types of insulation available, including extruded foam boards (in sheets), reflective bubble insulation (in rolls) and blanket type insulation (in rolls). Installing bubble and blanket type radiant insulation is the easiest and can be done by simply unrolling the insulation over the slab area and taping the seams with moisture-resistant 3-4” wide tape (non-aluminum). Make sure that insulation overlaps the edges of the slab. When installing foam board insulation, a thicker (1.5” or 2”) sheet should be installed along the perimeter, covering about 4ft of space from the slab edge. The rest of the insulation is usually 1-1.5” thick foam boards. Seams are taped with 3-4” wide adhesive tape. Edges of the slab should also be insulated to minimize the heat loss.
  6. To assure stability and long life of the slab, reinforcement needs to be installed. A typical reinforcement is done with welded wire mesh or rebar and the choice usually depends on the size and thickness of the concrete slab. For residential applications, wire mesh is most common and cost effective. Note, that it is recommended to use wire mesh in sheets, rather than rolls, as they are easier to install and provide a more even surface for the PEX tubing. Sheets of welded wire mesh have to be overlapped and tied together. Right before the slab is poured, install spacers (“elevating chairs” or stanchions) to lift the PEX tubing with the reinforcement to half the height of the slab. If spacers alone do not provide necessary elevation, use a non-abrasive handheld hook to pull the reinforcement up during the pour.
  7. Determine the future location of the manifold(s) – refer to the PEX tubing layout. Prepare PEX manifold stations. Most radiant manifolds can easily be secured to a piece of plywood, so having one nearby is a good idea.
  8. Using tubing layout and spray paint, draw PEX tubing circuits on the insulation according to scale. A good idea is to use 2 or more colors of spray paint for different tubing circuits, as it will help to visualize the actual tubing layout. Mark the runs with arrows showing the direction of the water flow. Since all welded wire mesh is usually screened 6”x6”, drawing and PEX tubing installation is made easier, as tubing is normally spaced at 6” OC (On Center) near the edges of the slab and 12” OC in all other areas.
  9. The next step is to install the radiant manifold(s) in designated locations. If radiant manifold is not available, it can be installed later, but having at least a temporary manifold version with test plugs is necessary for pressure testing purposes. Make sure to leave enough PEX tubing to be able to connect to the manifold later.
  10. PEX tubing installation can now begin. First, select the circuit to install and determine an appropriate coil of PEX tubing. A designer usually specifies which coil is used for which circuit. If such information is not available, a rule of thumb is to make sure that the coil length is at least 15-20ft longer than the selected circuit. A good starting point is to connect the tubing to the supply manifold and gradually uncoil and secure it, while following the spray paint marks. It is recommended that at least 2 people do the installation, where one uncoils the PEX tubing and the other one secures it to the reinforcement at every 2-3ft. For a single-person installation, use PEX uncoiler. To secure PEX tubing, use wire ties, zip ties, mesh clips, foam board staples or other manufacturer-approved method. For 90 degree tubing turns, use galvanized steel PEX bend supports of appropriate sizes. Wherever expansion joints or any slab-to-slab tubing transitions are present, PEX tubing has to be sleeved at least 6” on both sides. Sleeves can be larger diameter split plastic pipes and have to fit loosely over the tubing.
  11. Pressure-test the system. Many manufacturers supply a test kit with the radiant manifold for this purpose. According to RPA Standard Guidelines, the pressure test must be at 1.5 times the operating pressure or 100psi – whichever is greater and must last at least 30 minutes. Verify the integrity of the system and repair leaks if present. A good idea is to have a few splicing kits and a PEX tool handy, or a few of the push-fit fittings (such as SharkBite). If making repairs, the fitting should be isolated from the concrete with electrical tape to prevent chemical reaction.
  12. Pour the cement. An overhead pump truck is the best option, as it will minimize the traffic over the installed PEX tubing and the chance of damage. Make sure to keep the system under pressure and monitor it when pouring concrete. If PEX tubing is damaged, test gage will display a pressure drop and a bubble will burst at that place, which makes it easier to identify the location and repair any leaks. The concrete can then be finished in a normal way.

In most cases, since you will only have 1 chance to pour the concrete slab, you will only have 1 chance to put the PEX tubing in it. So even if there are no existing plans for the radiant floor heating system, installing PEX tubing in it may turn out to be a good decision.

 

Here's a list of basic materials you'll need for a concrete slab radiant heating project:

Oxygen Barrier PEX Tubing

Make sure to select a proper tubing size for your project. 1/2" is the most common for residential and light commercial radiant heating. It is flexible enough and has high BTU output values. All PEX tubing sold by PexUniverse.com is manufactured to meet and exceed ASTM F876/F877 industry standards and is approved by all major plumbing and heating codes.

 

PEX Manifolds for Radiant Heating

Manifold is the central distribution station for all of your PEX tubing circuits. The size of the manifold must match the number of circuits in your radiant heating system. All our radiant heat manifolds are sold in pairs (supply and return) and supplied by default with 1/2" PEX compression adapters. Remember that you'll need to test the system before pouring concrete, so getting a pressure test kit is a good idea.

 

PEX Installation Accessories

For 90-degree tubing turns, use metal PEX bend supports of appropriate sizes. To tie tubing to the rebar, use nylon zip ties or metal wire ties. The fastest and easiest way to install PEX tubing in concrete is to use PEX Rails.

Components listed above are a must-have for every concrete slab radiant heating project and are required to complete the initial stage of the installation.

 

Other system components, such as Circulator Pumps, Mixing Valves, Controls, etc. can be installed later, at the final stage of the project.


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