Hydronics – Installation methods and spacing

There are numerous methods of how to physically install tubing in a radiant slab, and various spacing configurations and tube depths that can be used, and there are reasons and ramifications with every choice made on the jobsite which, for the purposes of the discussion at hand, we will focus on slab on grade applications for a typical warehouse or office.

The general rule of thumb for tubing installation depth is two to three inches below the surface of the slab. I wrote about the effects of various tube depths in the January/February 2012 edition of Mechanical Business (online at www.mechanicalbusiness.com).

Tube depth is always a consequence of the installation method that is being used for the radiant tubing. For example, if the tubing is going to be installed directly onto rigid foam insulation, or an existing surface, the final tube depth would depend on the depth of the overpour slab itself.

Sometimes foam insulation may be located on top of an existing structural slab and then only require a shallow two or three-inch overpour, but if the insulation is to be installed below the structural slab, that slab would typically be between four and six inches thick, if not thicker.

Other methods of installing radiant tubing to insulation include PEX rails, or to tie or clip the tubing to wire mesh that is laid directly on top of rigid insulation. Insulation panels with a knob pattern are also available from numerous manufacturers, allowing the tubing to be simply walked into place and friction fit. These panels typically offer patterns that allow tubing to be spaced in three-inch increments, allowing for six, nine and 12” spacing.

Advantages of installing the radiant tube directly to rigid foam insulation:

Consistent tubing depth.
When using chairs or lifting wire mesh during a pour, the actual depth of the tube may vary depending on if there are sags in the tubing or the mesh in various locations within the slab. When stapled directly to a rigid surface, there is a level of consistency that can be expected.

Added protection from future tube damage.
When installed directly on the foam insulation, there is a level of protection for the tubing during the installation and prior to the pour. The tubing is less likely to get damaged from sharp objects, such as rocks, or metal burs on wire mesh or rebar. Also, by being stapled directly to the insulation, the tube is that much deeper in the slab, and therefore is less likely to get damaged if the slab gets drilled or cut in the future.
Easier and faster installation.
When installing tubing to rigid insulation, installers typically use staple guns that allow then to remain upright, thus minimizing bending which can cause back and knee strain.
Improved radiant performance.
Whenever insulation is included in a radiant installation, the overall performance of the radiant system will be improved, in terms of response time and efficiency. The general rule of thumb is to have a minimum of five times more resistance below the slab than the surface cover that is above the slab, to ensure a good response from the radiant system.

Pick a depth
If the desire is to have the radiant tubing installed at a fixed depth within a thick slab, wire mesh or rebar would be required. This would be set on chairs to maintain the desired depth within the slab.

Wire mesh or rebar is often required within concrete slabs for structural integrity and is used as an anchoring point for connecting the radiant tubing. Connecting tubing to rebar can be accomplished with plastic zip ties or wire ties. This is a labour intensive process, but often the most cost effective.

When tying radiant tubing to rebar, always make sure the tubing does not stay in full horizontal contact with any long sections of rebar. Tubing should always be tied perpendicularly to rebar and be at least an inch from any horizontal runs.

Structural engineers require the concrete to have full contact with all rebar, and will insist on shifting any radiant tubing that has too much horizontal contact. This method of radiant tubing installation is labour intensive as it is, and not a fun job to have to disconnect and repeat.

Spaced out     
In addition to the method of radiant tubing installation and the tubing depth, tube spacing is another key consideration. The tube spacing is often determined based on the desired radiant output desired from the floor.

Radiant tubing installations are typically six, nine or 12”. Wider spacing requires less tubing and less installation labour, but does have a higher operating cost than installations with tighter tubing spacing.

Let’s assume you are building a 10,000 sq. ft. commercial building with radiant heating. The loop layout within that building would likely have 12” spacing in the slab. This design would produce approximately 30 BTUH per sq. ft. of heating at a supply water temperature (SWT) of approximately 41°C.

If that same building was designed with six-inch spacing, the SWT for the heating would become approximately 34°C to deliver the same 30 BTUH per sq. ft.

This does double up on the amount of tubing that would be needed, but it would also have the advantage of being used for radiant cooling, if desired. Chart 1 lists the SWT that would be required at various tube spacings to give a 30 BTUH per sq. ft. heating output.

On the straight and narrow
Regardless of the tubing spacing that is selected for a given job, it’s important to stay as consistent as possible to the original radiant layout and design, to ensure even heat distribution and to get the optimum performance from the radiant system.

Even spacing not only delivers a more uniform distribution of heat, but also minimizes risk if and when a slab is required to be drilled or cut.

Using such products as knob mats, PEX rails or prefabricated roll out PEX mats can help ensure proper tube spacing, while allowing the installation to go quickly. If tying the tubing to rebar or wire mesh is the chosen method, ensure a professional installation that is evenly spaced and at a consistent depth within the slab.

Jerry Leyte, P.Eng., MASc., is the sales manager for Central Canada at Uponor. He can be reached at [email protected]