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Commercial radiant questions?

Here are answers to FAQ about slab-on-grade techniques for a typical warehouse application.

APPLE VALLEY, MINNESOTA — When installing a radiant heating system in a commercial application, there are many important design details to consider, such as the radiant area square footage, type of floor covering, insulation levels, heating source, zoning requirements and manifold locations. All these aspects contribute to the performance as well as the cost of the radiant system.

There are numerous tubing installation methods, as well as various spacing configurations and tube depths, to consider when designing and installing a commercial radiant heating system. For this article, I’d like to concentrate 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. Tube depth is always a consequence of the installation method. For example, if installing the tubing directly onto rigid foam insulation or an existing surface, the final tube depth would depend on the depth of the overpour slab itself.

Sometimes the install includes foam insulation on top of an existing structural slab, and then only requires a shallow two- to three-inch overpour. But if installing the insulation below the structural slab, that slab would typically be between four to six inches thick, if not thicker.

Some of the advantages of installing radiant tubing directly to rigid foam insulation include:

·      Guaranteed tube depth within the slab: When using chairs or lifting wire mesh during a pour, the actual depth of the tube may vary, depending if there are sags in the tubing or the mesh in various locations within the slab. Stapling the tubing directly to a rigid surface, however, offers a level of consistency.

·      Added protection from tubing damage: When installing tubing directly on the foam insulation, the tubing is less likely to get damaged from sharp objects, such as rocks or metal burs on the wire mesh or the rebar. Also, by stapling directly to the insulation, the tube is that much deeper in the slab and therefore less likely to get damaged if the slab is drilled or cut in the future.

·      Easier, faster installations: When installing tubing to rigid insulation, installers typically use staple guns that allow them to remain upright, thus minimizing bending and reducing back and knee strain.

·      Improved radiant performance: Adding insulation will always improve the overall performance of a radiant system in terms of response time and efficiency. A general rule of thumb, to ensure a good response from the radiant system, is to have a minimum five times more resistance below the slab than the surface cover that is above.

Other methods of installing radiant tubing to insulation include the use of PEX rails or tying/clipping the tubing to wire mesh laid directly on top of rigid insulation. Knobbed insulation panels are also available that allow the tubing to be simply “walked” into place and friction fit. These panels typically offer patterns that allow tubing spacing in three-inch increments, allowing for six-inch, nine-inch and 12-inch spacing.

It’s important to stay as consistent as possible with the original radiant layout and design to ensure even heat distribution and optimal performance.

If the desire is to have the radiant tubing installed at a fixed depth within a thick slab, wire mesh or rebar set on chairs would 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, using plastic zip ties or wire ties. While this is a labor-intensive process, it is 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. Always tie tubing perpendicular to rebar and ensure it is at least an inch from any horizontal runs. (It is important to note that structural engineers require concrete to have full contact with all rebar and will insist on shifting any radiant tubing that has too much horizontal contact.)

Tube Spacing

In addition to radiant tubing installation methods and tubing depths, tube spacing is another key consideration. Tube spacing is typically six, nine or 12-inches on center, depending on the desired radiant output. While wider spacing requires less tubing and less installation labor, it equates to a higher operating cost than installations with tighter tubing spacing.

For example, a 10,000-sq.ft. commercial building with 12-inch spacing in the slab would produce approximately 30 Btuh per square foot of heating at a supply water temperature (SWT) of approximately 106°F. If that same building were designed with six-inch spacing, the SWT for the heating would be 94°F to deliver the same 30 Btuh/ft2.

The table below lists the SWT required at various tube spacings for a 30 Btuh per square foot heating output.

 

Regardless of the tubing spacing, it’s important to stay as consistent as possible with the original radiant layout and design to ensure even heat distribution and optimal performance. Evenly spacing the tubing not only delivers a more uniform distribution of heat, but it also minimizes the risk of accidental tubing puncture if a slab needs to be drilled or cut. Using products such as knobbed mats, PEX rails or prefabricated rollout PEX mats helps ensure proper tube spacing, while allowing faster installation.

The next time you’re planning a radiant heating project, take care to consider the radiant tubing installation method, tubing depth and spacing. Then incorporate best practices to ensure the system you install delivers the best performance for the building and its occupants.

Jerry Leyte, MASc., P.Eng., is a commercial sales engineer for Uponor. He can be reached at [email protected].

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