Hydronic radiant floor-heating basics
Eight installation applications.
Radiant floor heating is not a new concept. It’s been around since Roman times and it is the primary method for indoor comfort throughout Europe, where climates can be much colder compared to areas in North America. Radiant has been proven to be the more efficient, economical and comfortable way to condition a space, yet it continues to only be used in about 4% of homes and businesses in the United States.
Radiant systems offer numerous benefits, including greater system efficiencies, better indoor environmental quality and superior human thermal comfort. Radiant floor heating can work under any floor covering: carpet, tile, hardwood, slate, cork, bamboo and even concrete. Best of all, a radiant system is designed in zones, which means each room can have a different thermostat setting, adding to the system’s efficiency.
Several factors go into the design of a radiant floor-heating system, including room size, heat loss, solar gain, loop lengths, number of zones, heat source and pump sizing. However, this article will focus specifically on the various installation methods for a hydronic radiant floor-heating system.
This is one of the more common methods for new construction. The staple-down method incorporates either metal staples for wood subfloors or plastic staples for foam insulation on a concrete slab. The installer simply “fastens” a flexible plastic tubing called PEX to the surface using a special tool. The staples go around the outside of the tubing (not through the tubing) and then adhere to the surface below.
After the tubing is completely fastened to the surface, a lightweight concrete overpour goes over the tubing to create a smooth, solid surface for the flooring.
The tie-down method is used for installations on or below grade. It requires a compacted base material such as dirt, sand or gravel with a wire mesh or rebar grid placed over the ground. Wire ties affix the tubing to the wire mesh or rebar (typically every 3 feet) to ensure the tubing stays in place as the concrete is poured over the tubing and wire-mesh or rebar grid system.
It is important to note that when doing this type of installation there can be under-slab heat loss that can hinder the system’s performance. Under-slab insulation is typically necessary to ensure heat is not lost to the ground.
PEX rails are long pieces of plastic, nylon or metal that feature “channels” to hold the tubing at a consistent spacing distance. They are typically used in structural and non-structural concrete slab applications, and they can be fastened to almost any surface, including wood, foam-board insulation and concrete. PEX rails help to ensure an even spacing for the layout pattern and can add efficiencies to projects compared with the time it takes to staple or tie down the tubing manually.
Aluminum is an excellent conductor of heat, so using aluminum heat-transfer plates is an effective way to install a high-performing radiant floor-heating system when concrete overpour on the surface of the floor is not an option. The plates fasten to the underside of the subfloor between the joists and feature a channel down the middle where the tubing locks into place.
A “stamped” aluminum heat-transfer plate features the tubing channel on the face (or top) of the plate that fastens to the subfloor, so the tubing must be installed first and then the plate is fastened to the underside of the subfloor. An “extruded” aluminum heat-transfer plate has the channel on the underside of the plate, so the plate can first be fastened to the subfloor and then the tubing can be installed.
Note that after installation, it is important to add a minimum R-11 fiberglass insulation into the joist bay snug against the plates to prevent downward heat loss.
Knobbed mats are becoming a more widely used option for radiant floor heating due to the ease of use for tubing installation. The mats come preformed with knobs throughout the surface which makes it easy to “snap” the tubing into place. The installer simply adheres the mat to the concrete slab or wood subfloor and presses the tubing between the knobs.
Typically, an installer can simply “walk” the tubing into the mat by pressing on it with a foot, eliminating the need to bend down to touch the surface. After the tubing is installed, it is then covered with a lightweight concrete overpour to create a smooth, solid surface for the flooring.
With typical thicknesses of 1/2-inch, wood radiant panels are an ideal solution for remodel and retrofit applications. The panels simply fasten to the plywood subfloor and feature a groove down the center for the tubing placement. An aluminum sheet on the bottom of the panel increases the heat transferability.
This method offers several advantages in addition to the minimal increase in floor height, including no moisture from concrete overpours and increased Btu-output potential over joist heating.
This method is most appropriate for “floor conditioning” or “floor warming,” which does not actually provide heat into the space. The clips simply fasten to the underside of the subfloor between the joists and suspend the PEX from the subfloor.
Like aluminum plates, it is important to add fiberglass insulation after the tubing is installed. However, unlike aluminum plates where the insulation must be snug against the plate, in joist-clip installations there must be an inch gap between the PEX tubing and the insulation. This creates an air gap below the subfloor that will enable the heat to better transfer to the floor.
For large commercial applications, some manufacturers offer a custom-designed, prefabricated, pre-pressurized network of tubing all rolled up as a mat that can be quickly and easily unrolled and installed in a fraction of the time it takes for traditional installation methods.
These “rollout mats” greatly increase the efficiency and consistency of an installation and can also eliminate many product costs as well. For example, one manufacturer’s mat claims to reduce install time by up to 60%, manifold ports by up to 80% and wire ties or staples by up to 40%.
Since these mats are custom-made, it’s important to work closely with the manufacturer’s design department to ensure a proper design when ordering the product.
With radiant floor-heating systems offering greater occupant comfort and increased system efficiencies, it makes sense to learn the basics of installation for customers interested in this smart indoor climate solution.