EGEE 102
Energy Conservation and Environmental Protection

Radiant Heating Systems - Floors

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Types of Radiant Floor Heat

There are three types of radiant floor heat:

  • Radiant air floors (air is the heat-carrying medium)
  • Electric radiant floors
  • Hot water (hydronic) radiant floors.

Instructions: Compare conventional baseboard heating to radiant floor heat, by clicking on the button below.

Baseboard Versus Radiant Heat Systems

Click here to open a text description of the baseboard versus radiant heat activity

Baseboard Versus Radiant Heat Systems

In a baseboard heat system, heat spreads from the baseboard throughout the room. As you move further away from the heat source, the air becomes cooler. In a radiant heat system, the floor heats the air so that it rises throughout the room and distributes the heat evenly.

Finish: Comparison of Home Heating Systems

Types of Installation

All three types of radiant floor heat (air, electric, hot water) can be further subdivided by the type of installation:

  • Those that make use of the large thermal mass of a concrete slab floor or lightweight concrete over a wooden subfloor (these are called “wet installations”)
    Image of a concrete slab above the radiant water tubing.
    Concrete Radiant Floor Heat
  • Those in which the installer “sandwiches” the radiant floor tubing between two layers of plywood or attaches the tubing under the finished floor or subfloor (“dry installations”)
    Image of a plywood slab above the radiant water tubing.
    Wood Radiant Floor Heat

Cost Effectiveness

Radiant Air Floors

Because air cannot hold large amounts of heat, radiant air floors are not cost-effective in residential applications and are seldom installed.

Electric radiant floors

Electric radiant floors are usually only cost-effective if your electric utility company offers time-of-use rates. Time-of-use rates allow you to “charge” the concrete floor with heat during off-peak hours (approximately 9 p.m. to 6 a.m.). If the floor's thermal mass is large enough, the heat stored in it will keep the house comfortable for eight to ten hours without any further electrical input. This practice saves a considerable number of energy dollars compared to heating at peak electric rates during the day.

Instructions: Press the play button to observe how a concrete floor is charged during time-of-use rates.

The animation is described in the text above.
How a concrete floor is charged during time-of-use rates

Hydronic systems

Hydronic (liquid) systems, popular and cost-effective systems for heating-dominated climates, have been in extensive use in Europe for decades.

Hydronic radiant floor systems pump heated water from a boiler through tubing laid in a pattern underneath the floor. The temperature in each room is controlled by regulating the flow of hot water through each tubing loop via a system of zoning valves or pumps and thermostats.

Instructions: Press the play button to see how a hydronic radiant floor system works.

The animation is described in the text above.
Hydronic radiant floor system

Installation

Wet installations are the oldest form of modern radiant floor systems. In a wet installation, the tubing is embedded in the concrete foundation slab, or in a lightweight concrete slab on top of a subfloor, or over a previously poured slab.

Diagrams of wet installations on top of both a concrete foundation and a subfloor.
Wet Installations: Slab on Grade and Thin Slab Frame Floor

A new generation of in-floor hydronic heating that employs corrosion-proof, hot-water tubing has enjoyed widespread popularity in recent years. With this type of system, heat is evenly distributed and floors are warm under foot. A variety of heating equipment may heat water: natural gas or propane water heater or boiler, electric boiler, wood boiler, heat pump, solar collector, or even geothermal energy.

Tubing for a hydronic system may be installed in a conventional concrete slab or in a lightweight, gypsum-cement slab. It can also be stapled to the undersides of subflooring as shown in the image below:

Diagrams show "staple up" tubing in which tubes are suspended underneath the flooring and "sandwich over frame floor" tubing in which tubes are placed between flooring and subflooring.
Staple Up and Sandwich Over Frame Floor Installations

A new generation of hydronic heating: This photograph depicts corrosion-proof, hot-water tubing stapled to the underside of subflooring.

Corrosion-proof, hot-water tubing stapled to the underside of subflooring
Corrosion-free Hot-water Tubing

Radiant Floor Coverings

Although ceramic tile is the most common floor covering for radiant floor heating, almost any floor covering can be used. However, some perform better than others. Common floor coverings like vinyl and linoleum sheet goods, carpeting, wood, or bare concrete are often specified.

  • Carpeting
    It is wise to always remember that anything that can insulate the floor also reduces or slows the heat entering the space from the floor system, which in turn increases fuel consumption. If carpeting is required, a thin carpet with dense padding is preferred. If some rooms, but not all, will have a floor covering, then those rooms should have a separate tubing loop to make the system heat these spaces more efficiently, because the water flowing under the covered floor will need to be hotter to compensate for the floor covering.
  • Wood Flooring
    Most radiant floor references also recommend using laminated wood flooring instead of solid wood, thus reducing the possibility of the wood shrinking and cracking from the drying effects of the heat. While solid wood flooring can be used, the installer is strongly advised to be very familiar with radiant floor systems before attempting to install natural wood flooring over a radiant floor system. Most manufacturers and manuals relating to radiant floors offer guidelines to help you resolve these issues.

Instructions: Dr. P. is doing some remodeling and needs to purchase new flooring. Assuming his home uses radiant heat, help him select the most appropriate flooring option.

Flooring Exercise

Click here to open a text description of the Dr. P Flooring activity

Flooring Exercise

Dr. P. is doing some remodeling and needs to purchase new flooring. Assuming his home uses radiant heat, help him select the most appropriate flooring option.

Questions:

1) Which type of flooring is the least effective for his home heating system?

a. Ceramic tile

b. Vinyl

c. Linoleum

d. Solid wood

2) What type of carpeting should be used in his family room?

a. Thick carpeting with minimal insulation

b. Thick carpeting with dense insulation

c. Thin carpeting with minimal insulation

d. Thin carpeting with dense insulation

3) What type of wood flooring would be best for his kitchen?

a. Solid wood

b. Laminated wood

c. None- you cannot use wood because it will crack

Answers:

1) D: Solid wood

2) D: Thin carpeting with dense insulation.

3) B: Laminated wood

Radiant Floor Tubing

There are various types of tubing used in Radiant floor heating systems.

  • Copper or Steel Tubing - Older radiant floor systems used either copper or steel tubing embedded in the concrete floors. Unless the builder coated the tubing with a protective compound, a chemical reaction between the metal and the concrete often led to corrosion of the tubing, and to eventual leaks.
  • PEX or Rubber Tubing - Major manufacturers of hydronic radiant floor systems now use cross-linked polyethylene (PEX) or rubber tubing with an oxygen diffusion barrier. These materials have proven themselves to be more reliable than the older choices in tubing. Fluid additives also help protect the system from corrosion.
  • Defective Tubing - There have been recent reports of problems with rubber tubing produced by one chemical manufacturer. Leaks develop at the metal connections or fittings, and, in some cases, the tubing becomes rigid and brittle. It is still not clear what causes this problem, but, theoretically, excessively high water temperatures may be to blame. Tightening the connections and clamps only temporarily fixes the leaks. Remember, this problem only concerns a specific brand of rubber tubing; it does not have anything to do with the PEX tubing, which has performed very reliably for many decades. Since the price of copper tubing is considerably lower now than several years ago, it is again gaining some popularity because of its superior heat transfer abilities over plastic-based tubing.

How Radiant Heat Systems are Controlled

Instructions: Move your cursor over the numbers on the image below to find out how radiant heat systems are controlled.

How Radiant Heat Systems are Controlled

Click here to open a text description of how radiant floor systems are controlled

How Radiant Heat Systems are Controlled

A radiant floor that uses a concrete slab will take many hours to heat if it is allowed to become cold. Once cold, the floor will require a waiting period to heat up and warm the interior space. Consequently, most radiant floor systems are not permitted to go into a very deep night setback. Depending on how the floor is constructed, the time it takes to reheat the floor is sometimes longer than the occupant's sleep period.

Many floor systems are controlled by a floor thermostat instead of a wall thermostat. The system is often designed to keep the circulation pump(s) running while the thermostat only controls the boiler's burner. Other, more sophisticated, types of controls sense the floor temperature, outdoor temperature, and room temperature to keep the home comfortable. Such a system may also use less fuel.

Although radiant floor systems are usually heated by a boiler, they can also be heated with a geothermal heat pump. Such a system offers even greater energy savings in climates where the heating and cooling loads are similar in size. Another alternative for those with small houses, or those with small heating loads, is to use an ordinary gas water heater to supply the radiant floor system.

Advantages and Disadvantages of Radiant Floor Heating Systems

Advantages and disadvantages of radiant floor heating systems
Advantages Disadvantages
Radiant floor systems allow even heating throughout the whole floor, not just in localized spots as with wood stoves, hot air systems, and other types of radiators. Does not respond quickly to temperature settings.
The room heats from the bottom up, warming the feet and body first. Relatively expensive to install but can save money in the long run.
Radiant floor heating also eliminates the draft, dust, and allergen problems associated with forced-air heating systems. Requires professional installers.
With radiant floor heating, you may be able to set the thermostat several degrees lower, relative to other types of central heating systems.  
There are no heat registers or radiators to obstruct furniture arrangements and interior design plans.