Radiant Floor System
Heating Selection Summary
Our team has selected a hydronic radiant floor heating system to heat our home. Radiant floor heating is a key element for a small home because there is no limit on the placement of furniture since heat comes from the floor instead of a duct or baseboard heater. It is also a silent system, so you will hardly even notice it is there. Radiant floor heating is also easily zoned to heat only select areas of the house that are currently in use for more energy savings. Two additional venues for energy savings are through the use of solar hot water and the integration of thermal mass. Solar hot water collectors are used to heat the water running through floor tubing, essentially a free form of heat. Thermal mass can be used to reduce heat fluctuations occurring from the natural diurnal temperature variation.
How does it work?
Before discussing radiant floor heating it is important to have a basic understanding of the three methods of heat transfer: conduction, convection, and radiation. In conduction, heat energy is transferred from one substance to another when they are in direct contact. When gas or a liquid is heated, hot areas of the material flow and mix with the cool areas. This type of heating is called convection. In radiation a warm or hot object gives off infrared electromagnetic radiation, which can be absorbed in another object, heating it up.
Hydronic radiant floor heating essentially uses all of these heat transfer methods. First the water is pumped from your hot water heater into small flexible tubes embedded in the floor of your home. The most common tubing for this application is PEX tubing. Conduction is used to transfer the heat from the water to the PEX tubing and also transfers heat from the tubing to the rest of the flooring material. This flooring material is insulated at all perimeters except for the face closest to the home. The 2nd law of Thermodynamics tells us that heat must flow from warmer surfaces to colder surfaces. Hence, when the flooring material becomes warmer than the inside air temperature of the house the heat will radiate from the floor into the air space very close to the floor. Once this air space heats up, convection circulates the warm air to the rest of the room. Since the density of warm air is lower than the density of cooler air, the warm air will tend to rise in this convection process.
An Ancient Technology
Radiant technology is actually and old technology; in fact, ancient Romans used this method to heat public baths. In the mid 1960’s and 70’s radiant floor gained popularity. During this time, copper piping was typically used for its heat emitting properties. However, the main problem with copper is corrosion.
Today, polyethelene is used in place of copper tubing, noted highly for its greater flexibility, corrosion resistance and retaining high heat emitting properties. This polyethelene tubing is cross-linked, labeled as PEX tubing in most practices, to enhance the material's heat output per square foot. According to the Radiant Panel Association, cross-linking is: "a three dimensional molecular bond created within the structure of the plastic which dramatically improves a large number of properties such as heat deformation, abrasion, chemical and stress crack resistance. Impact and tensile strength are increased, shrinkage decreased and low temperature properties improved. Cross-linked tubes also have a shape memory which only requires the addition of heat to return it to it's original shape when kinked". If you're interested in a more detailed technical explanation of the various cross-linking processes, try this link:
The Flooring Material
There are a variety of different choices of flooring material you can use when you put in radiant floor heating. These choices include:
- a concrete slab on grade
- a concrete suspended slab
- a staple up system
There is a drastic performance difference between these choices. Concrete slabs, both suspended and on grade, put out more heat than suspended floor systems. (50 BTU vs. 35 BTU per square foot)
Installing radiant tubing within a concrete slab is probably the easiest, most cost effective and highest performance application of the science. The thermal benefits are unsurpassed. The suspended slab incorporates sand, cement, concrete or Gypcrete. Also, you can use 2 x 4 sleepers within the material for additional support. With this method, 2 x 4’s are laid out across the existing floor, 16” on center, to create “sleeper bays”.
Department of Energy Solar Decathlon Website