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Watts Radiant | Floor Heating & Snow Melting

Knowledge Base

About Radiant Heat

In a resting position the human body will produce about 400 Btu's/hour. This is about 300 Btu's more than we need to survive.

In order to "feel" comfortable, we need to shed these extra Btu's. A heating system is a mechanism is which we control the rate excess heat is lost.

The slower we lose heat, the warmer we feel.

In order to heat a space, something within that space has to be warmer than the desired space temperature (hot to cold).

In a forced air environment, the air coming from the duct is between 120° and 140°, assuming a 72° desired room temperature.

In a radiant floor system the floor temperature is between 72° and 85°, assuming a desired 68° room temperature.

Forced Air System

Heat Transfer

All forms of heating work on three basic modes of heat transfer: Convection, Conduction, Radiant.

Convection Heat Transfer is the most familiar type of heat. All forced-air systems are convective heat transfer systems. This includes hydronic baseboards and fan coils.

Conductive Heat Transfer is energy moving through an object. Place a metal pot on the stove and in a few minutes the handle is hot.

Radiant Heat Transfer is the exchange of energy from a hot source to a cold source. The sun is typically used to illustrate this mode of transfer.

Regardless of the type of heating system used, all follow one basic rule. Hot always moves to cold. Place your hand under a lamp and your hand begins to get warm. This is because the lamp is hotter than your hand and is trying to lose energy to its cooler surroundings.

Floor Covering

Any floor covering can be used with a radiant heat system. The key is to ensure the radiant design uses the correct floor covering. Different floor coverings will have different R-values (their ability to restrict energy transfer). Carpet is more restrictive than tile, but can still be used. The difference is usually a slightly higher supply water temperature.



There are various reports that carpet can not be used over a radiant system. The truth to the matter is that carpet works just fine, if the system is designed with carpet in mind. There is a greater range for error with carpet, depending on the combination of pad and carpet used. An ideal R-value for a carpet and pad is around 2.0 or less.



Wood is what is referred to as being hydroscopic, which means it will act like a sponge. If the wood is installed dry, it will absorb moisture and expand. To help eliminate errors associated with hardwood floor installations:

  1. Use strips, not planks. Ideal width is 3" to 3.5" in width.
  2. Use kiln dried wood.
  3. Make sure the wood is between 7% and 10% moisture content when it is installed.
  4. Make sure the support floor below the wood is no more than 4% higher moisture content than the hardwood.
  5. Quarter sawn wood will respond better than a plain sawn wood.
  6. Try to keep the room's relative humidity between 35% and 50%.



One of the main concerns with regard to tile is cracking. There are three main reasons why tile cracks: deflection, moisture and crack migration from the substrate. Some simple guidelines can be followed to minimize these concerns.

  1. Always install the substrate per TCA (Tile Council of America). This may include a cement backerboard, thick set, double layer of plywood or a thin slab.
  2. Do not run the radiant system until the substrate has cured. A minimum of 7 days is required, 15 days is better and 28 days is ideal.
  3. Install a crack suppressant membrane. This will help retard any crack progress that may originate in the substrate.
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