Radiant Heating

radiant hydronic heating a steel re-enforced concrete slab

What is Radiant Heating?

Hydronic radiant heating is an energy-efficient method of home heating in which water, housed in tubing throughout the floor system, is heated and circulated. The water heats the mass of floor, which then radiates warmth into the home. At Greenovision, we typically design homes that combine hydronic radiant heating with passive solar heating for maximum efficiency.

Radiant heating has a number of advantages. It is more efficient than baseboard heating and usually more efficient than forced-air heating because it eliminates duct losses. People with allergies often prefer radiant heat because it doesn’t distribute allergens like forced air systems can. Hydronic (liquid-based) systems use little electricity, a benefit for homes off the power grid or in areas with high electricity prices. Hydronic systems can use a wide variety of energy sources to heat the liquid, including standard gas- or oil-fired boilers, wood-fired boilers, solar water heaters, or a combination of these sources.

Despite its name, radiant floor heating depends heavily on convection, the natural circulation of heat within a room as air warmed by the floor rises. Radiant floor heating systems are significantly different from the radiant panels used in walls and ceilings.

thin slab stained concreteA minimum of 1.5″ of concrete slab thickness as the flooring works very well because it holds sufficient heat.  This is considered a “thermal mass floor”. “Thin slabs” can also have “radiant hydronic heating coils, or “Pex,” laid into it. It can be applied over typical 3/4″ plywood subfloor as long as the joist span meets the increased load rating.  A Note on this:  a tile grid of scores must be cut into “thin slabs” to reduce the number of random crack that occur.  We recommend using gyp-crete as the mass with antifracture membrane and tile overlay in thin slab heating.

On some South-facing floors, it is possible to pour “slab on grade” if the elevations of the building works out correctly. This can be poured up to 8″ thick, although this is considered almost wasteful and has a slow “thermal lag time”. The slab must be insulated to the ground   and exterior walls with hard insulation or the floor and perimeter concrete walls will wick cold in and heat out, AKA “cold bridging”. Proper insulation below such radiant gain surfaces is a must. Again, heating hydronics in a slab makes sense because relying solely on the sun’s energy can be difficult when the sun doesn’t shine for extended periods of time.

Through combining passive solar heating and hydronic slab heating, there is much to be gained. Hydronic coils bring heat back into the system, which flows throughout the floors of the home, helping to reduce the need to frequently run the boiler. If relying on mass alone, it must be thicker. It is possible in Northern climates to rely on an insulated 6″ concrete slab with Southern glazing and just a wood stove. People in strictly passive solar heated homes live very comfortably, even through the coldest months. The concrete slab stores solar heat so well if insulated properly, such homeowners often don’t have to light their wood stove for days at a time.

Floor Coverings

Ceramic tile is the most common and effective floor covering for radiant floor heating because it conducts heat well and adds thermal storage. Common floor coverings like vinyl and linoleum sheet goods, carpeting, or wood can also be used, but any covering that insulates the floor from the room will decrease the efficiency of the system.

If you want carpeting, use a thin carpet with dense padding and install as little carpeting as possible (throw rug). If some rooms, but not all, have a floor covering, then those rooms should have a separate tubing loop “zone” to make the system heat the spaces more efficiently. This is because the water flowing under the covered floor will need to run at a different rate to compensate for the floor covering. Wood flooring should be laminated wood flooring instead of solid wood to reduce the possibility of the wood shrinking and cracking from the drying effects of the heat.  Better yet there are plentiful examples of porcelain wood tile which conducts heat much better and adds to the mass of the floor.