Hydronic (liquid) radiant floor heating

Hydronic (liquid) floor heating are by far the most popular and competitive radiant heating system.

Common hydronic heating systems are based on hot water pumped from a boiler through tubing underneath the floor; the hot water heats the slab, and the thermal mass of the slab holds the heat and drives it slowly into the house rooms.

Advantages of Hydronic Radiant Heating

Hydronic Radiant Heating offers top comfort, extremely quiet operation, improved air quality (no dust, no eventual toxic or chemical infiltrations…), significant energy savings and is particularly suitable to work with solar water heater systems or geothermal heat pumps.

Comfort

Most of all, hydronic radiant floor heating means enhanced comfort. The large radiant surface (the whole floor), the nature of the heating (radiation) and the fact that floor surface is itself warm (allowing people to walk around barefoot) explain that enhanced comfort.

Energy savings

The delivery of heat at floor level and the absence of significant airflow provide comfort at relatively low thermostat temperatures (70°F / 20°C).

Unlikely forced-air heating systems, radiant heating doesn't increase the rate of air infiltration, which is also a source of energy savings.

Disadvantages

The initial high investment and the long lag-time between the activation of the radiant floor system and the heating of the rooms are the two main disadvantages of radiant heating.

Radiant wet heating floor installation

Images source: Radiant Panel Association

Wet installations

The so-called "wet" radiant floor installation is based on tubing embedded in a thick concrete slab or in a layer of concrete, gypsum or other material installed on top of a sub-floor.

Concrete slab systems have an excellent heat capacity and can be associated to solar powered water systems.

Dry installations

Radiant floor “dry" installations are relatively new approaches in which the tubing runs in an air space under the floor.

Some installations involve suspending the tubing under the sub-floor between the joists and require reflective insulation under the tubes (in order to direct the heat upward).

Tubing may also be installed between layers of sub-floor, with tubing fitted into aluminum diffusers (to spread the water's heat across the floor).

Note: Dry radiant floor heating needs to operate at a higher temperature and demands more energy than “wet” systems, but some manufacturers are presenting cheaper “dry” packages with faster response times and less tubing (or cabling)…

PEX Tubing

Cross-linked polyethylene (PEX) tubing is a very common solution for tubing. Rubber tubing is also used, though in a much smaller scale. Copper piping is a solution of the past.

Design

Radiant floor design can be rather complex. The length of tubing per square foot of floor depends on many variables: climate, tubing diameter, type of boiler and controls…

Some manufacturers present packages integrating the various components, which simplifies the design of radiant-floor systems.

Insulation

Insulation beneath the tubing or the heated slab is crucial for radiant-floor heating systems performance. Minimums of 1”/25 mm of extruded polystyrene are often applied, but these levels should be higher in colder climates.

Zoning the radiant floor heating system & controls

The temperature in each room of the house can be controlled by regulating the temperature of the flow of water through each tubing loop – which can be accomplished by valves, pumps and thermostats.

Zoning the system is important, since maintaining different temperatures in different parts of the house allows energy savings and can respond to different needs (spaces with and without solar gains, for instance).

Temperatures, thermostats and controls

To provide maximum comfort and to regulate where the heated water should be driven, radiant-floor systems may use temperature sensors: outdoors, in the rooms, in the floor slab... Traditional standard thermostats do not provide the necessary flexibility to this regulation.

These specific controls can also be used to overcome the long lag-time of radiant-floor heating systems, since the radiant floor system can be programmatically activated in advance.