Hydronic Radiant Floor Heating
Hydronic radiant floor systems heat water in a boiler, heat pump, or water heater and force it through tubing arranged in a pattern of loops located beneath the floor surface. These systems can be clas- sified as being either wet installations or dry installations depend- ing on how the tubing is installed.
In wet installations, the tubing is commonly embedded in a con- crete foundation slab or attached to a subfloor and covered with a lightweight concrete slab. Dry installations are so called because the tubing is not embedded in concrete.
System Components
The principal components of a typical hydronic radiant floor heating system can be divided into the following categories:
1. Boilers, water heaters, and heat pumps
2. Tubing and fittings
3. Valves and related controls
4. Circulator
5. Expansion tank
6. Air separator
7. Heat exchanger
8. Thermostat
Boilers, Water Heaters, and Heat Pumps
The boilers used in hot-water radiant heating systems are the same types of heating appliances as those used in hydronic heating systems. Information about the installation, maintenance, service, and repair of hydronic boilers is contained in Chapter 15 of Volume 1.
Gas-fired boilers are the most widely used heat source in hydronic radiant heating systems. Oil-fired boilers are second in popularity and are used most commonly in the northern United States and Canada. Coal-fired boilers are still found in some hydronic radiant heating systems, but their use has steadily declined over the years.
Note
Hydronic radiant floor heating systems operate in an 85–140ºF (29–60ºC) temperature range. This is much lower than the 130– 160ºF (54–71ºC) temperature operating range required in other hydronic systems. As a result, the boilers used in floor systems operate at lower boiler temperatures, which results in a much longer service life for the appliance.
The electric boilers used in hydronic radiant floor systems are competitive with other fuels in those areas where electricity costs are low. Their principal advantage is that they are compact appliances that can be installed where space is limited.
Radiant floor systems can also be heated with a geothermal heat pump. In climates where the heating and cooling loads are equal or almost equal in size, a geothermal heat pump will be very cost effective.
Most standard water heaters produce a maximum of 40,000 to 50,000 Btu/h. This is sufficient Btu input to heat a small house or to separately heat a room addition, but it cannot provide the heat required for medium to large houses. As a result, some HVAC manu- facturers have developed high-Btu-output dedicated water heaters for radiant heating systems. These water heaters are designed specifically as single heat sources for both the domestic hot water and the space- heating requirements. As is the case with boilers used in hydronic radiant heating systems, they operate in conjunction with a circulating pump and an expansion tank. See Chapter 4 (“Water Heaters”) for additional information about combination water heaters.
Tubing and Fittings
The tubing in a radiant heating system is divided into the supply and return lines. The supply line extends from the discharge open- ing of a boiler to the manifold. It carries the heated fluid to the loops (circuits) in the floors, walls, or ceilings. A return line extends from the return side of a manifold to the boiler. It carries the water from the heating panels back to the boiler where it is reheated.
Hydronic radiant floor heating systems use copper, plastic (PEX or polybutylene tubing), or synthetic-rubber tubing to form the loops. Because of space limitations, only the two most commonly used types are described in this chapter: copper tubing and PEX (plastic) tubing. Information about the other types of tubing used in hydronic heating systems can be found in Chapter 8 (“Pipes, Pipe Fittings, and Piping Details”) of Volume 2.
Loops or Circuits
The words loop and circuit are synonyms for the length of tubing within a zone. Sometimes both are used in the same technical publication. At other times, one or the other is used exclusively. Many loops or circuits of the same length will form a zone. Circuits also refer to the electrical circuit required to operate the heating system.