Radiant Floor Company Logo
leftcorner rightcorner

Radiant Dictionary

Air Eliminator
A device for removing small amounts of air from a closed radiant system. An important component of the Expansion and Purge Kit. (return to top)
Anti-freeze
For the purposes of radiant heat, either Propylene Glycol (non-toxic) or Ethylene Glycol (automotive/toxic). Most commonly used with heat exchangers for freeze protection or snowmelt applications. (return to top)
Bleeding the System
Removing air from a closed radiant system using the Expansion and Purge Kit. For more information see Filling the Closed System in the Installation Details section of this site. (return to top)
Brass Adaptors
Brass fittings used to convert from 3/4" rigid copper pipe to plastic Polyethylene tubing. (return to top)
Brass Couplings
Brass fittings used to join two pieces of plastic heat exchanger tubing. (return to top)
BTU
British thermal unit. A way of measuring heat energy. Or if you want to get technical: the quantity of heat required to raise the temperature of 1 pound of water by 1 degree F. (return to top)
Circuit
A length of tubing within a zone. Also called a "loop". Often, many circuits of the same length constitute a zone. (return to top)
Circulator Pump
A small, low wattage water pump used to circulate heated fluid through tubing in a radiant system. (return to top)
Closed System
A radiant heating system that uses a dedicated heat source to warm a space. A closed system re-circulates the same fluid around and around in a continuous circuit, completely separate from the domestic water supply. (return to top)
Dry Mix
A mixture of 4 parts sand and 1 part Portland cement often used as thermal mass in a "suspended slab" installation. For more information see The Suspended Slab in the Installation Details section of this site. (return to top)
XPS Foam
Short for Extruded Polystyrene. A rigid insulation often called Pink board or Blue board and used to insulate around the edges of, and below, radiantly heated slabs. It is used primarily for its high R-value and resistance to water saturation. (return to top)
Expansion Tank
A tank containing a pre-pressurized rubber membrane. It is used in closed radiant systems to absorb the expansion of water that results when liquid is heated from cold to hot. (return to top)
Expansion and Purge Kit
A factory assembled plumbing package containing fill and drain valves, expansion tank, air eliminator, pressure gauge, and pressure relief valve. Its function is to facilitate the removal of air from the newly installed radiant tubing and to maintain a balanced and efficient heating system. (return to top)
Glycol
Anti-freeze, either Propylene (non-toxic/domestic use) or ethylene (toxic/automotive use) glycol. (return to top)
Gypcrete
A lightweight concrete product often used when pouring a thin slab over an existing floor. (return to top)
Header
A copper supply line or return line used mainly in radiant staple-up systems. Basically an elongated site-built manifold, headers are useful because they allow the installer to tap off of a single, long copper line to feed multiple circuits of tubing within a single zone. Headers bring the heated fluid to the circuits instead of forcing each circuit to return to a small, centralized manifold. This method prevents congestion and snarl and gives the installer more flexibility in the placement of feed and return lines. (return to top)
Heat Diffusion Plates
Pure aluminum plates, 16" long by 8" wide, used to attach radiant tubing to the underside of the floor to be heated. The thermal properties of aluminum guarantee that heat drawn from the tubing will transfer efficiently to the floor. (return to top)
Heat Exchanger
A device used in some radiant systems to separate dissimilar fluids such as anti-freeze and water. Heat exchangers transfer heat from one fluid to another without allowing direct contact between the fluids. (return to top)
Heat Exchanger System
A type of radiant heating system that uses a heat exchanger to separate the anti-freeze in the floor from the potable domestic hot water supply. This allows one heat source to provide hot water for both applications. (return to top)
Heat Source
A water heater or boiler used to provide 120 to 130 degree water to a radiant floor. A heat source can be fed by any fuel, including gas, solar, wood, geo-thermal, oil, or electric. (return to top)
Loops
See Circuit. (return to top)
Mixing Valve
A device used to supply a consistent, lower, pre-regulated water temperature to a radiant system. Mixing valves are most often used in conjunction with high temperature boilers, many designed to heat water to temperatures in excess of 160 degrees. In radiant floor applications, mixing valves do not lower the boiler temperature. Instead, they are plumbed so that the water returning to the boiler from the heated floor branches off to the "cold" side of the mixing valve. In this way the "return" water, after losing much of its heat to the living space, is re-heated to the proper temperature by bleeding small amounts of super hot boiler water into the mixing valve. (return to top)
Open System
A radiant system integrated into the home's plumbing system. In other words, the same water that ends up in your hot shower has passed through the floor first. Open systems use one heat source to heat both the floor and the domestic water. It's an efficient system because one properly sized water heater doing two jobs eliminates the "standby loss" of a second unit. (return to top)
Poly Tubing
Slang for Polyethylene tubing. Poly tubing retains many of the heat exchanging properties of copper, but exhibits much greater flexibility and longevity. Poly tubing will last well over 100 years in radiant heat applications. (return to top)
Pressure in System
"Closed" radiant systems circulate the same fluid around and around in a closed circuit, using the fluid as a heat transfer medium. A "closed" system generally operates at a pressure of around 15-psi. "Open" systems are basically part of the domestic plumbing system. They operate at the same pressure as the house supply, generally around 40 to 50-psi. (return to top)
Pressure Testing
A feature built into a slab manifold. It is used to test for leaks prior to pouring a concrete slab and for maintaining a positive pressure in the tubing during the pour. (return to top)
R-value
A measure of a given material's ability to resist heat flow. Fiberglass has a very high R-value. That's why it's used in walls to slow the transfer of heat to the outside. Wood has a very low R-value. That's why the walls need the fiberglass. (return to top)
Radial Flooring
cartoon (return to top)
Radiant Barrier
Generally a paper thin, pure aluminum material used to reflect 97% of the infrared heat spectrum back up to the heated floor. For more information see The Floor Joist Installation in the Installation Details section of this site. (return to top)
Recovery Rate
Basically, how fast a water heater or boiler can heat water. Recovery rates are generally measured by how many gallons of water can be raised 90-degrees in one hour. If a water heater, for example, stores 50 gallons of water, a recovery rate suitable for radiant heating would be the ability to heat 1 1/2 times its storage amount within one hour. In other words, it could heat 75 gallons each hour. The best water heaters on the market can heat almost three times their capacity. Recovery rate is important in radiant systems because the water in the radiant tubing cools down to room temperature between heat cycles. Depending on the size of the zone, this could be as much as 20 or 30 gallons of water. So, when the system kicks on, this 70-degree water enters the water heater and "dilutes" the temperature. A water heater with a good recovery rate will heat the water back up quickly and return the radiant system to its desired operating temperature (125 degrees). Gas and oil fired water heaters offer the quickest recovery rates, electric water heaters the slowest. (return to top)
Repair Coupling
A brass fitting used to repair a damaged section of radiant tubing, generally in a slab application. The coupling is sheathed with rubber tape to prevent the concrete from corroding the metal fitting. (return to top)
Resistance
In radiant heating applications, resistance refers to pressure within multiple circuits of tubing. In practice, this simply means that all circuits should be the same length, within about 20-ft. No single circuit should be shorter than the others and thereby offer the water a path of least resistance to follow. With even circuits, water flows through the system in a balanced fashion and distributes heat evenly throughout the zone. (return to top)
Return Line
An insulated copper line leading back to the heat source. (return to top)
Slab Manifold

A copper manifold used to distribute heated fluid to a concrete slab. Slab manifolds come in many configurations, the simplest being the "1-loop" manifold. This is simply a supply connection to a single loop of tubing and its return. A "6-loop" manifold is generally the largest. Bigger than "6-loop", and the manifold becomes unwieldy. Imagine 6 supply connections and 6 return connections on one manifold and you'll get the idea. Super large zones will often use multiple manifolds spread throughout the heated space to achieve coverage. Slab manifolds should do more than split one supply and one return

into multiple branches. An important component of our slab manifolds is the pressure-testing feature. Testing for leaks prior to pouring a slab is mandatory. Also, the box containing our manifold acts as the form around which you pour the concrete. This creates a "manifold well" in the finished slab and keeps the multiple connections below floor level. Everything remains visible and accessible, but only the supply and return pipes stick up above the slab. Concrete never touches the copper manifold. (return to top)

Sleepers
Strapping placed on an existing floor to create "sleeper bays" in which to run radiant tubing. Most often, 2 by 4's laying flat, 16" on center, act as the best sleepers. They raise the floor 1 1/2" and allow adequate room for even the largest tubing. Sand or concrete is generally placed over the tubing, between the sleepers, and the sleepers are then used : 1) to screed across in the case of a concrete floor, or 2) for nailing down the finished floor . For more information see The Suspended Slab Installation in the Installation Details section of this site. (return to top)
Staple-up System
Tubing stapled to the underside of the sub-floor. For more information see The Floor Joist Installation in the Installation Details section of this site. (return to top)
Supply Line
Generally, an insulated copper line used to feed multiple circuits of tubing. Or, any water line running from the discharge side of a radiant circulator pump. (return to top)
Suspended Slab
A radiant floor system installed on top of an existing floor. For more information see The Suspended Slab Installation in the Installation Details section of this site. (return to top)
Thermal Mass
In the context of radiant heat, materials capable of absorbing and storing heat energy. Concrete, sand, slate, and tile possess a greater thermal mass than wood. The greater the mass, the longer stored heat will remain in the floor. (return to top)
Tubing Sizes
For most practical applications, two tubing sizes are best. 7/8" Poly and 1/2" PEX offer the greatest flexibility. The 7/8" Poly provides the highest heat output (50 BTU's per ft.) and can be spaced 16" on center. But a compromise is made when it comes to bending diameter. However, in a wide open slab environment, bending diameter is not a problem. In floor joists, spacing of 16" on center or greater is the most practical use for the 7/8" size. The 1/2" PEX ( 25 BTU's per ft.) can also be used in virtually any application, but it should be spaced 8" on center. It will heat the same as the 7/8" Poly tubing, but you have to use twice as much of it. It can raise the cost of a radiant system substantially. However, 1/2" PEX has thicker walls, and hence, a greater pressure and temperature rating. In high temperature applications it's the best way to go. Its great durability is also ideal for snowmelt situations. (return to top)
Zone
Any heated area regulated by one thermostat and supplied by one circulator pump. One zone may contain many parallel circuits of tubing, literally thousands of feet of tubing, and encompass an entire living space, but the whole area will maintain the same, even temperature. (return to top)
Zone Manifold
Most often a factory built manifold that contains the circulator pumps, ball valves, check valves, in line thermometers, pump flanges, drain valves, and all the plumbing hardware necessary to effectively distribute heated fluid to multiple heating zones. (return to top)
cartoon