Active Loop Length

Refers to the portion of PEX Tubing in a Radiant Heat Loop that is actually in the space being heated. It is equal to the total loop length minus the leader length.


A radiant heat product that controls the valve of an individual manifold loop. These controls are wired to the thermostat and will open or close the valve depending upon when heat is needed. The actuators come in fast opening motorized versions and slower opening thermal versions.

Air Eliminator

Removes air bubbles from pipes in hydronic systems. This helps preserve proper flow in closed heating systems.

Amperage (amps)

The amount of electricity flowing through wires.


Aquapex is potable water PEX Tubing, made by Uponor Wirsbo. The Tubing is most commonly used for PEX Plumbing applications, but may also be used for Radiant Heat Applications where no ferrous materials are present.


Measures the temperature of water in hydronic systems. Calls for a boiler to ignite until the aquastat's upper temperature limit is reached. Prevents water from getting much cooler than the lower temperature limit.


Bend Support

A bend support is used in Radiant Heat and PEX Plumbing systems to make turns with PEX Tubing. The bend supports are sold in plastic, PVC, and metal. The PVC bend supports are designed for exiting concrete slabs.


An acronym for British Thermal Unit, it is a measure of the amount of energy needed to raise the temperature of one pound of water by one degree fahrenheit. Many of the heat related items in the Hydronic and Radiant Heat industry are measured in BTU's, BTU per hr, or BTU per sq. ft.


Circulator Pump

Pushes water in a hydronic system. Activated when the thermostat calls for heat. Circulator pumps can also be used with water heaters to ensure hot water is always available at the turn of the faucet.

Closely Spaced Tees

A term used for two tees used in a hydronic system to separate a primary (heat source) loop from a secondary (distribution) loop.

Condensing Boiler

A boiler that takes advantage of the condensing gases that accumulate in a boiler. This type of boiler makes use of the extra heat that is created when the gas turns to liquid. This liquid is very corrosive and therefore the boilers are created to withstand corrosion.

Counter-Flow Spiral Pattern

A pattern used to lay out PEX Tubing in a Radiant Heat System. This pattern is most often used when there are no exposed walls, or in a Snow Melt System. This pattern allows for even heating throughout a floor space. Counter-flow is not recommended for rooms with one, two, or three exposed walls.


Dead Head a Pump

When a pump such as a Taco Pump, does not have any flow of water coming to it. The pumps' motor continues to spin without any water and eventually burns out.

DIN 4726

A standard that is often mentioned when referring to PEX Tubing. This is a measure of the ability of PEX Tubing to prevent oxygen diffusion. DIN 4726 is a German Standard, which all of the oxygen barrier PEX and PEX-al-PEX on meet. This standard says that no more than .1 milligrams of oxygen will enter a system per liter of water in the system per day.

Direct-Piped System

A Radiant Heat or other hydronic heating system in which the radiant panel and the heat source are connected directly to each other. This is the simplest piping strategy possible.

Direct Vent

Venting system that vents outside and pulls air from the outside.



Indoor unit of a mini-split air conditioning system.


Refers to a type of oxygen barrier that is commonly used in PEX Tubing. EVOH stands for ethylene vinyl alcohol, this compound prevents oxygen diffusion. This compound is bonded to the PEX during the manufacturing process.


Fixed Temperature Control

A control used in a Radiant Heat or other hydronic heating system that centers the supply water temperature around a single number when heat is called for. This temperature is often referred to as the target temperature.



"Gallons per minute;" the maximum amount of hot water a tankless water heater can produce in one minute. This is the only relevant factor when sizing a tankless unit. Along with head loss, flow rate (measured in GPM) is a determining element in circulator pump sizing.


Hardy Board

A board placed over a slab, thin-slab, quik trak, or other form of heat source prior to placing tile down. This board is commonly seen in Radiant Heat Systems, as tile is an excellent floor covering type for radiant.

Head Loss

The pressure available on the outlet side of a hydronic pump. It is expressed in terms of feet of head. Feet of head represents the height of a column of water that is supported by a hydronic pump under standard atmospheric conditions.


hePEX is the brand name for the Oxygen Barrier PEX-a tubing that is made by Uponor Wirsbo. Wirsbo hePEX is designed for use in radiant heat systems. This PEX Tubing comes in several sizes and lengths.



"Indoor air quality;" a line of Honeywell products that allow temperature, humidification, dehumidification, and ventilation to be managed by a single control.

Indirect Water Heater

Uses hot water from a boiler to heat domestic water indirectly with a coil (heat exchanger) inside a tank.

Injection Mixing

A process in which hot water is injected from a primary loop into a secondary loop, using closely spaced tees, a pump, and a mixing control. This is the most preferable way to operate a hydronic heating system. Injection mixing reduces the need for controls in a system that may go bad over time. Injection mixing also increases the level of control over a hydronic heating system. A small pump such as a Taco 007 or Taco 005 Pump will supply more than enough flow.

Line Set

Insulated copper tubes that keep refrigerant cool while running it between the indoor and outdoor units of a mini-split air-conditioning system. Each indoor unit requires an individual line set.

Loop (Circuit)

One run of PEX tubing in a radiant heat system.


Leader Length

Refers to the portion of PEX Tubing that is needed to get from a manifold, to the space to being heated in a radiant heat system. Leader Length varies depending upon the layout of the building or residence being heated.

Loop Length

Describes the amount of PEX Tubing in a Radiant Heat System that is run from the supply manifold to the space being heated and back to the return manifold. The maximum loop length varies depending upon the size of the PEX Tubing being used. For 1/2" PEX Tubing the maximum loop length is 300'. When loops are run over maximum loop length, head loss becomes too high and flow rate becomes too low for a system to operate correctly.


Mean Radiant Temperature

The average temperature of all the surfaces in a room being heated by a Radiant Heat System. These temperatures will be relatively consistent in a Radiant Heat System, however in a forced hot air, or baseboard system the temperatures will vary substantially. This thermal comfort is one of the key benefits of Radiant Heat.

Mini-Split System

An air-conditioning system that combines the zoning efficiency of traditional window units with the quietness of ducted central air. Consists of an outdoor condensing unit that is connected to indoor units with insulated line sets.

Mixing Valve

A Device that blends hot and cold water. Mixing valves can be used to temper hot water in order to prevent scalding or to re-heat return water in a hydronic heating system.


One-Directional Serpentine

A term used for a pattern used to lay out PEX Tubing in a Radiant Heat system. This method is used when there is one exposed wall, or one main area of heat loss. The PEX tubing is taken to that wall and then "snaked" away in a serpentine fashion.

Open System

A Radiant Heat or Hydronic Heating system that is open to atmospheric conditions. In an open system components must be resistant to corrosion related to oxygen.

Outdoor Reset

Modifies boiler temperature based on outdoor weather conditions; particularly useful on unseasonably warm days.


The term used for a radiant heat installation in which lightweight concrete (or concrete substitute) is poured over the PEX Tubing. This installation uses a relatively thin layer of concrete as opposed to a slab installation.

Oxygen Barrier

Prevents oxygen from entering PEX tubing in heating systems. Oxygen can corrode the iron and steel in these systems.

Oxygen Diffusion

It refers to the ability of oxygen to pass through certain materials such as PEX.



PEX is an acronym for cross-linked polyethylene. PEX Tubing is now being widely used for Plumbing and Radiant Heating applications. PEX is desirable due to its flexibility, durability, and thermal memory. Synonyms for PEX include PEX Tubing, PEX Pipe, and PEX Tube.


Refers to PEX Tubing that is made using the Engel Method. This includes Wirsbo PEX Tubing and MrPEX Tubing.


PEX tubing with an embedded layer of aluminum. The aluminum serves as an oxygen barrier and helps keep the tubing straight.

PEX Clips

Used to hang PEX in joist bay installations; prevent contact between the tubing and wood.

Pneumatic Thermostat

A thermostat in which air-filled tubes register temperature due to associated changes in pressure.

Polybutylene (PB) Tubing

PB Tubing is manufactured from an oil-derived plastic resin. PB Tubing that carries the ASTM D-3309 spec may be suitable for radiant heating. However, PB Tubing is not cross-linked so it can't withstand the high temperatures that PEX Tubing can, and does not have thermal memory.

Power Vent

Venting system that vents outdoors, but uses a fan to pull air from the inside.

Primary Loop

A loop in a Hydronic Heating System that is connected to a boiler or other heat source and is isolated from the secondary loop which feeds the radiant panels or other heat output locations. The idea of using a primary loop is to be able to control the flow and temperature of this loop and offer protection to a boiler.

Primary/Secondary Piping

A piping strategy often used in hydronic heating systems in which the flow resistance of the heat source is separated from the distribution circuit. This generally involves the use of a primary pump for the loop containing the heat source. Separation is generally accomplished through the use of closely spaced tees. These tees allow heat to pass through from the heat source to the distribution circuit without allowing the primary and secondary pumps to interfere with one another.

Programmable Thermostat

Allows users to schedule different temperature settings at specific times. Most units come with four programming "steps" (wake up, leave in morning, return home in the evening, and sleep). Different schedules can be programmed for weekends. The ability to set lower nighttime and midday temperatures can result in significant reductions in energy use and utility costs.


proPEX is a PEX Tubing connection system developed by Uponor Wirsbo. This system requires the use of a Wirsbo expansion tool, proPEX Rings, and proPEX Fittings. This system may be used for Radiant Heat or PEX Plumbing applications.

Propylene Glycol

Antifreeze used in radiant heat systems; used in various proportions with water to minimize risk of freezing.


A pump is another name for a circulator. Pumps are used in hydronic systems to move water through the system at a desired rate. Two of the largest manufacturers of circulator pumps are Taco and Bell & Gossett. Taco Pumps and Bell & Gossett Pumps come in several models and should be selected based on the needs of the system.


Quik Trak

Quik Trak is a product made by Uponor (formerly known as Wirsbo). This product consists of 1/2" plywood with a thin sheet of aluminum underneath. Quik Trak may only be used with 5/16" PEX Tubing. Quik Trak is ideal for Radiant Heat systems where there is an existing slab or sub-floor.



A refrigerant used in mini-split air conditioning systems.


A measure of a material's ability to resist the flow of heat. The formula for an R-Value is R=1/U. This value is expressed as BTU/h/square ft.

Radiant Heating

The process in which heat leaves the surface of one object and travels to the surface of another. This takes place in the form of electromagnetic waves.

Reset Control

A control used in a Radiant Heat or other hydronic heating system that attempts to equate the heat loss of the building with the heat output of the Radiant Heat Panel. This type of control generally makes use of an outdoor sensor.

Reset Ratio

A description of the relationship between the supply water temperature of a radiant heat system vs. the outdoor temperature. It is the slope of the line formed by the change in supply temperature divided by the change in outdoor temperature. Most low temperature systems will be between .4 and .8, while high temperature systems will have a range of 1 to 2.

Return Line

In a hydronic heating system this is the line that brings the cold water from the space being heated back to the heat source. In a radiant heat system, the return line brings cold water from the manifold back to the heat source. It may be copper, PEX, or some other form of pipe. Sizing of return lines does vary, but is normally 3/4" or 1".

Rotated Flange

Allows for inverted bolt orientation when mounting a circulator pump (horizontal instead of vertical).


Secondary Loop

A loop in a Hydronic Heating System that is separated from the heat source, so that the system is not a direct heat system. This allows a specific flow rate and temperature to be set for the secondary loop. The secondary loop has the heat output sources, such as Radiant panels, running off of them.


"Seasonal energy efficiency ratio;" a measure of efficiency for air conditioners.

Slab Below Grade

An entire concrete slab that is greater than 4 feet below the surface.

Slab On Grade

A concrete slab that is less than 4 feet below the surface.


This is an abbreviation for Stainless Steel Clamp, a system created by Uponor Wirsbo. This system requires the stainless steel clamping tool and stainless steel clamps to make a PEX Connection. The SSC PEX Fittings are designed with barbs in order to work with the SSC Clamps.


A source of heating or cooling that needs to be controlled by a single thermostat. Many thermostats can control multiple stages. For example, a zone with radiant heating and an electrical backup with ducted central air conditioning would be 2 heat, 1 cool (2H/1C).

Standby Loss

The amount of energy wasted in hot water heaters as the water sits in the tank. This is not a concern with tankless water heaters.

Supply Line

In a hydronic heating system this is the line that supplies hot water to the system. The supply line carries hot water from the heat source to the space to be heated. In a radiant heat system, the supply line brings hot water to the manifold. It may be copper, PEX, or some other form of pipe. Sizing of return lines does vary, but is normally 3/4" or 1".

Suspended Floor

A floor that is not resting on the earth's surface. Therefore the floor is suspended above the surface.


Temperature Rise

The difference between the temperature of supply water entering a system and the desired temperature of the water once it is heated.

Thermal Radiation

Radiant Heat that can't be seen, this is what is used in radiant panel heating.


Initiates calls for heat or cooling within a zone in order to maintain desired temperature.

Three-Directional Serpentine

A term used for a pattern used to lay out PEX Tubing in a Radiant Heat system. This method is used when there are three exposed walls, or three main areas of heat loss. The PEX tubing is taken to the three walls and then "snaked" away in a serpentine fashion.

Two-Directional Serpentine

A term used for a pattern used to lay out PEX Tubing in a Radiant Heat system. This method is used when there are two exposed walls, or two main areas of heat loss. The PEX tubing is taken to the two walls and then "snaked" away in a serpentine fashion.



A measure of a material's ability to transfer heat. Describes how well an item conducts heat. The formula for U-Value is U = 1/R. This value is expressed as BTU/h/square ft.


Uponor, formerly known as Wirsbo, is the world's leading manufacturer of PEX Tubing. Uponor also makes a full line of PEX Fittings, Manifolds, and Controls. Uponor is a Swedish company with US headquarters in Apple Valley, Minnesota.


Variable Tube Spacing

A method of laying out PEX Tubing, in which the tubing is spaced closer together near high heat loss areas. For example for the first 3 to 4 feet along an exposed wall tubing may be spaced 6" apart. Once the tubing moves away from the wall 12" tube spacing may be used. This is a common technique used in Radiant Heat Applications.

Voltage (volts)

A measurement of the difference between two electric fields (in heating terms, the difference between the Earth's electrical field and the electricity running through your wires).


Wattage (watts)

The amount of electrical power that an appliance uses or generates.



Zurn is a manufacturer of plumbing products. Zurn makes a full line of PEX Products including Zurn PEX Tubing. Zurn PEX Tubing is designed for use in potable water systems.


A section within a heating or air conditioning system. Zoning can reduce energy costs by allowing different temperature settings in different areas of a building. Each zone must be controlled by its own thermostat.

Zone Valve

Used to control flow of water into zones within one-pump hydronic systems. Normally closed valves open when heat is needed in a particular zone.