Radiant Floor Heating - Process of embedding tubing  directly in a concrete floor so that hot water can be pumped through the tubing for the purpose of heating the floor and thus the building.

Radiation - The passage of heat from one object to one other without warming the space between.

Refrigerant - The medium of heat transfer in a refrigeration system that picks up heat by evaporating at a low temperature and rejects heat by condensing at a high temperature.

Refrigeration system - A combination of parts in witch a refrigerant is circulated for the propose of extracting heat.

Relative Humidity- The amount of moisture in the air compared with the amount of moisture the air can hold.

Relay - An electromechanical device containing a coil and a set of contacts. The contacts close when the coil is energized.

Relief Device - A device designed to open under a high pressure.

Resistance - The opposition to current flow. or the reciprocal of thermal conductivity.

Rotary Compressor- A compressor in witch a compression is attained in a cylinder by rotation of a semi radial member.

Reversing Heat Pump - A heat pump in which the condenser and evaporator coils of the unit reverse roles in response to a reverse in the direction of the flow of refrigerant in the machine.

Running Time - The percentage of time a refrigeration compressor runs.