Infra-Red 3" Coupler for Radiant Tube Heater. The Reznor tubular radiant heaters are available for use with natural gas or optional propane gas. Tubular radiant heaters are engineered to provide quiet, reliable, energy-efficient, comfort level heating for both spot and space applications. This Infra-Red 3" Coupler for Radiant Tube Heater is designed to connect multiple heater together. This coupling is designed to connect two runs as if they were one. Heat will not be lost or disbursed as it passes through the coupling.