They were working on a scaffold, performing routine sandblasting inside the 22-by-27 foot boiler, according to David Harpole, a spokesman for the Equistar Chemical LP plant.
"What typically is done when you have workers in a confined space is, you have another worker who stays outside the confined space outside the entry," he said. "That worker is there to make sure the oxygen line doesn't become tangled."
When the workers inside failed to respond, the third worker informed emergency personnel.
Plant officials are not sure how much time elapsed between the loss of communication and the discovery of the men.
Harpole added that information was still sketchy and the cause of the deaths was not yet known.
Harpole said that the unit does not handle chemicals and that officials have no reason to suspect any release of hazardous materials.
Charles Perkins, 51, and Brian Turner, 37, were unconscious and not breathing when a rescue team found them on a scaffold inside the boiler heating box, according to The Houston Chronicle.
Harpole told The Chronicle the two men were employees of AIS Basic, a contractor which provides scaffolding erection and sandblasting services.
Equistar and OSHA are investigating the accident.
Channelview is approximately 15 miles east of Houston.