The current shutdown at Jackfish II is the first since it began operations in May 2011.
Hanson said workers installed new convection boxes, a better design available from a Tulsa-based manufacturer, after the plant went offline Aug. 16.
Officials have described the oil sands operations as massive water treatment plant, turning brackish water from nearby sand formations into steam that is pumped underground to soften the thick oil.
Hanson said it will take about a month to get back to full production at Jackfish II once the shutdown is completed, based on past experience.
He said at the peak of the maintenance operation, Devon had 655 workers on site.
The project likely will exceed 90,000 man hours by the time it is completed, but Hanson said there have not been any safety incidents so far.