Rittich is 11-7-4 with a 2.76 goals-against average and .913 save percentage for the Flames, 2-0-1 in their past three games following a six-game losing streak (0-5-1).
Geoff Ward will be behind the Calgary bench for the first time after being named coach to replace Bill Peters, who resigned Friday, three days after the NHL and Flames announced they were investigating a claim Peters directed a racial slur at a player while coaching in the American Hockey League during the 2009-10 season.
Ward, who had been Calgary associate coach, filled in for Peters in a 3-2 overtime win at the Buffalo Sabres on Wednesday.
The Senators have lost three straight, outscored 10-3, after going 5-1-0 in their previous six games.
Goalie Marcus Hogberg could start for Ottawa after Anders Nilsson made 28 saves in a 7-2 loss at the Minnesota Wild on Friday.