Not only did Hartnell record his second hat trick in five games, the veteran left winger scored all of his goals within a span of 11:40 in the third period, the first time he had pulled the trick off in one period.
"It's been a good couple of weeks," said Hartnell, who scored three goals in a 6-5 win over Carolina on Dec. 11. "I just want to keep going, stay positive, and stay confident."
Hartnell, who has five career hat tricks, scored on a scramble to give the Flyers a 4-0 lead at 4:45, hammered in a rebound at 7:29, and poked in a shot from the front of the left circle at 16:25.
Joffrey Lupul added a goal and three assists, and Simon Gagne also scored for the Flyers, who had won five straight games before losing at Montreal 5-2 on Thursday.
Philadelphia is unbeaten in regulation in its past 11 home games (9-0-2).
"We're just coming out with determination in our home building," Hartnell said. "People aren't going to come in here and have an easy game. We're getting physical. The coaches have us prepared and 20 guys are coming to play. We want nothing short of that."
Carter, who also had two assists, scored his goals within a span of 3:54 in the second period to move into a tie for the NHL scoring lead with Buffalo's Thomas Vanek. Both have 24 goals.
Carter put a wrist shot from the left circle off Brent Johnson's stick and between his pads at 13:11. At 16:05, he scored a short-handed goal.
It was the league-leading 13th short-handed goal for the Flyers, who matched their entire 2007-08 total.
Niittymaki, who has started the past three games in place of flu-ridden Martin Biron, was spectacular, especially in the first period when he stopped 25 shots, a Washington record for most shots in one period on the road.
The Capitals tied their franchise record for shots in one period, set on Feb. 3, 1998, against Tampa Bay.
It was also a Flyers home record for shots allowed in one period. Philadelphia's record for shots allowed is 32 at Minnesota on Dec. 13, 1972.
Brooks Laich scored for the Capitals, who had won seven of their previous eight games. Afterward, he praised Niittymaki.
"It wasn't like we weren't trying to score," Laich said. "We pressed hard. We were throwing everything at him. Guys were shooting. He was making glove saves when he wasn't seeing them."
Niittymaki, who has won six of his past seven starts, made a number of highlight-reel saves, including three on Alexander Ovechkin.
"Niitty won us that game," Carter said. "They could have had four or five or six goals in the first period. Niitty stood tall and gave us the momentum."
Johnson, who allowed three goals on 13 shots, was replaced at the start of the third period by Jose Theodore. He gave up four goals on 15 shots.
"I wasn't as sharp as I needed to be," Johnson said. "It was an odd hockey game. We had all the pressure in their zone in the first 40 minutes at least and just couldn't get one in."
Lupul put the Flyers ahead at 4:29 of the first period, scoring his 11th goal on a wrist shot from the right circle.
Laich scored from in-close on Washington's 41st shot at 5:09 of the third period.
Gagne scored his 17th goal off a pass from Carter on a power play at 9:19 of the third.
It was the first meeting between the two teams since Game 7 of last year's Eastern Conference quarterfinals, when Lupul scored the game-winner in overtime.
Philadelphia C Glen Metropolit was a late scratch because of a lower-body injury. He is day to day. ... The Flyers recalled LW Josh Gratton from the Philadelphia Phantoms, their American Hockey League affiliate. ... Washington outshot the Flyers 25-6 in the first period, and 48-28 for the game. ... Washington RW Tomas Fleischmann batted the puck out of the air and hit the post during a Capitals' power play in the first period.
Copyright by STATS LLC and The Associated Press
Arena: Wells Fargo Center Location: Philadelphia, Pennsylvania
Referees: Fred L'Ecuyer, Dan Marouelli Linesmen: Ryan Galloway, Greg Devorski
Attendance: 19,897 (102.0% full)
Team Stat Comparison
Power Play Goals
Power Play Opportunities
Shots on Goal
Joffrey Lupul (11)
Assists: Scott Hartnell, Matt Carle
Jeff Carter (23)
Assists: Joffrey Lupul, Andrew Alberts
Jeff Carter (24)
(Shorthanded) Assists: Braydon Coburn, Mike Knuble
Scott Hartnell (11)
Assist: Joffrey Lupul
Brooks Laich (8)
Assist: Tomas Fleischmann
Scott Hartnell (12)
(Power Play) Assists: Matt Carle, Kimmo Timonen
Simon Gagne (17)
(Power Play) Assists: Jeff Carter, Kimmo Timonen
Scott Hartnell (13)
Assists: Joffrey Lupul, Jeff Carter