"Plumbing, now that's a man's job. You want someone to catch a butterfly, call my son."
Frank Duncan is the paternal grandfather of PJ, Teddy, Gabe, Charlie, and Toby Duncan. He is the father of Bob Duncan, the ex-husband of Linda Duncan, and brother of Mel Duncan. In the episode, It's a Charlie Duncan Thanksgiving, he first appeared and fell in love with Mrs. Dabney, but when the Duncan family left them alone for an hour, they had an argument and began to hate each other.
He is portrayed by Alan Rachins.
The kids seem to prefer their maternal grandfather, Hank Blankenhooper over him, as they react positively to hear their grandfather is coming over, but react negatively when they learn it's actually Grandpa Duncan.
Frank appears to be grumpy. He doesn't even let the kids say hi to him when he walks in the door. When Amy introduces him to Charlie, he thinks that Charlie is a boy and asks why "he" is dressed like a girl. When Amy says it's because Charlie is short for Charlotte, he asks her why her name isn't pronounced "Sharlie." Then he criticizes Bob and Amy's choice of naming their children, and suggests that PJ's new name is Teddy, Gabe's new name is Charlie, and Teddy should be named PJ, but Bob says they are not renaming their kids.
When Mrs. Dabney comes over, she asks Bob if she could look at her broken garbage disposal, but Frank says he'll do it, since he has owned a plumbing business for thirty years. When he comes back, he plays with Charlie and Bob notices that he's happy, which is unsettling. Then Mrs. Dabney comes over and tells Gabe and Bob that they're dating. Bob and Gabe try to get them to break up by letting them get to know each other, but they ended up liking each other more. Gabe says that Frank hates cats, and Mrs. Dabney said she was willing to get rid of her cat Kaboodle for Frank. Bob says that Frank snores so loud it rattles the whole house, but Mrs. Dabney liked that because her ex-husband was a quiet snorer.
When Teddy is hit by the turkey and goes to the hospital, Frank and Mrs. Dabney were left alone for an hour. And they got into a fight, which makes Gabe and Bob happy. However, when Mrs. Dabney reveals she was going to move to Arizona with Frank, Gabe tries to get them back together, but it doesn't work.