Dave ...
I'm not sure that WGBH does produce the show anymore. They did in the beginning and Russ Morash, the producer, worked for them. But at some point, due to whatever contractual agreements he made with WGBH, he apparently became the "owner" of the show. WGBH still "presents" the show because, unlike the way other networks operate, all PBS shows must have one of their local affiliates be the presenter. Another indication that WGBH does not actively produce the program and that there might be some bad blood between Morash and PBS is that the PBS website
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does not even have any This Old House related pages, nor does it link to them. Time Warner publishes This Old House Magazine and is also responsible for the official website
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The program itself, as well as Ask This Old House, and a new program starting next month called "Find" featuring the Keno Brothers of Antiques Roadshow fame, are all produced by "This Old House Ventures" which I believe is a partnership between Morash and Time Warner.
Lee