It's hard to have a discussion on modern hydronic heating and not talk about primary/secondary (P/S) piping. Listening in on such a conversation, someone new to hydronics might conclude that P/S is one of the latest innovations in hydronics.
Not so. According to the Bell & Gossett Co., P/S piping dates back over 50 years (to 1954). You can read about this in the B&G publication, Primary Secondary Pumping Applications Manual (Bulletin No. THE-775), which I consider one of the classic references on the subject.