There are "2 wire" zone valves [2 wires, or, terminals]: 2 wires, operate the valve, nothing else. The valves that I mentioned are actually 4 wire, with 2 from an isolated end switch going to TT etc.
Afaik, there are 3 wire valves actually operate like those with 4 wires and an end switch. Those are usually wired with more than one circuit sharing a terminal. I have a bit of gray matter deficit, so I have a hard time understanding this, but.....http://www.houseneeds.com/upload/pdf/ta ... iescat.pdf
I'm about to wire up a 24v "3 wire" Taco and a 24v "4 wire" honeywell, both will have a pair going back to "TT". Wish me luck. LOL
Fwiw, some other wiring info;http://s3.pexsupply.com/manuals/1283431 ... D_FILE.pdfhttp://s3.pexsupply.com/manuals/1350288 ... D_FILE.pdf