Perhaps you can search your website using keywords from the question and see the original response? If not, I'm including a cut/paste transcript of the discussion as I see it when I go to the page. The original technician appears to be recommending the use of a relay. You'll notice that they are using a white rodgers valve too. They are using model 1311-103, while I'm using model 1311-102 (not -02 as I said earlier, that was a typo). The conversation eventually drifts off and I can't tell if the recommended solution worked out or not. Perhaps we can contact the original HVAC technician who was answering the question? The website makes it seem like Douglass was the technician, but as far as I can tell, that's not a real person. It's the same picture/name used when you sign up at the website and I think that's just a marketing thing. I suppose he could be real though. Anyways, here's the transcript (abridged):I have a white rodgers 1F35-910 3 wire thermostat, (red,White, Greeen wires) Can I use the NEST programable, and how would I wire it. I have been unsuccessful. The furnaces stays on after set point is reached.Douglas: Hi, I'm Douglas and I am here to help you resolve problems with your heating and cooling equipment. I love the nest thermostat but we need to know what we are controlling. Is this a forced air system or a boiler
Customer: It is a boiler with zone valves. I'm only using it in 1 zone
Douglas: Ok… I need to do a little checking to see how we can get around the issue of not having a common wire. In the mean time, have you noticed if there is a spare, unused wire in the bundle coming out of the wall?
Customer: There is no spare. 3 wires only. They correspond to red connected to #4,white to #5 and Green to #6 on current thermostat. The wiring at zone valve has similar numbering, then the additional zone valves have wiring connecting each one.
Douglas: The red to white drives the valve open with a call for heat, the red to green drives the valve home. The nest doesnt have a way to do this internally but it can be done by adding a relay near the boiler... I'll work on a diagram but basically, the thermostat will now control a relay with multiple contacts and that will control the zone valve....The relay is wired in such a way that Red from the transformer goes to the relay contacts common and up to the thermostat and the green goes on the normally closed set and on to the zone valve. The white to the valve connects to the normally open contacts and the white from the thermostat drives the coil. A common must go to the other side of the coil on the relay. This way on a call for heat, the relay switches and powers the white at the zone valve to open it, when the thermostat satisfies, the relay returns to its unpowered state and feeds power to the green driving the valve closed. Otherwise we would need a zone valve that is powered open and has a spring return close function. Do you know where the transformer is that powers the low voltage system?
Customer: I am fairly good at this stuff. I can find the transformer if you describe it. I've looked at all the connections and taken photos of all. Is the transformer attached to the boiler directly or on a seperate mount.
Douglas: It can be on the boiler... or if you have multiple zones. It should have a wire leading to the zone valves… In the drawing below, I used black to be the white wire since white wont show up.
Customer: Would this affect the other zone valves if I didn't want to change them? The transformer wires attach to the 1st zone valve and I suspect the others are jumped from valve to valve? What would be the description of the relay when I go to but it? Electrical store or HVAC?
Douglas: No, it wouldnt impact them at all, just leave any wires that lead to other zones alone. They are parallel circuits.. You may end up with multiple wires on the transformer terminals.
Customer: What type of relay switch do I buy ?
Douglas: Its going to be a 24v coil with either single pole double throw or double pole double throw.. let me find a part number. you will probably have to go to an HVAC supply house to get it or order it online. here is an example...http://www.grainger.com/Grainger/Magnetic-Relay-6AZU4