Taco needs to create an animation of how the 570 series zone valve works.

They are a bit confusing at first — I certainly was anyway! But the secret is that the three terminals are really two completely different circuits. One (1 and 2) powers the zone valve to open when it gets power — from somewhere. The other 2 and 3 — is a simple limit switch which closes when the valve is open. Apply power to 1 and 2, and the valve motor is energized and the valve opens. When it is opened, the limit switch closes between 2 and 3 and you can use that closure for whatever.

Now there is a ringer in the deck: terminal 2 is common to two circuits. Whatever is sending the signal to open the valve is one circuit. Whatever the valve is telling to operate is the other.

Those circuits must be independent. In theory, under certain miswiring conditions, it is possible to set them up so that they can short out, but if both the independent 24 VAC circuits are intact and NOT GROUNDED this won't happen.

Basically it works like this:

Terminal 2 receives 24v from R on the transformer. It is connected to the heat motor and the end switch.

Terminal 1comes from the other side of the heat motor and goes to the thermostat. When the thermostat switch closes, it connects to C on the transformer, and activates the heat motor to open the valve. The heater switch keeps the heat motor from overheating.

Terminal 3 comes from the end switch. When the valve opens, the end switch closes, outputting 24V to terminal 3. This tells the boiler to start. But the connection between the 570 and the boiler control can be problematic.

The danger comes from connecting two transformers together via terminal 3. This can burn the transformers up if they are not connected properly. I always use an isolation relay between the 570 and the boiler control, to prevent this. I've used a single relay to handle the Terminal 3 outputs from several 570s that are fed from the same transformer. Yes, it's an extra item and costs a few bucks, but it can save a lot of aggravation.

This is where the confusion starts: "Terminal 2 receives 24V from R on the transformer."

Don't think of it that way. That's how transformers get the factory-installed smoke to leave them.

Think of Terminal 2 as a common, or C terminal. When you look at it that way, it always makes sense. The problem is that not everyone thinks that way, and that's where the trouble begins.

When you come across a Taco 570 Series three-wire motor, where the transformer's 24V R is connected to #2 or common (C), things can get confusing. Eventually, when diagnosing a problem involving a transformer with R connected to the #2 or C terminal on a zone valve motor, you can end up connecting an R and a C from a transformer together.

We all know what happens when you connect R and C together on a transformer and then apply 120V to the primary winding. the factory installed smoke is released.

So think of the taco 570 series motor like this. R from the transformer goes to R on the thermostat. That makes sense all the time weather toy are wiring a heater or air conditioner or heat pump. R is always connected to R and if you use a Red insulated wire that makes it even easier to remember.

Now that you have connected one of the transformer wires to R on the thermostat, you need to connect W (the heat terminal on the thermostat) to something. That should be terminal #1 on the Taco 570 motor. This means that whenever there is a call for heat, the thermostat will send 24 VAC to the Taco motor.

However, that does not make a complete circuit for the Taco motor. You need a return path for the circuit to be complete. This is where terminal #2 becomes a Common (C) terminal. All you need to do is connect terminal #2 on the motor to C on the transformer. This completes the circuit.

That is really all you need to know about the Taco 570 series valve motor. There is more to learn, but if you want the basics to get it right every time, remember this:

Transformer R → Thermostat R (use the Red insulated wire)
Thermostat W → Taco 570 Motor Terminal #1 (use the White insulated wire)
Taco 570 Motor Terminal #2 → Transformer C (I like Blue for this, but use what ever color you have left in the bundle if spaghetti because electric is colorblind)

Once you have this circuit in place, you can then use terminal #2 (Common) and terminal #3 on the Taco 570 motor as an end switch, independent of the thermostat circuit.

Please forgive my ignorance, but I'm not seeing how you can hook the two transformers up the wrong way. Their phasing doesn't matter because they are actually two independent circuits, no?

Them being tied together on T2 doesn't really mean much.

What did I miss?