As twack indicated, Zwave switches are very different than standard switches. You can not just replace one switch but not the other. Those “two packs” that you see are usually a Z-wave master switch and a Z-wave AUX switch. I’m a visual guy, so hope this helps:
On the top of this image is a typical 3-way setup. This is by no means the only way you can do it, but often this is how it’s done. Okay, so going left to right in a standard setup we have:
Black “HOT” line running from the circuit breaker to the first box. The hot line connects to one side of the switch, then two lines, called Traveled wires, are connected on the other side. In my house these have always been red and white, which can make things really confusing because white is usually used for neutral but in this case white is NOT neutral. In this case the first switch is sending the electricity down either the white line or the red line depending on which way the switch is turned.
In the second box you have the situation reversed. The traveler wires connect to one side of the switch. This switch takes power from the red or white wires and depending it’s position passes it thru to the black that runs to the light. Now the takes the power from the black and to complete the circuit it’s connect to a neutral (the long white wire) which goes back to the circuit breaker. This neutral may or may not pass back through the first and/or second box. This will be very important later.
Now, let’s look at the bottom row, again left to right. The first box here is the Master Switch. We’ve got black (hot) going in. We’ll only be sending this down ONE line. In a perfect world there would be a black wire headed out that we’d connect this too. In my house I don’t have this so I just connect that white traveler wire here where it should be black.
Now notice in the second box how the black line goes right thru the box. You do NOT connect this line to the AUX switch at all. It just passes thru on the way to the light. Again in my house what I did was connect the white traveler wire (which I turned into line) straight to the black wire leading to the light. So how does the AUX fit in here? The AUX goes in the second box and is connected tot he red wire. This wire is also connected to the ‘traveler’ spot on the master switch.
As you can see the AUX switch is never connected to the line that turns on or off the light. Instead the AUX switch, when pressed, sends a signal to the master switch which turns on or off. This is why you have to change both ends of a 3way. A “regular” three way switch doesn’t have any ability to send this signal to the master.
Now here’s the fun part… remember that talk about the neutral coming back from the light? Most Z-wave switches REQUIRE a neutral! If they don’t have a neutral they won’t work, so you’ll need to check your boxes to see if they have neutral before doing too much work here. Note that the traveler wire (often white) is NOT a neutral. Also note that is doesn’t have to be the same neutral that comes from the light… it can be any neutral that you can access from the box.
Now remember I said MOST z-wave switches. There are some masters that do NOT require a neutral. However, these switches are only dimmers and are only rated for incandescent lights, not CFLs or LEDs. Note also that even though the masters don’t require a neutral, the AUX still needs neutral.
I’ve probably made this sounds a lot more confusing rather than helped… sorry about that. Nature of the beast a little bit. When you get ready to replace your switches if you need help come on back. Takes some pictures of the wiring and I’ll see if I can help sort it out.