I am replacing a Mears w/ a Lux LV2-1042: Double Pole, Line Voltage, Heating Only Thermostat. This is radiant floor elec heat. Am not sure which wires connect to which. The thermostat has 4 wires (2 red, 2 black) Out of the wall comes, from the top- 1 black wire (hot), 1 white (neutral) and the ground. Then there are 2 groups of 2 wires that head down out of the box (they are all transparent gray in color) In 'dummy' terms which wires connect to which? Any help is appreciated, thanks

Just so you realize, you are probably working with 240 volts, and you should have the circuit breaker off to prevent woking on live lines.

The white you see is NOT a neutral, which would imply 120 Volts.

You need to find which set of wires that are in the box is the feed. Could be the one cable on top, or it could be the two splced on the bottom.

The thermostat should have "Line" and "Load" wires identified by labels on the wire, or stamped on the stat near the terminals, and in the instructions.

Usually the blacks are the "Line" but no always, depends on the maker.

One set of wires in the box went to a certain set of terminals on the old stat, again this should be labeled "Line" and "Load".

You need to carefully test each set of wires in the box to determine which are live, and these get connected to the "Line" wires on the stat.

Do you have a voltage tester that can handle 240 Volts? Without one you are only guessing. Since you have in floor heating , I cannot even guess which are Line or Load. Best I can offere is to get a voltage tester rated for at least 240 volts AC.

I do have the circuit breaker off when working w/ this.
I've determined that on the stat, the 2 red top wires are marked LINE and the lower 2 black wires are LOAD. If I understand correctly I need to determine which wires from the box are live and attach to the LINE wires. I don't have a voltage tester.

I had assumed the wires from the top of the box (black, white & ground) were live and the other 4 transparent wires exiting the bottom of the box were down to the radiant floor. If I get a voltage tester how exactly would I use it?

Thanks

Hmmm you said "transparent" wires twice. These may be the wires to the radiant heat, they would. Or could be wires the manufacturer permanently connects to the heat element in the floor. I am not familiar with the actual materials used in electric radiant heat, but I do know that there is no building wire that uses transparent insulation.

My guess is the single cable at the top is the feed, or line

If you use a multimeter, or any thing that has a selection knob, be sure it set to 240 volts or more AC. Use the two probes across the wires and read your volts.
There is a tester available that you only put near a live wire and do not need to touch the wire to check if there is voltage.

I was able to determine that the multi 'transparent' wires are indeed to the radiant heat. We are A OK as the heat is working. Part of my confusion/doubt was the fact that it takes so bloody long for the heat to get going. I wasn't patient enuf to wait for the heat to kick in.

Thanks for your efforts and help, much appreciated:)

When I worked for a wire and cable manufacturer, their products included cross linked polyethylene. It is more translucent than transparent. You may see wire labeled XLPE. It resists high temperatures, high voltage, and hard radiation. Similar to the PEX water pipe.