Electrical – How to do with the white wire when wiring a 240 volt garage heater

If the old fixture worked, at least one of the white wires is hot.

The easiest way to check which is hot is to use a non-contact tester. If the fixture is already removed, leave the two wires bundled together with a wire nut over the end. Put a wire nut on the single white wire. Turn the breaker back on. Only one of those should register on tester. That is the hot side. The other should not register. That is the neutral.

Once you know which wire is hot, mark it with black tape or a permanent marker.

If the the fixture is still attached, you can do the same thing with the fixture hanging down. Once you know what is what, turn the breaker back off before wiring.

The black wire goes to the hot wire, the white to the neutral. The ground wire should be attached to a green or bare wire if there is one in the box, or to the box itself if it is metal. If not, just cap it.

There's not really enough detail to give a solid answer. The answer will depend on the type of thermostat you purchased and what the existing wires are connected to. There's a good chance that you have a switched (red), neutral (white), and hot (black) wire in the wall since baseboard heat often works line voltage. This would be a single pole thermostat with a neutral which I'm not used to seeing (perhaps the old thermostat had any electronics within it). Typically you see 2 or 4 wire thermostats for baseboard heat.

The new thermostat you purchased sounds like a double pole thermostat that supports two hot connections and two switched connections. This would be for a 220v vs 110v baseboard wiring.

You'll need to do some more analysis, the easiest method being to look up the details on wiring your old thermostat and understand what each wire should be. If the white is an unneeded neutral, you'd cap it. And if the black/red you have are hot/switched then those can be wired to one half of the thermostat (only use the black or red wires on the thermostat). Double check the manual on your new thermostat to be sure it supports a single pole baseboard wiring.

If information on the old thermostat isn't available, you can at least verify the hot with a non-contact voltage tester being careful not to electrocute yourself when the breaker is energized for testing. A neutral should have continuity with a known good ground if you have a voltage tester. With the baseboard connected the switched will also show continuity like the neutral. If you somehow reverse the neutral and switched, the breaker will immediately trip when you turn on the thermostat (since you've created a short circuit).

Please don't hesitate to contact an electrician if you can't identify the wiring on the old thermostat and you don't have the equipment to safely test the wiring to verify what each wire does.