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anon5005 t1_jc20487 wrote

I agree with RobBears. Many thermostats just act as a simple switch, in that case all that matters is which two wires actualy go to the furnace, and not which order. I note online that red is usually the low voltage (typically 24V) power supply input to the thermostat and the white is the output wire from the thermostat back to the furnace that calls for heat, with yellow and green not actually connected to any part of the heating system. So for many thermostats, all they do is connect the red wire to the white wire when the room is below the target temperature and disconnect if the room is above it. I'd imagine for some solid state thermostats the polarity might matter. Then you'd want not to interchange red and white. I note some online diagrams say a thermostat can sometimes independently control a fan of the furnace or air conditioning. In my country I've honestly never seen that, because the PCB of the boiler is more intelligent and more apt to know when it wants the fan running or not, and because maybe we worry about carbon too much to run air conditioners from a thermostat; but anyway in that case, either the green or yellow wire would be connected to terminals on the heating/AC system and when needed is ALSO shorted, by the thermostat, to the red wire. Even if you mess up and accidentally connect the wires wrong it will not cause any damage to your furnace/boiler/AC because some thermostats in some situations under normal operations actualy will connect all the wires together; and the only risk would be if you reverse polarity you might damage a solid state thermostat if you attach the red wire to the wrong terminal of the actual thermostat.

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Jwarenzek t1_jc20qdt wrote

That’s an oversimplification. The wires OP has can power the thermostat, provide fan only control, call for heat, and call for cool. It may even be two stage. You are correct in that each of those signal wires needs only to be shorted to the power to close the circuit and activate a particular function.

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anon5005 t1_jc22mmv wrote

>each of those signal wires needs only to be shorted to the power to close the circuit and activate a particular function.

 

Yes, exactly (I do not know what two-stage is, so likely I over-simplified something).

For years, here in England, I was frightened to mess with landine phone circuits until I noticed that all the wires excpt two are just snipped off.

 

By the way, by contrast, the thing a home user (here) should never try to mess with is ordinary electrical power sockets, because they have an unbelievably subtle configuration. All the live, neutral, and ground wires are connected in a intentional circles called 'ring mains' (with redundancy and an intentional ground loop) so you can depend on how it will work even if any wire is cut, and, secondly all the neutral wires and all the ground wires are connected together...but they are connected outdoors. Even before RCD there were almost never any fatalities because of this strange system. So for example, with two leads coming into a house, to connect two sockets, you need nine wires, with most in parallel with other wires, where only six, or actually four, would suffice. I won't try to explain the theory behind this but it is non-trivial. Essentially, neutral and ground are two independent versions of ground that are redundant but isolated from each other in such a way that significant current cannot flow between them without being noticed at the fusebox.

 

Another weird thing is, the ground wire of an electrical shower was bare, but covered by a decorative green/yellow striped tube which did not totally cover it. This meant, because of having that large bare wet electrical wire next to you, you were sure you were completely safe. And, it worked. Where, back in the USA with half the voltage (one-quarter the power into any given resistance) there was actual significant danger just because of people mowing their lawn or trying to install an electrical shower when users were allowed to wire things up. They may still have those polarized but ungrounded plugs which go in either way, but you have to press harder to get them to go in with the reverse polarity.

 

Actually, that is a sense in which any of the wires to the boiler/furnace could, in the USA, be dangerous because the ground of the furnace/boiler could well be live compared to the ground of some other room. Something that would be dangerous but you'd never notice it without stretching a wire between the rooms for some purpose.

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