Just match up each of the wires to the same letters on the new thermostat. The letters describe the function of the wires, not their colours.

Label them with whatever letter they are associated with currently. Then put them back into the corresponding letters of the new thermostat. Colors don’t matter.

Ok, I’ll bite. What do they stand for?

Black, blue, green, orange, yellow, and white.

And yellow 2 which is sometimes blue.

Note, cut power to your boiler or furnace before touching the wires. I had learned this the hard way and changed a thermostat without cutting power. While it is low voltage, two wires must have touched and blew a fuse in my furnace causing it to stop working. It cost me $300 for someone to come out and swap what was a basic car fuse.

The wire colors help you identify them on each end, if you want to be completely sure what letters they correspond to then open up your furnace/blower and see what each color is attached to on the unit. You can change them if you want.

If the thermostat works as wired, follow that same pattern with the new one. Don’t overthink this.

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.

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.

>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.

Yeah 5amp blade style fuse usually. I wish more people knew this.

I’ve saved a lot of people a lot of money (while pocketing a $20 each time) by knowing this.

This is why most new thermostats come with stickers that have the letters on them. Put the stickers on the wires according to the terminal letters before disconnecting them. Then put a piece of clear tape over them because the stickers don’t adhere very well.

E: Emergency heat Aux: Auxiliary heat Y: Cooling G:Fan O: Heat pump reverse valve switch L: Indicator lights R: Power C: Common

Thank you all for all the comments, sugestions and tips. Here is the thermostats I was gonna replace which has only 4 matched letters. Should I just connect 4 wires and tape the rests or consider a better matching thermostat? pic

I'll properly take sometimes to check it out. Thanks!

The worst part is that I talked to someone and explained what happened. They told me they would have to come look at it but it would have to wait until Monday. It was Saturday night and I live in the Buffalo NY area and it was winter. I couldn't get anyone to come look at it so I ended up using a kerosene heater. (the indoor type). When I found out it was a fuse that he could have just told me to replace I was pissed. I understand it was my own fault but it seems like he could have told me over the phone to check that and still have charged me.

Question: I have a boiler (1972) and the gas co is offering me a new programmable thermostat but I have tried this before and failed. Someone told me I need a third wire from the furnace to make this work. Any body familiar with this situation?

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Assuming someone didn't just hook wires up however they liked... ask me how I know

Since you figured out the wiring situation, I'm going to throw this in unsolicitedly: it's probably a good thing to replace the thermostat with a smart thermostat at the same time. Energy companies typically team up with Home Depot and Lowes to offer instant rebates on smart thermostats, anywhere from 50-150$ off sale value. Check on your energy company's website for a coupon. I picked up a couple ecobee smart thermostats for 15 bucks each when I rewired my furnace.

If you still have a heat pump system I'd recommend a pro to come and install a new thermostat for you. There may be internal settings that have to be adjusted for proper function.

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You can always check on the other side

There’s already quite a few data communicating thermostats on the market!

I changed guys and houses haha

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Most manufacturers have them on their higher end equipment, but yes less common on shelves because they’re all proprietary for the specific equipment.

Generally you need a common wire.

Old tstats were basically switches.

New tstats need a power circuit in addition to the switched outputs.

Please shut the power off to your furnace before doing this!!! R will be energized and you can blow a fuse if you're not careful :)

I matched the wires when I replaced my thermostat a couple of months ago. Didn’t take long at all.

Thank you. I bought the basic $29. version of Honeywell and have it working already. But I would love a programmable in the future. Thanks for the response. Now I know why the ones in the past did not work.

And it's a good idea to do so. Trust, but verify.