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swehModern smart thermostats are typically powered from the 24VAC transformer that's used for the relay. This is commonly called a "C" wire, and is from the opposite side of the transformer to the "R" wire.
One of the better descriptions I've seen is here
Now there's a second way of powering these thermostats; that's to use
an external 24VAC supply such as this one, which can be wired like this
So now the original HVAC wiring is still the R/W pair, but the additional power needed for the smarts comes from the wall-wart.
I currently only have a 2 wire system, but I can see a third (truncated) wire, so I could easily(ish) add a C wire.
One thing I can't decide, though, is which of these two options is better.
Normally, during the summer, I turn the power off to the furnace. (It's a gas furnace with electronic ignition, so it's simple; just throw the big red switch; power up in winter is just as simple). But if I do that then the thermostat will lose power and I'll lose benefits (eg ability to record temperature).
Alternatively the wall-wart would require an unsightly wire running up the wall to the thermostat... which I might be able to hide.
I'm also trying to think what would happen if the thermostat was wired backwards. In a 2-wire system this doesn't matter, but what will the consequences be if I added a C wire, or a wall-wart?
I should be able to check the wires are right, though.
One of the better descriptions I've seen is here
Now there's a second way of powering these thermostats; that's to use
an external 24VAC supply such as this one, which can be wired like this
So now the original HVAC wiring is still the R/W pair, but the additional power needed for the smarts comes from the wall-wart.
I currently only have a 2 wire system, but I can see a third (truncated) wire, so I could easily(ish) add a C wire.
One thing I can't decide, though, is which of these two options is better.
Normally, during the summer, I turn the power off to the furnace. (It's a gas furnace with electronic ignition, so it's simple; just throw the big red switch; power up in winter is just as simple). But if I do that then the thermostat will lose power and I'll lose benefits (eg ability to record temperature).
Alternatively the wall-wart would require an unsightly wire running up the wall to the thermostat... which I might be able to hide.
I'm also trying to think what would happen if the thermostat was wired backwards. In a 2-wire system this doesn't matter, but what will the consequences be if I added a C wire, or a wall-wart?
I should be able to check the wires are right, though.
