Some advanced furnaces use lower voltage DC, and use relays, thermostats etc for DC.
Most other furnaces use 24VAC
Nice explanation raindog. The question is then - if the batteries die while the furnace is running - the R & Y wires are connected via a relay - does the relay go back to the off position? My experiment last night with my thermostat showed that it does not.
So if the batteries are needed to keep actual temperature, what happens if the batteries die but the C wire is connected? How will the thermostat know whether it’s cold enough to turn the heat on or hot enough to turn it off? Wouldn’t it make more sense to run the internal thermometer (heck, everything) from the house power if it’s connected?
And yes, I’m sure if I wired the C wire up to my thermostat, it would run without batteries. That’s what the manual says to do. Unfortunately the cable up to it is 3 wire only (I don’t use the G wire, the furnace control board handles the blower), so short of running new wiring, I’m stuck with batteries.
I’m certain that all of these relays are “normally open” (NO) meaning they power closed, and would return to open without power.
I’ve never had to check, or ask an engineer (and I talk to engineers regularly about issues like these) but I’d be surprised if they were “normally closed,” or had to be powered open. I dunno.
I misspoke I think.
I’m sure the common wire would take over the reading of the actual ambient temperature. I know from experience that these can be operated without batteries with a common wire, and theres no way they could work if the stat couldn’t read the temperature. My bad. :smack:
Gotta run
[rant]And all those functions could have been handled by the same old 3 wire 24v standard and a little bit of electronics at the furnace/ac. Sheesh, some companies… :smack:[/rant]
Now that the rant’s out of the way, if you only have 3 wires, then your furnace/ac works by the old standard. You can either get new programmable thermostats that use that, or you can run a 4th wire, or you can keep changing the batteries. That’s all I can offer. Sorry.
The backstory - when our house was built, the builder offered programmable thermostats as an option, at some outrageously inflated price - like $800 for the 3 zones. Doing some research, I found that I could replace them myself for under $100 and half an hour of work, so I opted to keep the old style t’stats. I replaced them soon after we moved in, only to find that he used 3 wire cable that worked fine with a manual t’stat, but didn’t provide that 4th wire to power a programmable.
You can get 3 wire programmable ones. I used to have one, although I only used 2 wires because I didn’t have A/C. It’s actually quite trivial, from an electronics standpoint, to power it with the same wires you are switching. There is no good reason to require a separate power wire.
Those are electro-mechanical switches.
You can use 2 or 3 wires on any digital stat but you will lose some functionality.
2 Wires: Heat or A/C only. No fan operation. No common wire so batteries required.
3 wires: Heat and A/C, but no fan operation. Or…You can have Heat or A/C and use the 3rd wire for fan operation or common to get rid of batteries.
4 wires: Heat and A/C and Fan operation. You can use one of those wires to power for common if you want to lose one option; e.g. no A/C but 4th wire for common.
5 wires: Heat, A/C, Fan and common for digital thermostats.
For heat pumps, and gas furnaces and A/C systems that have multi-stages (or other features) it is common to need more. We run 8 conductors minimum, and when its a heat pump we run 10. (and regularly need 11)
Shoot, somebody beat me to it. I was just chiming in to say that my heat pump uses something like 7 or 8 wires, and a non-standard color code. In fact, the documentation for the thermostat mentions several times that there is no “official” color code, so you’d better use the little letter labels on the wires.
They are not electro-mechanical switches. I have one, all electronic. You don’t need a common wire. If the circuit, when closed is closed thru a low value resistor (say 1-2 ohms), you can then draw power for the electronics from the voltage drop across that resistor. The resistor is too small to effect the relay at the furnace. The only reason you need more than 3 wires is if you want a separate fan control.
Requiring a separate power line is just breaking a perfectly good standard.
Let me correct what I wrote.
The coils and other components that control [most] furnaces are electro-mechanical; many of them solenoids. Those are the switches I was referring to. The switches in the stat are electronic.
Some of the more advanced furnaces now use a communication bus and can use only 3 wires for many functions—including heat pumps and other appliances that would normally need 8 or more wires.
In any event, the different ways that furnaces can be wired ------including with different amounts of conductors--------is in post 48.
Furnaces, and A/C systems have running a 24V power to be switched for the various functions-----heat, A/C, fan and common for digital-------for decades.
That is the standard.
in the discussion there seems to be communication difficulties.
there are consumers who might be able to match wire colors to a number/letter on a terminal/screw. manuals for furnace/boiler/heat pumps might present lots of hookup options which can be confusing in relation to manuals for thermostats and its hookup options.
there are people familiar with heating/cooling systems.
there are people who might know electronics or electricity not necessarily specific to current products on the market for heating and cooling.
discussion so far seems not detailed enough to communicate all the way around.
Yes, that is the standard I’ve been talking about.