The furnace gets its signals from a transformer located in the furnace which sends low voltage (24V) to the thermostat.
The thermostat is simply then a switch which is sensitive to temperature. In any event, the batteries and the 24V are fully segregated and all of the controls----like the gas valve as the best example in this situation------only operate on the 24V AC. The batteries have nothing to do with the operation of the furnace.
It may have been the thermostat. That would be my first guess. But it couldn’t have been the batteries.
But the thermostat also compares the temperature to the desired temperature to decide whether to switch on and call for heat. So if the battery fails on a programmable thermostat, what temperature setting is used? Last one? Or some default? Or something unpredictable?
The 24v totally powers the thermostat, also. The battery is just there to keep the program in the event of power failure (which will kill the 24v circuit).
I understand all that. But inside the thermostat is a relay that signals the furnace to turn on or off. If the relay is closed, and the furnace is on, and the battery dies, does the thermostat automatically open that relay, or does it stay closed forever/until you change the batteries? I can easily see a poorly designed thermostat leaving the relay closed.
Not necessarily. If your house was built with manual thermostats, they may not have run the permanently powered 24v line up to thermostats. I know in my house, if the batteries die I lose the programming regardless of the status of the house power.
I don’t have a programmable thermostat. All the battery does is the temperature readout, for all I can tell. Maybe it does other stuff.
The manual ones NEED the 24v to operate. They are just temperature sensitive switches. Their purpose is to switch the 24v. If a house was made exclusively with electronic thermostats, I can see how it could be done having the thermostat just send a control signal, but a thermostat made that way couldn’t easily be retrofitted to an older system, so no manufacturer makes them that way. They just conform to the old “switch the 24v at the thermostat” standard, which also gives them a convenient power source. I’ve actually wired thermostats, both old manual, and new electronic, in old houses. That’s the ONLY type I’ve ever seen. Why would anyone violate a perfectly good existing standard (except Microsoft, of course)?
I have 3 programmable thermostats in my house for zoning. 2 of them (the non-main zones) have only 3 wires going to them - common, hot, cold. You’re absolutely right that the only thing the thermostat does is make connections between those wires. If hot & common are connected, the heat comes on. If cold & common are connected together, the A/C comes on. I understand that the common line is at 24VAC from the others. But that 24VAC is not also being used to power the programming on the thermostats I have. The data sheet for the control board on the furnace and the instructions for the thermostat both call out a separate pin that is designed to supply permanent 24v for powering a thermostat. But the builder of my house used 3 wire thermostat cable, so short of running a new wire from the basement, I cannot get power to the thermostat to keep the programming if the batteries die. The manual quite clearly states that if you don’t connect a 24V supply to that separate pin, you will need to use batteries all the time. Trust me - I’ve had the batteries die and been stuck reprogramming the damn thing. Now it’s quite possible that I could also use the common line to connect to the power pin on the thermostat. But given that batteries are cheap, and furnace repairs are not, I’ve chosen not to try that out.
Does it have a backlight?
Even if it does nothing else, it powers a chip that remembers what temperature you had it set at in the event of a power failure, I’d wager.
I don’t think you do understand. 
The thermostat is powered by the furnace. Not by the battery.
So if the furnace is on, and the battery dies, it makes not a jot of difference, because the thermostat gets its power from the furnace and is not using the battery. So the relay will continue opening and closing just like before.
The only situation in which the battery dying would stop the thermostat from working would be if there was a power failure, in which case the furnace wouldn’t be on either.
Trust me, I understand how the thermostat that I have in my house works. It was stated by Cheshire Human that the batteries are only a backup for the power from the furnace. This is absolutely, positively, not the case for my thermostat. If you don’t wire a certain pin from the furnace to the thermostat, a pin that is called out in both the thermostat manual and the furnace spec sheet, and is not necessary for the standard operation of the furnace, then the thermostat absolutely requires battery power at all times to keep the program in memory. This pin is completely separate from any of the pins that actually control the operation of the furnace. If I pull the batteries out of the thermostat, it loses it’s programming. I tested this last night.
Now it may be the case that the power from the furnace to the thermostat over the control lines is actually powering the relay, as well as the thermostat thermometer and enough memory to keep the temperature setting that was in place when the batteries died. I have no idea if this is true or not. But I know that when I pulled the batteries last night, the heat did not shut off. I pressed the temperature down key many times, and the heat still didn’t shut off, so it certainly wasn’t accepting user input at that point. I wasn’t willing to wait long enough to see if it would shut off when the room temp came up to the last setting before I pulled the battery, but it is certainly within the realm of possibility that it would not - that in the absence of battery power, that the thermostat would just leave the relay in the same state it was in when the batteries died.
I also know that my thermostat CAN control the relays on battery power alone. It can be removed from the wall for programming, and is fully operational on just batteries. I can hold it in my hand, and play with the temperatures, and hear the relays click open and shut as I run the set temperature up and down.
As to the OP, I’d still speculate that listing the brand names is a way for the thermostat manufacturer to reap some financial gain from those companies. It would be just as easy for them to generically inform the purchaser of their unit to be sure to use high-quality or premium alkaline batteries to avoid the likelihood of leakage and damage. Some products do come with that type of legend, so I’m suspicious of this one.
I think the wording is just to ensure that people use alkaline batteries instead of lead-acid ones. If it said “use alkaline batteries”, half the people who read it wouldn’t have a clue, but everyone knows Duracell.
I stand corrected, there ARE companies other than Microsoft who would mess up a perfectly good standard. I haven’t wired an electronic one in a few years, so I have never encountered it, but it would appear from your description that the manufacturer has indeed done so. (4 wires needed??? What idiot thought of that one???)
your statement is incomplete and that causes confusion.
if you were to externally power your thermostat would it need 24VAC, 24VDC?
Don’t have the manual in front of me, but I’m assuming 24VAC - I merely wrote 24V as shorthand.
if you have 24VAC on the common wire why do you need another wire from the basement? you have 24VAC at the thermostat for both the sensing loop and the power for the thermostat circuit.
If that is true, it is not what the manual for the thermostat, and the data sheet for the furnace control board, claim. They both call out a 4th wire to power the thermostat. Again, given the great difference in both price and convenience between AA battery replacement and furnace repair, I decline to experiment with powering the thermostat with one of the control lines.
And in fact, I am assuming based on statements from others in this thread that any of the wires are actually 24VAC from ground, or from the other wires. Certainly one could design a circuit that detects if 2 wires are shorted without keeping a measurable voltage difference between them. If I get the time, I’ll try measuring the voltage of all 3 wires this weekend.
Let me apologize for not being clearer and more comprehensive.
On a typical stat the following wires are needed:
R (red) 24V Power (Hot)
**Y ** (yellow) send power out to your A/C unit
W (white) sends power to your gas valve or electric heat etc
G (green) sends power to your fan/blower
**C ** (blue) the “common” side of the 24V power. (more on this wire below)
Your digital programmable thermostat has essentially 2 functions:
1)It keeps Time and [actual] Temperature.
Because those are changing-----and therefore can’t be stored in memory------ it keeps time/temp.
2) It actually switches those wires based on what the thermostat calls for, in some/most cases. (more on that in a minute…)**
So…as an example…if your furnace calls for heat because the space is getting colder, the batteries “throw the switch” that allows the 24V to flow through the stat to the gas valve. The batteries are not operating the furnace directly, although you might say it is indirectly because it is throwing the switch.
So muldoonthief is correct that in his house batteries are required to operate the furnace [indirectly]. muldoonthief may have the 24V available to power up the fan/gas valve/ A/C etc-------- but in his house (and most houses, really…) it is the batteries that throw the switch.
So, no batteries, no furnace, A/C etc.
At the risk of being redundant then, it is accurate to say that dead/weak batteries in a thermostat will cause the furnace or A/C not to work.
There is an exception:
Most digital thermostats have a provision for a “common” wire; the “common” side of the 24V----one is “Hot” (the “R” terminal) and one is “common” (the **“C”**terminal, usually a blue wire)
Do you remember the 2 functions listed above?
Well, when you add the 5th wire----the common wire C/Blue-----the presence of the common wire allows the 24V to [essentially] “take over” the switching function and now the only function left for the batteries is to keep Time/Temp.
So if have the additional wires available and you wire in the Common wire into the stat and the furnace the stat will now work without batteries.
Does that make sense? I hope so.
One last thing…
The program memory is written on an eprom chip (…erasable, permanent memory…, iirc…), so most [reasonably] modern digital stats do not use the batteries to “hold on” to what you programmed.
muldoonthief
I’d say that theres a pretty good chance that if you wired your stat (and furnace) with these designations**:
R (red)
Y (yellow)
G (green)
W (white)
C (blue) (to common side of the 24V in the furnace)
that your furnace/stat would work without batteries, because the presence of both the Hot/ R/ 24V and Common/ C/ Blue would likely take over the switching functions in the stat. (depending how new it is)