Did you forget to change the batteries again!

[Vern Heiler]

This is the second time I have found a digital thermostat without a common wire, today's was the first time I have noticed the note on the base stating it is ok. I am posting this to make all aware that a non-working thermostat may be nothing more than dead batteries.

I can't imagine how cold my wife's shoulder would be on a hot summer night, if the a/c quit because I forgot to change the batteries:(.

[Jim Luttrall]

I have one in the place I rent that has needed battery replacement about once a year. This last time the a/c would not shut off!
I have no explanation since this is totally counter intuitive to how I think it should work but once I replaced the batteries, all is well.
This is an older programmable. I have also encountered several other types out in the field on vacant houses that refuse to work without batteries.

[Jerry Peck]

I have one in the place I rent that has needed battery replacement about once a year. This last time the a/c would not shut off!
I have no explanation since this is totally counter intuitive to how I think it should work but once I replaced the batteries, all is well.

Those digital thermostats probably have a flash type memory, i.e., the bytes are either 'on' or 'off' and remain in that state until changed. That allows memory to retain settings and automatically restore the settings when power comes back on. The A/C was probably 'on' when the battery voltage dropped too low, that prevented the thermostat from switching the memory to the 'off' setting.

Kind of like a USB memory stick - no power required to keep the memory intake, only needs power to read or change what is retained on the USB memory stick.

[Vern Heiler]

Those digital thermostats probably have a flash type memory, i.e., the bytes are either 'on' or 'off' and remain in that state until changed. That allows memory to retain settings and automatically restore the settings when power comes back on. The A/C was probably 'on' when the battery voltage dropped too low, that prevented the thermostat from switching the memory to the 'off' setting.

Kind of like a USB memory stick - no power required to keep the memory intake, only needs power to read or change what is retained on the USB memory stick.

My guess would be that the switches are solenoid toggle type rather than solenoid hold on, that way there would not be as much drain on the batteries. (I would still prefer a common wire!)

[Scott Cook]

Most old thermostats just had four wires. To get power for a microprocessor based digital stat, you have to have either the fifth ("C" wire), batteries, or you would have to use a "power robbing" thermostat. The latter are not popular because they don't work with many systems. Some older "clock type" stats have to have both 24VAC power and batteries for backup because they used a lot of power for the clock motor.

Most of the newer ones can run 24VAC or just batteries. That is what the big-box stores sell to the DIY homeowners, since running a new thermostat wire is usually beyond their abilities.

When I changed-out stats for someone wanting setback abilities (the most common reason) I first looked to see if the installer ran a 5 wire line, as many did, and just didn't connect the fifth wire. If so, I hooked it up. If not, I looked at the distance from the stat to the furnace and did a fast estimate of changing out the wire. I then gave the H.O. the choice of batteries only, at no extra charge, or if a new wire could be installed in an hour or less, I gave a price for that. If it is going to take over an hour most H.O.'s will not spend the extra money as opposed to a dollar or two on batteries every year.

[Mike Clarke]

most will have either micro relays or solid state device called a triac
either of which require power to be energized so without power the default is a open circuit so no fan/heat/cool operation when the batteries dies and no hard wired power of which usually requires the common wire to complete the circuit.