Last night it began getting cold in my house. I went to the thermostat and found out the Digital readout was blank. I pushed the UP button and the furnace did not turn on. I popped it off the wall and found two AAA batteries. Of course this is 3am and there are no stores open closer than a 25 mile drive (I live in a rural area).
Then I remembred that my small LED flashlite has AAA batteries, so I used those. The thermostat had alkaline batts, but the ones in the flashlite were carbon-zinc, but they worked fine and I had heat again.
That thermostat was installed in here about 4 years ago, when I had some furnace work done. I never knew it even had batteries, thinking it was powered from the transformer on the furnace. It's NOT a programmable t-stat, so aside from the digital readout, it's nothing more than a plain thermostat.
I never knew those thermostats would completely fail to start the furnace when the batteries went dead. Apparently so.
Now that I know it has batteries, I will replace them yearly, but I'm actually thinking about replacing that t-stat with a standard analog one. The last thing I need is for the pipes to freeze because the batteries failed, when I was not at home. (Even new batteries can be bad). There is no advantage to having a digital readout for me. I hardly ever change the temperture anyhow.
My question is how does this sort of thermostat switch the furnace on and off? Is it a semiconductor type of switch or what?
Yea, I did buy some new alkaline batteries today and put those in this thermostat.
(I still think the best Thermostats made were those old round Honeywell ones with the mercury switch. They lasted decades and never needed any repairs. So much for so called "improved technology").