New to forum; please forgive me if I’m posting to wrong place.
I have an old Honeywell CT3400 thermostat that I would like to replace with a Nest. The Nest site says I’m compatible but the old wiring looks a bit dodgy to me.
I installed a Nest at home no problem. The wiring at home was definitely low voltage single strand wire. The wiring go to the Honeywell unit looks more like lamp cord. My voltmeter indicates low voltage.
Just looking for some reassurance.
Goudy
Hello and welcome Goudy. I would suggest you start a new thread in General Questions. There are a lot of very smart people who would be looking at your new thread instead of the end of a zombie one.
Hi Goudy. My advice to you would be to get an electrician if you’re not sure - messing with the wiring in your house is a good way to burn the place down if you get it wrong.
Or start a thread in GQ. 
I’ve never seen a line-voltage stat wired with anything that looks like lamp cord. Usually, they use regular #14 solid wire and the terminals are big screws meant for holding big wires.
I’m guessing that some yutz installed the system with lamp cord because that’s what they had on hand, but the recommendation to have a pro confirm is a good one, since none of us are able to see your wiring.
I know I’m tagging on an old thread, but I stumbled on this one with a similar problem. I was sitting in my living room and it was getting pretty cold and it was warm for hours. The temperature outside, in November, was in the 60s.
It started getting pretty cold in the hose, and it was about 69 degrees out. I looked at my Honeywell LCD thermostat and it was blank. I checked my breakers an they were all ok. I was puzzled an noticed a battery slide out and there were two double AA batteries. They were pretty corroded.
I changed them out and the display re-appeared. The time was off and the temperature showed 75 deg for all stages of settings. Shortly the heat turned on.
I was surprised that the temperature of my house depended on the double AA batteries, regardless of the outside temperature. I figured the batteries would be there to maintain the time and such but the programming an control functions would be non-volatile. I called my AC service folks but no return call as of yet. Apparently, the heat is dependent on the battery power. I find that amazing considering how far gone the batteries were.