Several times in the past week, it has forgotten the time and now it is forgetting the programmed temperature settings. Rather than wait for it to fail completely, it seems prudent to replace it now.
So we have bought a new Nest Learning Thermostat (Home Depot, $269.54) to replace it.
First we remove the old thermostat, exposing the 4 wires that it uses.
These four wires are a standard color mix (Red, Green, Yellow, White) and based upon how they were used before and an explanation on the Nest web site, we have:
R -- red -- the power wire for heating and cooling
W -- white -- controls the heating system
Y -- yellow -- controls the cooling system
G -- green -- controls the fan
These are still labeled from when the last thermostat was installed.
After patching and painting the area where the wires come out of the wall, we can install the Nest base.
and attach the wires to the Y1, G, W1, and RH tabs on the Nest base.
Then we attach the Nest thermostat on the base, and configure it. Since it is just down the hall from one of the Nest smoke detectors, the two work together to set up the network information.
and, in theory, that is all there is to it.
This Nest thermostat was removed in November 2020, when we got a brand new Trane Heating and Cooling system, which requires a Trane thermostat (but of course).