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wrote:
My round Honeywell 2-program thermostat seems to be loosing time, but
not the other settings. WHen I had problems with the HVAC fan
capacitor, thermostate went blank, but time & settings remained ok.
Have no idea if it has a battery or is flash memory.

Most likely, there is a NiCd battery inside the thermostat. There is
some chance that it is a coin-cell battery (CR2032 or similar) or has a
high-capacity "supercap". Usually there is a way to get at the battery
to replace it; you may have to take the thermostat off the wall. If you
do, make a note of where all the wires go.

Newer thermostats might use a supercap and I've never heard of one of
those going bad, but I guess it's possible. Usually these are
relatively large diameter (19 mm/0.75" or more) but not very tall (say
6 mm/0.25"). They will be marked with a relatively large capacitance,
like 0.1 F, 0.33 F, or 1 F. They are usually soldered in.

In the furnace there is a 24 V transformer and a fan relay coil in
series. The thermostat contacts complete this circuit to turn the
blower on. When the blower is off, the thermostat can draw a small
amount of current through the fan relay coil without turning on the
relay. This current keeps the clock running and retains the program
memory. When the blower is on, this current is not available, and is
instead provided by the battery (or supercap) in the thermostat.

If you turn off the AC power to the furnace, as you probably did when
fooling with the blower start capacitor, the 24 V transformer will be
de-energized, which is probably why the thermostat went blank. On some
furnaces, even taking off the blower door cuts all AC power to the
furnace. During that time, the battery in the thermostat was again
keeping the clock running and the program memory.

Matt Roberds