*+-> 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
Many Thanks
- = - Vasos Panagiotopoulos, Columbia'81+, Reagan, Mozart, Pindus, BioStrategist
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