thermostate & capacitor

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

*+-> 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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