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Posted by Dave Martindale on April 26, 2007, 12:27 am
>So I'm having an interesting problem with my new houses A/C. I turned
>on the A/C last night at the control unit and I found that once the A/
>C kicks on the control units LCD shuts off and none of the controls
>work.
Does the control unit have an internal battery? Is it dead? If so,
replace it. What happens now?
Some electronic thermostats "steal" their power from the control
circuit. When the thermostat is open, there's about 24 VAC across the
thermostat, and it can let a small amount of current flow without
energizing the gas valve or relay of the furnace or A/C. When the
thermostat contacts close, the voltage across the thermostat has to be
very low (so most of the voltage is dropped across the gas valve or
relay coil), and an electronic thermostat has to get its power from
somewhere else. If it's expecting to use an internal battery for power,
but the battery is dead, it shuts down. Normally, this would open the
controlled contacts, but perhaps you have a thermostat with some sort of
latching contacts.
Of course, the thermostat could also be defective, or wired wrong.
Dave
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