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. The A/C just cranks away getting the house as cold as
possible. The only way to turn it off is to pull the panel off and
push it back into the 7 prong socket and turn off the A/C within the
first 5 seconds before the LCD shuts off. Now from what I remember my
home inspector told me the internal and external coils are older while
my furnace is one of those 4 year old 93% efficiency gas furnaces.
Btw, if the heat is on this problem never occurs. Could this just be
a control problem? This doesn't happen if lets say I set it at cool
to 78 degrees and the current temp is 74 degrees. It only happens
once the set temp is lower than the current temp. Basically when the
A/C kicks on.
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
Of course, the thermostat could also be defective, or wired wrong.
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